[image: 10337.png] Diagnostic Code Explanations (2) Diagnoses for this EKG: ventricular premature complex (PVC), sinus rhythm (SR) Explanation for ventricular premature complex (PVC) On a standard 12-lead ECG, a Premature Ventricular Complex (PVC) is identified by a specific set of electrical features that distinguish it from normal sinus beats. These signs relate to timing, morphology (shape), and the pause that follows the beat. 1. Key Diagnostic Criteria The three most reliable signs of a PVC are: Prematurity: The complex occurs earlier than the next expected sinus beat. Wide QRS Complex: The QRS duration is prolonged, typically >120 ms (0.12 seconds). Because the impulse originates in the ventricle, it travels through the heart muscle (cell-to-cell) rather than the fast-conducting His-Purkinje system, resulting in a slower, wider depolarization. Absence of a P Wave: There is usually no P wave preceding the QRS complex because the impulse did not originate in the atria. (Note: A retrograde P wave may sometimes be seen after the QRS or buried within it). 2. Morphological Signs (Shape) Bizarre Appearance: The QRS complex often looks large, jagged, or distinctively different from the patient's normal sinus beats. Discordance: The ST segment and T wave are typically directed opposite to the main vector of the QRS complex. If the QRS is largely upright (positive), the T wave will be inverted (negative). If the QRS is largely downward (negative), the T wave will be upright. 3. Compensatory Pause Most PVCs are followed by a complete compensatory pause. This means the interval between the normal beat before the PVC and the normal beat after the PVC is exactly equal to two normal R-R intervals. Why? The PVC renders the ventricle refractory, blocking the next normal sinus impulse. However, the sinus node itself is usually not reset, so it fires the subsequent beat on schedule. 4. Localization Signs (Where the PVC comes from) The shape of the PVC on the 12-lead ECG can help identify its origin within the ventricles: Left Bundle Branch Block (LBBB) Pattern: If the PVC looks like an LBBB (dominant S wave in V1, wide upright R wave in V6), it likely originates from the Right Ventricle. Right Bundle Branch Block (RBBB) Pattern: If the PVC looks like an RBBB (dominant R wave in V1, wide S wave in V6), it likely originates from the Left Ventricle. 5. Patterns and Variants PVCs are often described by their frequency and pattern: Bigeminy: Every other beat is a PVC. Trigeminy: Every third beat is a PVC. Couplet: Two consecutive PVCs. Multifocal: PVCs with different shapes (morphologies), suggesting they originate from different irritable foci in the heart. Interpolated PVC: A rare variant where a PVC occurs specifically between two sinus beats without a compensatory pause, effectively "squeezing" in without disrupting the rhythm. R-on-T Phenomenon: A dangerous sign where the PVC occurs very early, landing on the T wave of the preceding beat. This can precipitate ventricular fibrillation. Read more on LITFL: https://litfl.com/premature-ventricular-complex-pvc-ecg-library/ Explanation for sinus rhythm (SR) Identifying Sinus Rhythm on a standard 12-lead ECG is the foundational skill of ECG interpretation. It essentially means the heart's electrical impulse is originating correctly from the Sinoatrial (SA) node. To confirm sinus rhythm, you must look for specific signs related to the P wave, the rhythm regularity, and the conduction intervals. 1. The P Wave (The most critical sign) The definitive sign of sinus rhythm is the "P wave axis." Because the SA node is located in the top right of the heart, the electrical current should flow down and to the left. Lead II: The P wave must be upright (positive). This is the most important lead to check. Lead aVR: The P wave must be inverted (negative). If the P wave is upright in aVR, the rhythm is likely not sinus (e.g., it may be a low atrial or junctional rhythm). Leads I and aVF: P waves are typically upright. Consistency: The P waves should all look the same (consistent morphology) within a single lead. 2. The Relationship Between P and QRS The SA node should be driving the ventricles. 1:1 Ratio: Every P wave must be followed by a QRS complex, and every QRS complex must be preceded by a P wave. PR Interval: The time between the start of the P wave and the start of the QRS complex should be constant and within normal limits (0.12 to 0.20 seconds, or 3–5 small squares). 3. Rhythm Regularity Regularity: The distance between R waves (R-R interval) and P waves (P-P interval) should be consistent. Note: Minor variation is normal due to breathing (called respiratory sinus arrhythmia), but the rhythm should look visibly regular to the naked eye. 4. Heart Rate While "Sinus Rhythm" describes the origin of the beat, the rate determines the specific diagnosis: Normal Sinus Rhythm (NSR): Rate between 60 and 100 bpm. Sinus Bradycardia: All sinus criteria met, but rate is < 60 bpm. Sinus Tachycardia: All sinus criteria met, but rate is > 100 bpm. Summary Checklist When looking at a 12-lead ECG, you can confidently state "Sinus Rhythm" if: P waves are upright in Lead II. P waves are inverted in aVR. There is a P wave before every QRS. The rhythm is regular. Read more on LITFL: https://litfl.com/normal-sinus-rhythm-ecg-library/

[image: 21828.png] Diagnostic Code Explanations (4) Diagnoses for this EKG: inferior myocardial infarction (IMI), ventricular premature complex (PVC), abnormal QRS (ABQRS), sinus rhythm (SR) Explanation for inferior myocardial infarction (IMI) An inferior myocardial infarction (MI) is a heart attack affecting the inferior wall of the heart, which rests on the diaphragm. It is most commonly caused by an occlusion of the Right Coronary Artery (RCA), though it can also result from a blockage in the Left Circumflex Artery (LCx). On a standard 12-lead ECG, you can identify an inferior MI by looking for specific changes in the "inferior leads" (II, III, and aVF). 1. Primary Diagnostic Signs The hallmark of an acute inferior MI is ST-segment elevation in the leads that view the bottom of the heart: Lead II Lead III Lead aVF Diagnostic Criteria: Significant ST-elevation (usually ≥1 mm) in at least two of these contiguous leads. 2. Reciprocal Changes Reciprocal changes are "mirror images" of the injury that appear in leads on the opposite side of the heart. These are critical for confirming the diagnosis and ruling out mimics like pericarditis. Lead aVL: This is the most sensitive lead for reciprocal changes in an inferior MI. You will typically see * ST-segment depression* here. Lead I: Often shows ST-depression as well, though usually less pronounced than in aVL. 3. Identifying the Culprit Artery (RCA vs. LCx) While an angiogram is definitive, the ECG can offer clues about which artery is blocked: RCA Occlusion (Most Common): ST elevation in Lead III > Lead II. Reciprocal ST depression in Lead I and aVL. LCx Occlusion: ST elevation in Lead II ≥ Lead III. May have ST elevation in lateral leads (I, aVL, V5, V6) or absence of reciprocal depression in Lead I. 4. Associated "Danger" Signs Inferior MIs are unique because they often extend to other areas of the heart. You must check for these two common complications immediately, as they change treatment: A. Right Ventricular (RV) Infarction Because the RCA supplies the right ventricle in most people, roughly 40% of inferior MIs involve the RV. ECG Clue: Look at lead V1. If you see ST elevation in V1 (or if V1 is isoelectric while V2 is significantly depressed), suspect RV involvement. Action: Perform a Right-Sided ECG. Look for ST elevation in V4R. Clinical Relevance: These patients are very sensitive to preload-reducing medications (like nitrates/nitroglycerin), which can cause severe hypotension. B. Posterior Extension The infarction may extend to the back of the heart (posterior wall). ECG Clue: Look at the anterior leads (V1, V2, V3). Instead of ST elevation, you will see the "mirror image": Horizontal ST depression Tall, dominant R waves (the mirror of a Q wave) Upright, prominent T waves (the mirror of T-wave inversion) Action: Perform a Posterior ECG (leads V7, V8, V9) to confirm ST elevation. 5. Rhythm Disturbances The RCA supplies the AV node in ~90% of people. Consequently, inferior MIs are frequently associated with: Bradycardia (slow heart rate). AV Blocks (First-degree, Wenckebach/Type I, or even Complete Heart Block). Read more: LITFL (ST segment): https://litfl.com/st-segment-ecg-library/ LITFL (MI localization): https://litfl.com/mi-localization-ecg-library/ LITFL (inferior STEMI): https://litfl.com/inferior-stemi-ecg-library/ Explanation for ventricular premature complex (PVC) On a standard 12-lead ECG, a Premature Ventricular Complex (PVC) is identified by a specific set of electrical features that distinguish it from normal sinus beats. These signs relate to timing, morphology (shape), and the pause that follows the beat. 1. Key Diagnostic Criteria The three most reliable signs of a PVC are: Prematurity: The complex occurs earlier than the next expected sinus beat. Wide QRS Complex: The QRS duration is prolonged, typically >120 ms (0.12 seconds). Because the impulse originates in the ventricle, it travels through the heart muscle (cell-to-cell) rather than the fast-conducting His-Purkinje system, resulting in a slower, wider depolarization. Absence of a P Wave: There is usually no P wave preceding the QRS complex because the impulse did not originate in the atria. (Note: A retrograde P wave may sometimes be seen after the QRS or buried within it). 2. Morphological Signs (Shape) Bizarre Appearance: The QRS complex often looks large, jagged, or distinctively different from the patient's normal sinus beats. Discordance: The ST segment and T wave are typically directed opposite to the main vector of the QRS complex. If the QRS is largely upright (positive), the T wave will be inverted (negative). If the QRS is largely downward (negative), the T wave will be upright. 3. Compensatory Pause Most PVCs are followed by a complete compensatory pause. This means the interval between the normal beat before the PVC and the normal beat after the PVC is exactly equal to two normal R-R intervals. Why? The PVC renders the ventricle refractory, blocking the next normal sinus impulse. However, the sinus node itself is usually not reset, so it fires the subsequent beat on schedule. 4. Localization Signs (Where the PVC comes from) The shape of the PVC on the 12-lead ECG can help identify its origin within the ventricles: Left Bundle Branch Block (LBBB) Pattern: If the PVC looks like an LBBB (dominant S wave in V1, wide upright R wave in V6), it likely originates from the Right Ventricle. Right Bundle Branch Block (RBBB) Pattern: If the PVC looks like an RBBB (dominant R wave in V1, wide S wave in V6), it likely originates from the Left Ventricle. 5. Patterns and Variants PVCs are often described by their frequency and pattern: Bigeminy: Every other beat is a PVC. Trigeminy: Every third beat is a PVC. Couplet: Two consecutive PVCs. Multifocal: PVCs with different shapes (morphologies), suggesting they originate from different irritable foci in the heart. Interpolated PVC: A rare variant where a PVC occurs specifically between two sinus beats without a compensatory pause, effectively "squeezing" in without disrupting the rhythm. R-on-T Phenomenon: A dangerous sign where the PVC occurs very early, landing on the T wave of the preceding beat. This can precipitate ventricular fibrillation. Read more on LITFL: https://litfl.com/premature-ventricular-complex-pvc-ecg-library/ Explanation for abnormal QRS (ABQRS) An abnormal QRS complex on a 12-lead ECG is typically identified by deviations in three main categories: Duration ( width), Amplitude (height/voltage), and Morphology (shape/contour). 1. Abnormal Duration (Wide QRS) A normal QRS complex lasts between 0.08 and 0.10 seconds (80–100 ms). A duration of > 0.12 seconds (>120 ms or 3 small squares) is considered abnormally wide. Bundle Branch Blocks (BBB): A blockage in the electrical conduction system causes one ventricle to depolarize later than the other, widening the QRS. Right Bundle Branch Block (RBBB): Characterized by an rSR' ("bunny ears") pattern in leads V1–V2 and a wide, slurred S wave in leads I and V6. Left Bundle Branch Block (LBBB): Characterized by a deep, broad S wave in V1 and a broad, notched, or " M-shaped" R wave in leads I, aVL, V5, and V6. Ventricular Rhythms: Rhythms originating from the ventricles (rather than the atria) do not use the fast conduction system, resulting in a wide QRS. Examples include Premature Ventricular Complexes (PVCs), Ventricular Tachycardia (VT), and Idioventricular rhythms. Hyperkalemia: High potassium levels can slow conduction, leading to a bizarrely wide QRS that may merge with the T wave (sine-wave pattern). Wolff-Parkinson-White (WPW) Syndrome: An accessory pathway allows early activation of the ventricles ( pre-excitation), causing a Delta wave (slurring of the initial upstroke) and a widened QRS. 2. Abnormal Amplitude (Voltage) The height of the QRS complex represents the electrical force generated by the ventricular muscle mass. High Voltage (Hypertrophy): Left Ventricular Hypertrophy (LVH): The muscle wall is thickened, generating stronger electrical forces. A common sign is the Sokolow-Lyon criteria: Depth of S wave in V1 + Height of R wave in V5 or V6 > 35 mm. Right Ventricular Hypertrophy (RVH): Often causes a dominant R wave in lead V1 (height > 7 mm) and a deep S wave in V5 or V6. Low Voltage: Defined as QRS amplitude < 5 mm in all limb leads and < 10 mm in all precordial (chest) leads. Causes: Anything that insulates the heart or dampens the signal, such as pericardial effusion (fluid around the heart), COPD (air trapping), obesity, or hypothyroidism (myxedema). 3. Abnormal Morphology (Shape) Even if the width and height are normal, the shape of the wave can indicate pathology. Pathological Q Waves: Q waves are the first downward deflection of the QRS. While small "septal" Q waves are normal in some leads, * pathological* Q waves indicate dead myocardial tissue (previous Myocardial Infarction). Signs: Duration > 0.04 s (40 ms) or depth > 25% of the following R wave height. Poor R Wave Progression: Normally, the R wave grows larger as you move from lead V1 to V6. If the R wave remains small or absent in leads V1–V3, it is termed "poor progression." Causes: Anterior Myocardial Infarction (old or new), LBBB, or lead misplacement. Fragmented QRS: Presence of additional spikes, notches, or slurs within the QRS complex (not fitting a typical BBB pattern). This often represents myocardial scarring or fibrosis. Electrical Alternans: The height of the QRS complex alternates between beats (large, small, large, small). This is a specific sign of a large pericardial effusion (cardiac tamponade) as the heart swings back and forth in the fluid. Read more: LITFL (interventricular conduction delay): https://litfl.com/intraventricular-conduction-delay-qrs-widening/ LITFL (low QRS voltage): https://litfl.com/low-qrs-voltage-ecg-library/ Explanation for sinus rhythm (SR) Identifying Sinus Rhythm on a standard 12-lead ECG is the foundational skill of ECG interpretation. It essentially means the heart's electrical impulse is originating correctly from the Sinoatrial (SA) node. To confirm sinus rhythm, you must look for specific signs related to the P wave, the rhythm regularity, and the conduction intervals. 1. The P Wave (The most critical sign) The definitive sign of sinus rhythm is the "P wave axis." Because the SA node is located in the top right of the heart, the electrical current should flow down and to the left. Lead II: The P wave must be upright (positive). This is the most important lead to check. Lead aVR: The P wave must be inverted (negative). If the P wave is upright in aVR, the rhythm is likely not sinus (e.g., it may be a low atrial or junctional rhythm). Leads I and aVF: P waves are typically upright. Consistency: The P waves should all look the same (consistent morphology) within a single lead. 2. The Relationship Between P and QRS The SA node should be driving the ventricles. 1:1 Ratio: Every P wave must be followed by a QRS complex, and every QRS complex must be preceded by a P wave. PR Interval: The time between the start of the P wave and the start of the QRS complex should be constant and within normal limits (0.12 to 0.20 seconds, or 3–5 small squares). 3. Rhythm Regularity Regularity: The distance between R waves (R-R interval) and P waves (P-P interval) should be consistent. Note: Minor variation is normal due to breathing (called respiratory sinus arrhythmia), but the rhythm should look visibly regular to the naked eye. 4. Heart Rate While "Sinus Rhythm" describes the origin of the beat, the rate determines the specific diagnosis: Normal Sinus Rhythm (NSR): Rate between 60 and 100 bpm. Sinus Bradycardia: All sinus criteria met, but rate is < 60 bpm. Sinus Tachycardia: All sinus criteria met, but rate is > 100 bpm. Summary Checklist When looking at a 12-lead ECG, you can confidently state "Sinus Rhythm" if: P waves are upright in Lead II. P waves are inverted in aVR. There is a P wave before every QRS. The rhythm is regular. Read more on LITFL: https://litfl.com/normal-sinus-rhythm-ecg-library/

[image: 10450.png] Diagnostic Code Explanations (3) Diagnoses for this EKG: anterior myocardial infarction (AMI), abnormal QRS (ABQRS), sinus arrhythmia (SARRH) Explanation for anterior myocardial infarction (AMI) On a standard 12-lead ECG, an anterior myocardial infarction (MI) is primarily identified by changes in the precordial leads overlying the front of the heart. The following breakdown details the specific leads involved, the characteristic waveform changes, and the anatomical context. 1. Affected Leads The primary diagnostic changes for an anterior MI appear in the precordial chest leads: V3 and V4: These are the specific "anterior" leads. V1 and V2: Often involved as well, indicating septal involvement (Anteroseptal MI). V5 and V6: Involvement here suggests the infarct extends to the lateral wall (Anterolateral MI). Extensive Anterior MI: If ST elevation is present across V1–V6, it indicates a large infarction affecting a significant portion of the left ventricle. 2. Characteristic Waveform Changes The ECG signs evolve over time, but the classic findings of an acute anterior STEMI (ST-Elevation Myocardial Infarction) include: ST-Segment Elevation: This is the hallmark sign. You will see significant elevation of the ST segment (the flat line between the QRS complex and the T wave) in leads V3 and V4 (and often V1-V2). Note: The elevation is often convex (rounded upward) or obliquely straight. Severe cases may show " tombstoning," where the ST segment rises high enough to merge with the T wave. Hyperacute T Waves: In the very early stages (minutes after occlusion), T waves in the anterior leads may become tall, broad, and peaked. This often precedes visible ST elevation. Pathological Q Waves: As the infarction progresses and heart muscle tissue dies (necrosis), deep Q waves may develop in leads V1–V4. Poor R-Wave Progression: Normally, the R wave (the upward spike) gets taller moving from lead V1 to V6. In an anterior MI, the R waves in V1–V4 may remain small or disappear entirely (becoming QS complexes), indicating loss of viable muscle in the anterior wall. 3. Reciprocal Changes "Reciprocal changes" refer to ST depression that appears in leads electrically opposite to the infarct. Inferior Leads (II, III, aVF): You will frequently see reciprocal ST depression in these leads during an anterior MI. Significance: While not always present, reciprocal depression in the inferior leads strongly confirms that the ST elevation in the anterior leads is due to an acute MI rather than a benign mimic (like pericarditis or benign early repolarization). 4. Culprit Artery The presence of these signs almost always indicates an occlusion of the Left Anterior Descending (LAD) artery. Proximal LAD Occlusion: Tends to cause widespread elevation (V1–V6, plus leads I and aVL). Distal LAD Occlusion: Changes are often more localized to V3–V4. Summary Checklist for Anterior MI Primary Elevation: V3, V4 (often V1–V4) Reciprocal Depression: II, III, aVF T Waves: Hyperacute (early) or Inverted (late) QRS Complex: Loss of R-wave height; development of Q waves Culprit Artery: Left Anterior Descending (LAD) Read more: LITFL (ST segment): https://litfl.com/st-segment-ecg-library/ LITFL (MI localization): https://litfl.com/mi-localization-ecg-library/ LITFL (anterior STEMI): https://litfl.com/anterior-myocardial-infarction-ecg-library/ Explanation for abnormal QRS (ABQRS) An abnormal QRS complex on a 12-lead ECG is typically identified by deviations in three main categories: Duration ( width), Amplitude (height/voltage), and Morphology (shape/contour). 1. Abnormal Duration (Wide QRS) A normal QRS complex lasts between 0.08 and 0.10 seconds (80–100 ms). A duration of > 0.12 seconds (>120 ms or 3 small squares) is considered abnormally wide. Bundle Branch Blocks (BBB): A blockage in the electrical conduction system causes one ventricle to depolarize later than the other, widening the QRS. Right Bundle Branch Block (RBBB): Characterized by an rSR' ("bunny ears") pattern in leads V1–V2 and a wide, slurred S wave in leads I and V6. Left Bundle Branch Block (LBBB): Characterized by a deep, broad S wave in V1 and a broad, notched, or " M-shaped" R wave in leads I, aVL, V5, and V6. Ventricular Rhythms: Rhythms originating from the ventricles (rather than the atria) do not use the fast conduction system, resulting in a wide QRS. Examples include Premature Ventricular Complexes (PVCs), Ventricular Tachycardia (VT), and Idioventricular rhythms. Hyperkalemia: High potassium levels can slow conduction, leading to a bizarrely wide QRS that may merge with the T wave (sine-wave pattern). Wolff-Parkinson-White (WPW) Syndrome: An accessory pathway allows early activation of the ventricles ( pre-excitation), causing a Delta wave (slurring of the initial upstroke) and a widened QRS. 2. Abnormal Amplitude (Voltage) The height of the QRS complex represents the electrical force generated by the ventricular muscle mass. High Voltage (Hypertrophy): Left Ventricular Hypertrophy (LVH): The muscle wall is thickened, generating stronger electrical forces. A common sign is the Sokolow-Lyon criteria: Depth of S wave in V1 + Height of R wave in V5 or V6 > 35 mm. Right Ventricular Hypertrophy (RVH): Often causes a dominant R wave in lead V1 (height > 7 mm) and a deep S wave in V5 or V6. Low Voltage: Defined as QRS amplitude < 5 mm in all limb leads and < 10 mm in all precordial (chest) leads. Causes: Anything that insulates the heart or dampens the signal, such as pericardial effusion (fluid around the heart), COPD (air trapping), obesity, or hypothyroidism (myxedema). 3. Abnormal Morphology (Shape) Even if the width and height are normal, the shape of the wave can indicate pathology. Pathological Q Waves: Q waves are the first downward deflection of the QRS. While small "septal" Q waves are normal in some leads, * pathological* Q waves indicate dead myocardial tissue (previous Myocardial Infarction). Signs: Duration > 0.04 s (40 ms) or depth > 25% of the following R wave height. Poor R Wave Progression: Normally, the R wave grows larger as you move from lead V1 to V6. If the R wave remains small or absent in leads V1–V3, it is termed "poor progression." Causes: Anterior Myocardial Infarction (old or new), LBBB, or lead misplacement. Fragmented QRS: Presence of additional spikes, notches, or slurs within the QRS complex (not fitting a typical BBB pattern). This often represents myocardial scarring or fibrosis. Electrical Alternans: The height of the QRS complex alternates between beats (large, small, large, small). This is a specific sign of a large pericardial effusion (cardiac tamponade) as the heart swings back and forth in the fluid. Read more: LITFL (interventricular conduction delay): https://litfl.com/intraventricular-conduction-delay-qrs-widening/ LITFL (low QRS voltage): https://litfl.com/low-qrs-voltage-ecg-library/ Explanation for sinus arrhythmia (SARRH) On a standard 12-lead ECG, sinus arrhythmia is characterized by a normal sinus rhythm that has an irregular rate. The "irregularity" is usually patterned and predictable, corresponding to the patient's breathing cycle. Here are the specific signs and criteria used to identify it: 1. The Core Criterion: R-R Interval Variation The hallmark sign of sinus arrhythmia is a variation in the timing between heartbeats (the R-R interval) that originates from the sinus node. Variation Threshold: The difference between the longest and shortest R-R interval (or P-P interval) is greater than 0.12 seconds (which equals 3 small squares on standard ECG paper). Pattern: The rhythm typically speeds up and slows down in a cyclical pattern. 2. Normal P-Wave Morphology Because the electrical impulse still originates from the sinoatrial (SA) node, the atrial depolarization looks normal. Appearance: P-waves are upright in leads I and II. Consistency: The P-waves have a constant shape and structure beat-to-beat. There is one P-wave for every QRS complex. 3. Constant PR Interval The time it takes for the impulse to travel from the atria to the ventricles remains stable. The PR interval is constant and typically within the normal range of 0.12 to 0.20 seconds. 4. Normal QRS Complex The ventricular contraction is unaffected, so the QRS complex is narrow (usually < 0.12 seconds) and follows every P-wave. 5. Correlation with Respiration (Respiratory Sinus Arrhythmia) This is the most common form, especially in children and young, healthy adults. You can often see this "phasic" change on the rhythm strip at the bottom of the 12-lead page: Inspiration (Breathing in): The heart rate increases (R-R intervals get shorter). Expiration (Breathing out): The heart rate decreases (R-R intervals get longer). Note on Clinical Context: While usually benign and respiratory in nature, non-respiratory sinus arrhythmia (where the irregularity does not align with the breathing cycle) can occur. This is more commonly seen in older adults and can be associated with underlying heart disease or digitalis toxicity. Read more on LITFL: https://litfl.com/sinus-arrhythmia-ecg-library/

[image: 8063.png] Diagnostic Code Explanations (2) Diagnoses for this EKG: left anterior fascicular block (LAFB), sinus rhythm (SR) Explanation for left anterior fascicular block (LAFB) Left anterior fascicular block (LAFB), also known as left anterior hemiblock, is a conduction abnormality where the anterior fascicle of the left bundle branch is blocked. This forces electrical impulses to depolarize the left ventricle via the posterior fascicle, altering the direction of the electrical vector. Diagnosing LAFB relies on specific changes in the QRS axis and morphology. Below are the primary signs seen on a standard 12-lead ECG. 1. Pathological Left Axis Deviation (LAD) This is the hallmark sign of LAFB. Because the anterior fascicle is blocked, the electrical wavefront travels from the posterior/inferior wall towards the anterior/superior wall. This creates a net electrical vector that points upward and to the left. Criterion: The axis is typically between -45° and -90°. Note: While an axis between -30° and -45° suggests possible LAFB, it is less specific and can be a normal variant or due to left ventricular hypertrophy. A shift beyond -45° is generally required for a definitive diagnosis. 2. Lead-Specific QRS Morphology The redirected electrical vector creates a "divergent" pattern between the lateral and inferior leads. Lateral Leads (I and aVL): Pattern: qR complex (small Q wave, tall R wave). Explanation: The initial forces go downwards/rightwards (creating the small 'q'), followed by the major forces going upwards/leftwards (creating the tall 'R'). Inferior Leads (II, III, and aVF): Pattern: rS complex (small R wave, deep S wave). Explanation: The initial forces move briefly towards these leads (small 'r'), but the bulk of the depolarization moves away from them (deep 'S'). Key Check: Lead aVF should be negative. Lead II should also be negative (which confirms the axis is more negative than -30°). 3. Normal or Minimally Prolonged QRS Duration Unlike a complete Left Bundle Branch Block (LBBB), the main conduction trunk is intact, so ventricular depolarization is not significantly delayed. Criterion: QRS duration is usually < 120 ms (typically 80–110 ms). Context: If the QRS is ≥ 120 ms, a complete bundle branch block is likely present (e.g., RBBB + LAFB). 4. Prolonged R-Wave Peak Time in Lead aVL This measures the time from the onset of the QRS to the peak of the R wave (intrinsicoid deflection), representing the time it takes for depolarization to reach the lateral wall. Criterion: R-wave peak time ≥ 45 ms in lead aVL. Comparison: This is often delayed compared to lead V6. Summary of Diagnostic Criteria (Rule of Thumb) To quickly identify LAFB, look for: Left Axis Deviation (Positive Lead I, Negative Lead aVF). Negative Lead II (Deep S wave). Small Q waves in I and aVL. Normal QRS width (< 120 ms). Important Clinical Note: LAFB is a diagnosis of exclusion. You must ensure that the Left Axis Deviation is not caused by other conditions, such as a preexisting inferior myocardial infarction (which causes deep Q waves in II, III, and aVF) or severe Left Ventricular Hypertrophy (LVH). Read more on LITFL: https://litfl.com/left-anterior-fascicular-block-lafb-ecg-library/ Explanation for sinus rhythm (SR) Identifying Sinus Rhythm on a standard 12-lead ECG is the foundational skill of ECG interpretation. It essentially means the heart's electrical impulse is originating correctly from the Sinoatrial (SA) node. To confirm sinus rhythm, you must look for specific signs related to the P wave, the rhythm regularity, and the conduction intervals. 1. The P Wave (The most critical sign) The definitive sign of sinus rhythm is the "P wave axis." Because the SA node is located in the top right of the heart, the electrical current should flow down and to the left. Lead II: The P wave must be upright (positive). This is the most important lead to check. Lead aVR: The P wave must be inverted (negative). If the P wave is upright in aVR, the rhythm is likely not sinus (e.g., it may be a low atrial or junctional rhythm). Leads I and aVF: P waves are typically upright. Consistency: The P waves should all look the same (consistent morphology) within a single lead. 2. The Relationship Between P and QRS The SA node should be driving the ventricles. 1:1 Ratio: Every P wave must be followed by a QRS complex, and every QRS complex must be preceded by a P wave. PR Interval: The time between the start of the P wave and the start of the QRS complex should be constant and within normal limits (0.12 to 0.20 seconds, or 3–5 small squares). 3. Rhythm Regularity Regularity: The distance between R waves (R-R interval) and P waves (P-P interval) should be consistent. Note: Minor variation is normal due to breathing (called respiratory sinus arrhythmia), but the rhythm should look visibly regular to the naked eye. 4. Heart Rate While "Sinus Rhythm" describes the origin of the beat, the rate determines the specific diagnosis: Normal Sinus Rhythm (NSR): Rate between 60 and 100 bpm. Sinus Bradycardia: All sinus criteria met, but rate is < 60 bpm. Sinus Tachycardia: All sinus criteria met, but rate is > 100 bpm. Summary Checklist When looking at a 12-lead ECG, you can confidently state "Sinus Rhythm" if: P waves are upright in Lead II. P waves are inverted in aVR. There is a P wave before every QRS. The rhythm is regular. Read more on LITFL: https://litfl.com/normal-sinus-rhythm-ecg-library/

[image: 15796.png] Diagnostic Code Explanations (1) Diagnoses for this EKG: atrial fibrillation (AFIB) Explanation for atrial fibrillation (AFIB) On a standard 12-lead ECG, atrial fibrillation (AFib) is characterized by three primary diagnostic signs: an " irregularly irregular" rhythm, the absence of P waves, and the presence of fibrillatory waves. The following details explain these signs and how to identify them. 1. Irregularly Irregular Rhythm This is the hallmark of AFib. What to look for: Check the R-R intervals (the distance between the spikes of the QRS complexes). In AFib, these intervals are completely unpredictable. There is no pattern to the irregularity—it is chaotic. Why it happens: The AV node is bombarded by chaotic electrical impulses from the atria (often >300 per minute). It blocks many of them but lets others through at random intervals, causing the ventricles to beat irregularly. 2. Absence of P Waves What to look for: Look at the baseline immediately before each QRS complex. In a normal sinus rhythm, you would see a small, rounded "bump" (the P wave). In AFib, these discrete, organized bumps are missing. Lead Specifics: This is often most obvious in Lead II and Lead V1, which are typically the best leads for viewing atrial activity. 3. Presence of Fibrillatory (f) Waves Instead of P waves, you will often see a wavy, chaotic baseline between QRS complexes. Appearance: These can look like fine, low-amplitude tremors (fine AFib) or more distinct, jagged oscillations (coarse AFib). Best Leads: Fibrillatory waves are usually most visible in Lead V1 or the inferior leads (II, III, aVF). Note: In long-standing AFib, the f-waves may be so fine that the baseline simply looks flat (isoelectric). 4. Ventricular Rate (Heart Rate) While the atrial rate is extremely fast (300–600 bpm), the ventricular rate (what you feel as a pulse) varies based on how many impulses pass through the AV node. AFib with RVR (Rapid Ventricular Response): Heart rate >100 bpm. This is common in untreated, new-onset AFib. Controlled AFib: Heart rate between 60–100 bpm (often achieved with medication). Slow AFib: Heart rate <60 bpm (may indicate medication toxicity or intrinsic conduction disease). 5. QRS Morphology (Shape) Narrow QRS: typically, the QRS complexes remain narrow (<120 ms) because the electrical impulse still travels through the ventricles normally (via the His-Purkinje system). Ashman’s Phenomenon: Occasionally, you may see a wide, bizarre QRS complex that looks like a PVC (Premature Ventricular Complex). If this occurs after a long R-R interval followed immediately by a short R-R interval, it is likely a benign conduction aberration known as Ashman's phenomenon, rather than a true ventricular beat. Read more on LITFL: https://litfl.com/atrial-fibrillation-ecg-library/

[image: 11052.png] Diagnostic Code Explanations (1) Diagnoses for this EKG: sinus rhythm (SR) Explanation for sinus rhythm (SR) Identifying Sinus Rhythm on a standard 12-lead ECG is the foundational skill of ECG interpretation. It essentially means the heart's electrical impulse is originating correctly from the Sinoatrial (SA) node. To confirm sinus rhythm, you must look for specific signs related to the P wave, the rhythm regularity, and the conduction intervals. 1. The P Wave (The most critical sign) The definitive sign of sinus rhythm is the "P wave axis." Because the SA node is located in the top right of the heart, the electrical current should flow down and to the left. Lead II: The P wave must be upright (positive). This is the most important lead to check. Lead aVR: The P wave must be inverted (negative). If the P wave is upright in aVR, the rhythm is likely not sinus (e.g., it may be a low atrial or junctional rhythm). Leads I and aVF: P waves are typically upright. Consistency: The P waves should all look the same (consistent morphology) within a single lead. 2. The Relationship Between P and QRS The SA node should be driving the ventricles. 1:1 Ratio: Every P wave must be followed by a QRS complex, and every QRS complex must be preceded by a P wave. PR Interval: The time between the start of the P wave and the start of the QRS complex should be constant and within normal limits (0.12 to 0.20 seconds, or 3–5 small squares). 3. Rhythm Regularity Regularity: The distance between R waves (R-R interval) and P waves (P-P interval) should be consistent. Note: Minor variation is normal due to breathing (called respiratory sinus arrhythmia), but the rhythm should look visibly regular to the naked eye. 4. Heart Rate While "Sinus Rhythm" describes the origin of the beat, the rate determines the specific diagnosis: Normal Sinus Rhythm (NSR): Rate between 60 and 100 bpm. Sinus Bradycardia: All sinus criteria met, but rate is < 60 bpm. Sinus Tachycardia: All sinus criteria met, but rate is > 100 bpm. Summary Checklist When looking at a 12-lead ECG, you can confidently state "Sinus Rhythm" if: P waves are upright in Lead II. P waves are inverted in aVR. There is a P wave before every QRS. The rhythm is regular. Read more on LITFL: https://litfl.com/normal-sinus-rhythm-ecg-library/

[image: 1639.png] Diagnostic Code Explanations (1) Diagnoses for this EKG: sinus rhythm (SR) Explanation for sinus rhythm (SR) Identifying Sinus Rhythm on a standard 12-lead ECG is the foundational skill of ECG interpretation. It essentially means the heart's electrical impulse is originating correctly from the Sinoatrial (SA) node. To confirm sinus rhythm, you must look for specific signs related to the P wave, the rhythm regularity, and the conduction intervals. 1. The P Wave (The most critical sign) The definitive sign of sinus rhythm is the "P wave axis." Because the SA node is located in the top right of the heart, the electrical current should flow down and to the left. Lead II: The P wave must be upright (positive). This is the most important lead to check. Lead aVR: The P wave must be inverted (negative). If the P wave is upright in aVR, the rhythm is likely not sinus (e.g., it may be a low atrial or junctional rhythm). Leads I and aVF: P waves are typically upright. Consistency: The P waves should all look the same (consistent morphology) within a single lead. 2. The Relationship Between P and QRS The SA node should be driving the ventricles. 1:1 Ratio: Every P wave must be followed by a QRS complex, and every QRS complex must be preceded by a P wave. PR Interval: The time between the start of the P wave and the start of the QRS complex should be constant and within normal limits (0.12 to 0.20 seconds, or 3–5 small squares). 3. Rhythm Regularity Regularity: The distance between R waves (R-R interval) and P waves (P-P interval) should be consistent. Note: Minor variation is normal due to breathing (called respiratory sinus arrhythmia), but the rhythm should look visibly regular to the naked eye. 4. Heart Rate While "Sinus Rhythm" describes the origin of the beat, the rate determines the specific diagnosis: Normal Sinus Rhythm (NSR): Rate between 60 and 100 bpm. Sinus Bradycardia: All sinus criteria met, but rate is < 60 bpm. Sinus Tachycardia: All sinus criteria met, but rate is > 100 bpm. Summary Checklist When looking at a 12-lead ECG, you can confidently state "Sinus Rhythm" if: P waves are upright in Lead II. P waves are inverted in aVR. There is a P wave before every QRS. The rhythm is regular. Read more on LITFL: https://litfl.com/normal-sinus-rhythm-ecg-library/

[image: 15989.png] Diagnostic Code Explanations (3) Diagnoses for this EKG: left anterior fascicular block (LAFB), incomplete right bundle branch block (IRBBB), sinus bradycardia (SBRAD) Explanation for left anterior fascicular block (LAFB) Left anterior fascicular block (LAFB), also known as left anterior hemiblock, is a conduction abnormality where the anterior fascicle of the left bundle branch is blocked. This forces electrical impulses to depolarize the left ventricle via the posterior fascicle, altering the direction of the electrical vector. Diagnosing LAFB relies on specific changes in the QRS axis and morphology. Below are the primary signs seen on a standard 12-lead ECG. 1. Pathological Left Axis Deviation (LAD) This is the hallmark sign of LAFB. Because the anterior fascicle is blocked, the electrical wavefront travels from the posterior/inferior wall towards the anterior/superior wall. This creates a net electrical vector that points upward and to the left. Criterion: The axis is typically between -45° and -90°. Note: While an axis between -30° and -45° suggests possible LAFB, it is less specific and can be a normal variant or due to left ventricular hypertrophy. A shift beyond -45° is generally required for a definitive diagnosis. 2. Lead-Specific QRS Morphology The redirected electrical vector creates a "divergent" pattern between the lateral and inferior leads. Lateral Leads (I and aVL): Pattern: qR complex (small Q wave, tall R wave). Explanation: The initial forces go downwards/rightwards (creating the small 'q'), followed by the major forces going upwards/leftwards (creating the tall 'R'). Inferior Leads (II, III, and aVF): Pattern: rS complex (small R wave, deep S wave). Explanation: The initial forces move briefly towards these leads (small 'r'), but the bulk of the depolarization moves away from them (deep 'S'). Key Check: Lead aVF should be negative. Lead II should also be negative (which confirms the axis is more negative than -30°). 3. Normal or Minimally Prolonged QRS Duration Unlike a complete Left Bundle Branch Block (LBBB), the main conduction trunk is intact, so ventricular depolarization is not significantly delayed. Criterion: QRS duration is usually < 120 ms (typically 80–110 ms). Context: If the QRS is ≥ 120 ms, a complete bundle branch block is likely present (e.g., RBBB + LAFB). 4. Prolonged R-Wave Peak Time in Lead aVL This measures the time from the onset of the QRS to the peak of the R wave (intrinsicoid deflection), representing the time it takes for depolarization to reach the lateral wall. Criterion: R-wave peak time ≥ 45 ms in lead aVL. Comparison: This is often delayed compared to lead V6. Summary of Diagnostic Criteria (Rule of Thumb) To quickly identify LAFB, look for: Left Axis Deviation (Positive Lead I, Negative Lead aVF). Negative Lead II (Deep S wave). Small Q waves in I and aVL. Normal QRS width (< 120 ms). Important Clinical Note: LAFB is a diagnosis of exclusion. You must ensure that the Left Axis Deviation is not caused by other conditions, such as a preexisting inferior myocardial infarction (which causes deep Q waves in II, III, and aVF) or severe Left Ventricular Hypertrophy (LVH). Read more on LITFL: https://litfl.com/left-anterior-fascicular-block-lafb-ecg-library/ Explanation for incomplete right bundle branch block (IRBBB) An Incomplete Right Bundle Branch Block (IRBBB) is a conduction delay where the electrical impulse travels to the right ventricle more slowly than normal, but not slowly enough to be classified as a "complete" block. On a standard 12-lead ECG, the diagnosis is made by looking for specific changes in the QRS duration and the wave shapes in leads V1 and V6. 1. QRS Duration (The Main Criteria) The primary difference between an incomplete and a complete block is the duration of the QRS complex. Incomplete RBBB: The QRS duration is prolonged but remains between 100 and 119 milliseconds (0.10 to < 0.12 seconds). Complete RBBB: The QRS duration is ≥ 120 milliseconds (≥ 0.12 seconds). 2. Signs in the Right Precordial Leads (V1 and V2) The most distinct sign of IRBBB appears in leads V1 and V2 (the leads placed over the right ventricle). RSR' Pattern ("M-Shaped" Complex): You will typically see a three-phase wave described as rSr', rsR', or * *rSR'**. The first wave (r) represents normal activation of the septum. The second wave (s) represents the normal activation of the left ventricle. The third wave (R') is a second upward spike that represents the delayed activation of the right ventricle. R' is often taller than r: The second upward spike (R') is usually larger than the initial one. 3. Signs in the Lateral Leads (V6 and Lead I) While V1 shows the delayed "push" of electricity toward the right, the leads on the left side of the chest (V6 and Lead I) show that same electrical force moving away from them. Slurred S Wave: You will see a wide, "slurred" S wave at the end of the QRS complex. This S wave is wider than the preceding R wave (or > 40ms) and looks "messy" or slow compared to the sharp upstroke of the R wave. Summary of Diagnostic Criteria | Feature | Incomplete RBBB Signs | QRS Duration: 100 – 119 ms (Narrower than complete block) Lead V1 / V2: rSR' pattern (Bunny ears / M-shape) Lead I / V6: Wide, slurred S wave Clinical Significance It is important to note that an incomplete RBBB is frequently a normal variant (benign finding), particularly in children, young adults, and well-conditioned athletes. However, it can occasionally be associated with conditions that strain the right side of the heart, such as an atrial septal defect (ASD) or right ventricular hypertrophy. Read more on LITFL: https://litfl.com/right-bundle-branch-block-rbbb-ecg-library/ Explanation for sinus bradycardia (SBRAD) On a standard 12-lead ECG, sinus bradycardia is identified by a heart rate of less than 60 beats per minute (bpm) while maintaining all the characteristics of a normal sinus rhythm. To diagnose it, you must confirm that the rhythm originates from the sinus node (the heart's natural pacemaker) and is conducting normally to the ventricles. Here are the specific signs and criteria to look for: 1. Heart Rate Criterion: Ventricular rate is < 60 bpm. Calculation: On standard ECG paper (25 mm/s), the distance between consecutive R-waves will be greater than 5 large squares (since $300 \div 5 = 60$). 2. Rhythm Criterion: The rhythm is regular. Check: The R-R intervals (distance between heartbeats) are consistent. Slight variation (respiratory sinus arrhythmia) is common and acceptable, but the overall pattern should be steady. 3. P-Wave Morphology (Crucial for "Sinus" Origin) To confirm the rhythm is sinus (coming from the sinoatrial node) rather than an atrial or junctional escape rhythm, the P-waves must have a specific vector: Leads I, II, and aVF: P-waves must be upright (positive). Lead aVR: P-waves must be inverted (negative). Consistency: All P-waves should look identical, indicating they originate from the same focus. 4. P-to-QRS Relationship Criterion: There is a 1:1 relationship. Check: Every P-wave is followed by a QRS complex, and every QRS complex is preceded by a P-wave. This rules out Complete Heart Block (3rd-degree AV block), where P-waves and QRS complexes beat independently. 5. Intervals PR Interval: Normal and constant (between 0.12 and 0.20 seconds; 3–5 small squares). This rules out 1st-degree AV block (where the PR is > 0.20s). QRS Duration: Usually normal (< 0.12 seconds; less than 3 small squares), unless there is a pre-existing bundle branch block. Note on Differential Diagnosis: If the rate is slow but the P-waves are inverted in lead II or absent entirely, the rhythm is likely a Junctional Escape Rhythm, not sinus bradycardia. If there are multiple P-waves for a single QRS, you are looking at an **AV Block **. Read more on LITFL: https://litfl.com/sinus-bradycardia-ecg-library/

[image: 2388.png] Diagnostic Code Explanations (1) Diagnoses for this EKG: sinus rhythm (SR) Explanation for sinus rhythm (SR) Identifying Sinus Rhythm on a standard 12-lead ECG is the foundational skill of ECG interpretation. It essentially means the heart's electrical impulse is originating correctly from the Sinoatrial (SA) node. To confirm sinus rhythm, you must look for specific signs related to the P wave, the rhythm regularity, and the conduction intervals. 1. The P Wave (The most critical sign) The definitive sign of sinus rhythm is the "P wave axis." Because the SA node is located in the top right of the heart, the electrical current should flow down and to the left. Lead II: The P wave must be upright (positive). This is the most important lead to check. Lead aVR: The P wave must be inverted (negative). If the P wave is upright in aVR, the rhythm is likely not sinus (e.g., it may be a low atrial or junctional rhythm). Leads I and aVF: P waves are typically upright. Consistency: The P waves should all look the same (consistent morphology) within a single lead. 2. The Relationship Between P and QRS The SA node should be driving the ventricles. 1:1 Ratio: Every P wave must be followed by a QRS complex, and every QRS complex must be preceded by a P wave. PR Interval: The time between the start of the P wave and the start of the QRS complex should be constant and within normal limits (0.12 to 0.20 seconds, or 3–5 small squares). 3. Rhythm Regularity Regularity: The distance between R waves (R-R interval) and P waves (P-P interval) should be consistent. Note: Minor variation is normal due to breathing (called respiratory sinus arrhythmia), but the rhythm should look visibly regular to the naked eye. 4. Heart Rate While "Sinus Rhythm" describes the origin of the beat, the rate determines the specific diagnosis: Normal Sinus Rhythm (NSR): Rate between 60 and 100 bpm. Sinus Bradycardia: All sinus criteria met, but rate is < 60 bpm. Sinus Tachycardia: All sinus criteria met, but rate is > 100 bpm. Summary Checklist When looking at a 12-lead ECG, you can confidently state "Sinus Rhythm" if: P waves are upright in Lead II. P waves are inverted in aVR. There is a P wave before every QRS. The rhythm is regular. Read more on LITFL: https://litfl.com/normal-sinus-rhythm-ecg-library/

[image: 2585.png] Diagnostic Code Explanations (3) Diagnoses for this EKG: inferior myocardial infarction (IMI), first degree AV block (1AVB), sinus rhythm (SR) Explanation for inferior myocardial infarction (IMI) An inferior myocardial infarction (MI) is a heart attack affecting the inferior wall of the heart, which rests on the diaphragm. It is most commonly caused by an occlusion of the Right Coronary Artery (RCA), though it can also result from a blockage in the Left Circumflex Artery (LCx). On a standard 12-lead ECG, you can identify an inferior MI by looking for specific changes in the "inferior leads" (II, III, and aVF). 1. Primary Diagnostic Signs The hallmark of an acute inferior MI is ST-segment elevation in the leads that view the bottom of the heart: Lead II Lead III Lead aVF Diagnostic Criteria: Significant ST-elevation (usually ≥1 mm) in at least two of these contiguous leads. 2. Reciprocal Changes Reciprocal changes are "mirror images" of the injury that appear in leads on the opposite side of the heart. These are critical for confirming the diagnosis and ruling out mimics like pericarditis. Lead aVL: This is the most sensitive lead for reciprocal changes in an inferior MI. You will typically see * ST-segment depression* here. Lead I: Often shows ST-depression as well, though usually less pronounced than in aVL. 3. Identifying the Culprit Artery (RCA vs. LCx) While an angiogram is definitive, the ECG can offer clues about which artery is blocked: RCA Occlusion (Most Common): ST elevation in Lead III > Lead II. Reciprocal ST depression in Lead I and aVL. LCx Occlusion: ST elevation in Lead II ≥ Lead III. May have ST elevation in lateral leads (I, aVL, V5, V6) or absence of reciprocal depression in Lead I. 4. Associated "Danger" Signs Inferior MIs are unique because they often extend to other areas of the heart. You must check for these two common complications immediately, as they change treatment: A. Right Ventricular (RV) Infarction Because the RCA supplies the right ventricle in most people, roughly 40% of inferior MIs involve the RV. ECG Clue: Look at lead V1. If you see ST elevation in V1 (or if V1 is isoelectric while V2 is significantly depressed), suspect RV involvement. Action: Perform a Right-Sided ECG. Look for ST elevation in V4R. Clinical Relevance: These patients are very sensitive to preload-reducing medications (like nitrates/nitroglycerin), which can cause severe hypotension. B. Posterior Extension The infarction may extend to the back of the heart (posterior wall). ECG Clue: Look at the anterior leads (V1, V2, V3). Instead of ST elevation, you will see the "mirror image": Horizontal ST depression Tall, dominant R waves (the mirror of a Q wave) Upright, prominent T waves (the mirror of T-wave inversion) Action: Perform a Posterior ECG (leads V7, V8, V9) to confirm ST elevation. 5. Rhythm Disturbances The RCA supplies the AV node in ~90% of people. Consequently, inferior MIs are frequently associated with: Bradycardia (slow heart rate). AV Blocks (First-degree, Wenckebach/Type I, or even Complete Heart Block). Read more: LITFL (ST segment): https://litfl.com/st-segment-ecg-library/ LITFL (MI localization): https://litfl.com/mi-localization-ecg-library/ LITFL (inferior STEMI): https://litfl.com/inferior-stemi-ecg-library/ Explanation for first degree AV block (1AVB) First-degree atrioventricular (AV) block is characterized by a delay in the electrical conduction from the atria to the ventricles. Despite the name "block," the electrical impulse is not actually blocked; it is simply slowed down as it passes through the AV node. The hallmark sign on a standard 12-lead ECG is a prolonged PR interval that remains constant. Key Diagnostic Criteria To diagnose first-degree AV block, the ECG must show the following features: Prolonged PR Interval: Adults: The PR interval is consistently greater than 200 milliseconds (ms). On standard ECG paper (where 1 small square = 40 ms), this is wider than 5 small squares (or 1 large square). Pediatrics: The threshold is lower and age-dependent. For example, a PR interval >160 ms in an infant or >180 ms in a young child may qualify as first-degree block. 1:1 Relationship (One P for every QRS): Every P wave is followed by a QRS complex. Every QRS complex is preceded by a P wave. There are no dropped beats (unlike second-degree or third-degree heart blocks). Constant PR Interval: The duration of the PR interval is fixed and does not change from beat to beat. (If the PR interval progressively lengthens before a dropped beat, that is Second-Degree Type I / Wenckebach). How to Measure Properly Best Lead: Look at Lead II (or sometimes V1), as P waves are typically most visible there. Measurement: Measure from the beginning of the P wave (where it first leaves the isoelectric line) to the beginning of the QRS complex (the very first deflection, whether it is a Q wave or an R wave). Consistency: Verify the measurement across multiple beats to ensure it is fixed. Clinical Context & Nuances "Marked" First-Degree Block: If the PR interval exceeds 300 ms, it is often termed "marked" first-degree AV block. While usually asymptomatic, intervals this long can cause "pacemaker syndrome-like" symptoms (fatigue, dizziness) because the atria contract while the mitral/tricuspid valves are still closed from the previous beat. P Wave Location: In cases of marked block, the PR interval may be so long that the P wave is buried in the preceding T wave. You may notice a "camel hump" appearance on the T wave or a T wave that looks different than usual. Causes: Common causes include high vagal tone (common in athletes), medications (beta-blockers, calcium channel blockers, digoxin), electrolyte imbalances, or age-related degeneration of the conduction system. Summary PR Interval: > 0.20 seconds (more than 5 small boxes) Rhythm: Regular (assuming the underlying sinus rhythm is regular) P Waves: Normal upright P waves in Lead II; always present Conduction Ratio: 1:1 (Atria : Ventricles) Dropped Beats? None Read more on LITFL: https://litfl.com/first-degree-heart-block-ecg-library/ Explanation for sinus rhythm (SR) Identifying Sinus Rhythm on a standard 12-lead ECG is the foundational skill of ECG interpretation. It essentially means the heart's electrical impulse is originating correctly from the Sinoatrial (SA) node. To confirm sinus rhythm, you must look for specific signs related to the P wave, the rhythm regularity, and the conduction intervals. 1. The P Wave (The most critical sign) The definitive sign of sinus rhythm is the "P wave axis." Because the SA node is located in the top right of the heart, the electrical current should flow down and to the left. Lead II: The P wave must be upright (positive). This is the most important lead to check. Lead aVR: The P wave must be inverted (negative). If the P wave is upright in aVR, the rhythm is likely not sinus (e.g., it may be a low atrial or junctional rhythm). Leads I and aVF: P waves are typically upright. Consistency: The P waves should all look the same (consistent morphology) within a single lead. 2. The Relationship Between P and QRS The SA node should be driving the ventricles. 1:1 Ratio: Every P wave must be followed by a QRS complex, and every QRS complex must be preceded by a P wave. PR Interval: The time between the start of the P wave and the start of the QRS complex should be constant and within normal limits (0.12 to 0.20 seconds, or 3–5 small squares). 3. Rhythm Regularity Regularity: The distance between R waves (R-R interval) and P waves (P-P interval) should be consistent. Note: Minor variation is normal due to breathing (called respiratory sinus arrhythmia), but the rhythm should look visibly regular to the naked eye. 4. Heart Rate While "Sinus Rhythm" describes the origin of the beat, the rate determines the specific diagnosis: Normal Sinus Rhythm (NSR): Rate between 60 and 100 bpm. Sinus Bradycardia: All sinus criteria met, but rate is < 60 bpm. Sinus Tachycardia: All sinus criteria met, but rate is > 100 bpm. Summary Checklist When looking at a 12-lead ECG, you can confidently state "Sinus Rhythm" if: P waves are upright in Lead II. P waves are inverted in aVR. There is a P wave before every QRS. The rhythm is regular. Read more on LITFL: https://litfl.com/normal-sinus-rhythm-ecg-library/

[image: 2761.png] Diagnostic Code Explanations (5) Diagnoses for this EKG: inferior myocardial infarction (IMI), left ventricular hypertrophy (LVH), atrial premature complex (PAC), Q waves present (QWAVE), sinus tachycardia (STACH) Explanation for inferior myocardial infarction (IMI) An inferior myocardial infarction (MI) is a heart attack affecting the inferior wall of the heart, which rests on the diaphragm. It is most commonly caused by an occlusion of the Right Coronary Artery (RCA), though it can also result from a blockage in the Left Circumflex Artery (LCx). On a standard 12-lead ECG, you can identify an inferior MI by looking for specific changes in the "inferior leads" (II, III, and aVF). 1. Primary Diagnostic Signs The hallmark of an acute inferior MI is ST-segment elevation in the leads that view the bottom of the heart: Lead II Lead III Lead aVF Diagnostic Criteria: Significant ST-elevation (usually ≥1 mm) in at least two of these contiguous leads. 2. Reciprocal Changes Reciprocal changes are "mirror images" of the injury that appear in leads on the opposite side of the heart. These are critical for confirming the diagnosis and ruling out mimics like pericarditis. Lead aVL: This is the most sensitive lead for reciprocal changes in an inferior MI. You will typically see * ST-segment depression* here. Lead I: Often shows ST-depression as well, though usually less pronounced than in aVL. 3. Identifying the Culprit Artery (RCA vs. LCx) While an angiogram is definitive, the ECG can offer clues about which artery is blocked: RCA Occlusion (Most Common): ST elevation in Lead III > Lead II. Reciprocal ST depression in Lead I and aVL. LCx Occlusion: ST elevation in Lead II ≥ Lead III. May have ST elevation in lateral leads (I, aVL, V5, V6) or absence of reciprocal depression in Lead I. 4. Associated "Danger" Signs Inferior MIs are unique because they often extend to other areas of the heart. You must check for these two common complications immediately, as they change treatment: A. Right Ventricular (RV) Infarction Because the RCA supplies the right ventricle in most people, roughly 40% of inferior MIs involve the RV. ECG Clue: Look at lead V1. If you see ST elevation in V1 (or if V1 is isoelectric while V2 is significantly depressed), suspect RV involvement. Action: Perform a Right-Sided ECG. Look for ST elevation in V4R. Clinical Relevance: These patients are very sensitive to preload-reducing medications (like nitrates/nitroglycerin), which can cause severe hypotension. B. Posterior Extension The infarction may extend to the back of the heart (posterior wall). ECG Clue: Look at the anterior leads (V1, V2, V3). Instead of ST elevation, you will see the "mirror image": Horizontal ST depression Tall, dominant R waves (the mirror of a Q wave) Upright, prominent T waves (the mirror of T-wave inversion) Action: Perform a Posterior ECG (leads V7, V8, V9) to confirm ST elevation. 5. Rhythm Disturbances The RCA supplies the AV node in ~90% of people. Consequently, inferior MIs are frequently associated with: Bradycardia (slow heart rate). AV Blocks (First-degree, Wenckebach/Type I, or even Complete Heart Block). Read more: LITFL (ST segment): https://litfl.com/st-segment-ecg-library/ LITFL (MI localization): https://litfl.com/mi-localization-ecg-library/ LITFL (inferior STEMI): https://litfl.com/inferior-stemi-ecg-library/ Explanation for left ventricular hypertrophy (LVH) The signs of left ventricular hypertrophy (LVH) on a 12-lead ECG generally fall into two categories: **voltage criteria ** (increased amplitude of electrical signals) and non-voltage criteria (changes in wave shape, timing, or axis). Because the left ventricle is the largest and thickest chamber of the heart, its hypertrophy (thickening) generates stronger electrical forces that take longer to conduct. This results in taller R waves, deeper S waves, and specific repolarization abnormalities. 1. Voltage Criteria These are the most commonly cited signs. While there are many different formulas, the following are the most widely used in clinical practice. A. Sokolow-Lyon Criteria This is the classic "quick check" for LVH. Formula: Amplitude of S wave in V1 + Amplitude of R wave in V5 or V6 (whichever is taller). Threshold: A sum ≥ 35 mm (3.5 mV) indicates LVH. Note: A separate criterion is an R wave in aVL ≥ 11 mm. B. Cornell Voltage Criteria This criterion is often considered more accurate (higher sensitivity) than Sokolow-Lyon because it accounts for gender differences. Formula: Amplitude of S wave in V3 + Amplitude of R wave in aVL. Threshold (Men): > 28 mm. Threshold (Women): > 20 mm. C. Limb Lead Voltage Criteria High voltage in the limb leads alone can also indicate LVH. Lead aVL: R wave ≥ 11 mm. Lead I: R wave ≥ 14 mm. Lead aVR: S wave ≥ 14 mm. 2. Non-Voltage Criteria (Supporting Signs) Voltage criteria alone can sometimes be misleading (e.g., in very thin young patients). The presence of these non-voltage signs significantly increases the likelihood of true pathological hypertrophy. A. "Strain" Pattern (ST-T Wave Abnormalities) This is a sign of repolarization abnormality due to the thickened muscle. Appearance: You will see ST-segment depression and T-wave inversion in the lateral leads (I, aVL, V5, V6). Morphology: The depression is often down-sloping with an asymmetric T-wave inversion (the downslope is gradual, the return to baseline is steep). Discordance: The ST-T changes are usually "discordant" to the QRS complex (if the QRS is predominantly positive, the ST/T is negative). B. Left Axis Deviation (LAD) The QRS axis is shifted to the left (between -30° and -90°). On a standard trace: Positive QRS in Lead I, Negative QRS in leads II and aVF. C. Left Atrial Enlargement (LAE) Because a stiff, hypertrophied ventricle is harder to fill, the left atrium often enlarges. Lead II: A "P-mitrale" pattern (a broad, notched M-shaped P wave). Lead V1: A deep, wide terminal negative portion of the P wave (at least 1mm deep and 1 small box wide). D. Prolonged R-Wave Peak Time (Intrinsicoid Deflection) It takes longer for the electrical impulse to travel through the thickened ventricular wall. Measurement: Time from the onset of the QRS to the peak of the R wave in leads V5 or V6. Threshold: > 50 ms (more than 1.25 small boxes). 3. The Romhilt-Estes Score This is a point-scoring system that combines the above features to estimate the probability of LVH. Definite LVH: 5 or more points. Probable LVH: 4 points. Voltage Criteria (Any of: Limb R or S ≥20mm, V1/V2 S ≥30mm, V5/V6 R ≥30mm): 3 points ST-T Abnormalities (Strain pattern without digoxin): 3 points Left Atrial Enlargement (Terminal P in V1 ≥1mm deep/wide): 3 points Left Axis Deviation (≥ -30°): 2 points QRS Duration (≥ 90 ms): 1 point Delayed Intrinsicoid Deflection (V5/V6 ≥ 50 ms): 1 point Summary Checklist If you suspect LVH, scan the ECG for: Big Voltages: Deep S in V1/V2 + Tall R in V5/V6. Strain: Inverted T waves and depressed ST segments in V5, V6, I, and aVL. Left Axis: Negative QRS in aVF. Atrial Issues: M-shaped or biphasic P waves. Note: ECG has high specificity but low sensitivity for LVH. This means if the signs are present, the patient likely has LVH; however, a normal ECG does not rule out LVH. Echocardiography is the gold standard for confirmation. Read more on LITFL: https://litfl.com/left-ventricular-hypertrophy-lvh-ecg-library/ Explanation for atrial premature complex (PAC) Premature Atrial Complexes (PACs), also known as Premature Atrial Contractions, are early heartbeats originating from an ectopic focus within the atria (upper chambers) rather than the normal sinus node. On a standard 12-lead ECG, you can identify them by looking for a specific set of signs involving timing, P-wave morphology, and the post-beat pause. 1. Timing (Prematurity) The most defining feature is that the beat occurs earlier than the next expected sinus beat. If you measure the distance between the two previous R-waves (the R-R interval), the PAC will arrive sooner than that interval would predict. This disrupts the regular rhythm, making the heart rate appear irregular. 2. Abnormal P Wave Morphology Because the electrical impulse originates from a different location in the atria than the sinus node, the path of depolarization is different. This results in a P wave that looks different from the normal sinus P waves. Shape: The P wave may be flattened, peaked, notched, biphasic (part positive, part negative), or inverted, depending on where the ectopic focus is located. Axis: The axis of the P wave may change (e.g., becoming negative in leads where it is usually positive, like II, III, and aVF). "Buried" P Waves: If the PAC is very premature, the P wave may be hidden within the T wave of the preceding beat. This often creates a "camel hump" appearance or makes the T wave look taller and more pointed than usual. Always scrutinize the T wave of the preceding beat if you see a pause. 3. PR Interval The PR interval of a PAC is often different from the normal sinus PR interval. Normal or Prolonged: It is usually normal (0.12–0.20s) but can be prolonged if the AV node is still partially refractory (recovering) from the previous beat. Shortened: It may be shorter than the sinus PR interval if the ectopic focus is located closer to the AV node (low atrial origin). 4. QRS Complex Characteristics The QRS complex usually follows the P wave and typically looks identical to the patient’s normal sinus QRS complex ( narrow, <0.12s). This is because the impulse travels down the normal conduction pathways (His-Purkinje system) once it passes the AV node. However, there are two important variations: Aberrant Conduction: If the PAC arrives very early, one of the bundle branches (usually the right) may still be refractory. This forces the impulse to travel down the other bundle, resulting in a wide QRS complex that resembles a Bundle Branch Block pattern. Non-Conducted (Blocked) PAC: If the PAC arrives so early that the AV node is completely refractory, the impulse is blocked entirely. You will see a premature, abnormal P wave (often deforming the preceding T wave) followed by a * pause* with no QRS complex. 5. Non-Compensatory Pause Following the PAC, there is usually a pause before the next normal sinus beat occurs. Resetting the SA Node: PACs typically depolarize the sinus node prematurely, "resetting" its internal clock. Because of this reset, the pause is usually non-compensatory (or incomplete). Measurement: The interval between the beat before the PAC and the beat after the PAC is less than two full normal cardiac cycles (2x P-P interval). Read more on LITFL: https://litfl.com/premature-atrial-complex-pac/ Explanation for Q waves present (QWAVE) In a standard 12-lead ECG, a Q wave is defined as the first negative deflection of the QRS complex that is not preceded by any positive deflection (R wave). If there is any positive wave before the dip, that dip is an S wave, not a Q wave. While small Q waves can be normal in certain leads, "pathological" Q waves are a key sign of myocardial infarction ( heart attack) or other structural heart issues. The following breakdown details how to identify Q waves and distinguish normal from abnormal. 1. Identifying a Q Wave Visual Check: Look at the QRS complex. Does it start immediately with a downward line? That is a Q wave. QS Complex: If the entire QRS complex is just one deep downward stroke with no upward R wave at all, this is called a QS complex and is considered a Q wave equivalent. 2. Normal (Physiological) Q Waves Not all Q waves are bad. Normal "septal" Q waves represent the normal depolarization of the interventricular septum ( from left to right). Appearance: Narrow (short duration) and shallow. Duration: Less than 0.03 seconds (less than 1 small box). Amplitude: Less than 25% of the height of the following R wave. Location: Commonly seen in the lateral leads (I, aVL, V5, V6). Note: Deep Q waves can be normal in lead aVR (because it looks at the heart from the opposite angle) and sometimes in lead III (especially if they disappear when the patient takes a deep breath). 3. Pathological (Abnormal) Q Waves Pathological Q waves usually indicate that there is an electrical "hole" in the heart tissue, often scar tissue from a previous myocardial infarction (MI). Since dead tissue cannot conduct electricity, the electrode "looks through" the dead patch and records the electrical forces moving away from it on the opposite side of the heart, resulting in a deep negative deflection. Criteria for Pathological Q Waves: Duration (Width): ≥ 0.04 seconds (1 small box or wider). This is the most specific sign of pathology. Amplitude (Depth): ≥ 2 mm deep OR > 25% of the height of the subsequent R wave. Location: Leads V1–V3: Any Q wave in these leads (except a QS complex in V1) is generally considered abnormal. Contiguity: To be significant, Q waves must appear in two or more contiguous leads (leads that look at the same area of the heart). 4. Locating the Damage (Infarct Localization) The leads where pathological Q waves appear tell you which part of the heart has scarring: Inferior: II, III, aVF (Right Coronary Artery - RCA) Lateral: I, aVL, V5, V6 (Left Circumflex - LCx) Anterior: V3, V4 (Left Anterior Descending - LAD) Septal: V1, V2 (Left Anterior Descending - LAD) Posterior: Tall R wave in V1, V2* (RCA or LCx) (Note: Posterior MI is often indicated by a "reciprocal" change: a tall, wide R wave in V1 or V2, which is essentially an upside-down Q wave.) 5. Other Causes of Q Waves (Mimics) While a prior heart attack is the most common cause, other conditions can create Q waves or Q-wave-like patterns: Lead Placement Errors: Switching limb leads can create fake Q waves. Left Bundle Branch Block (LBBB): Can cause QS complexes in leads V1–V3. Ventricular Hypertrophy (LVH/RVH): Enlarged heart muscle can distort electrical forces, creating deep Q waves in certain leads. Wolff-Parkinson-White (WPW) Syndrome: The delta wave (pre-excitation) can sometimes be negative, mimicking a Q wave (pseudo-Q wave). Pulmonary Embolism: Classic "S1Q3T3" pattern includes a Q wave in lead III. Read more on LITFL: https://litfl.com/q-wave-ecg-library/ Explanation for sinus tachycardia (STACH) To identify sinus tachycardia on a standard 12-lead ECG, you must confirm that the rhythm originates from the sinus node (normal P-wave morphology) but fires at a rate faster than 100 beats per minute. The following is a breakdown of the specific signs and criteria. 1. Heart Rate Rate: > 100 beats per minute (bpm). Typical Range: Usually between 100 and 160 bpm. Note: In extreme physical stress or younger patients, it can exceed 160 bpm. If the rate is resting and >140-150 bpm, clinicians often consider other tachyarrhythmias (like atrial flutter or SVT) until P waves are clearly identified. 2. Rhythm Regularity: The rhythm is regular. The R-R intervals (distance between consecutive QRS complexes) are constant. Onset/Termination: unlike other supraventricular tachycardias (which start/stop abruptly), sinus tachycardia typically has a gradual onset ("warm-up") and gradual termination ("cool-down"). 3. P-Wave Morphology (The Key Identifier) To confirm the rhythm is "Sinus," the P waves must show that depolarization is starting in the Sinoatrial (SA) node and moving downward and to the left. Lead II: Upright (positive) P wave. This is the most important lead to check. Lead aVR: Inverted (negative) P wave. Leads I and aVF: Typically upright (positive). Consistency: All P waves look identical (monomorphic) and clearly precede every QRS complex. 4. Intervals PR Interval: Normal duration (0.12 – 0.20 seconds) and constant. Note: As the heart rate increases, the PR interval naturally shortens slightly but usually remains within the normal range. QRS Complex: Narrow (< 0.12 seconds), assuming there is no pre-existing bundle branch block. QT Interval: Shortens as the heart rate increases (rate-related shortening). 5. Visual Challenges at High Rates "Camel Hump" Appearance: At very fast rates (>140 bpm), the P wave may occur so early that it falls on top of the preceding T wave. This creates a fused "camel hump" look between the T wave and P wave. If the P wave becomes completely buried in the T wave, it can be difficult to distinguish sinus tachycardia from a re-entrant SVT. Clinical Context Note: Sinus tachycardia is usually a symptom of an underlying physiological stressor (e.g., fever, pain, dehydration, anxiety, exercise, PE) rather than a primary cardiac arrhythmia. Treatment focuses on the underlying cause, not the rhythm itself. Read more on LITFL: https://litfl.com/sinus-tachycardia-ecg-library/

[image: 223.png] Diagnostic Code Explanations (3) Diagnoses for this EKG: anteroseptal myocardial infarction (ASMI), left ventricular hypertrophy (LVH), sinus rhythm (SR) Explanation for anteroseptal myocardial infarction (ASMI) An anteroseptal myocardial infarction (MI) is characterized by ischemia and necrosis affecting the anterior wall of the left ventricle and the interventricular septum. It typically results from an occlusion of the Left Anterior Descending (LAD) coronary artery. On a standard 12-lead ECG, the signs of an anteroseptal MI are primarily observed in the precordial leads V1 through V4. Key ECG Findings ST-Segment Elevation Leads: The hallmark sign is ST-segment elevation in leads V1, V2, V3, and V4. Morphology: The elevation is often significant and may evolve into a "tombstone" pattern in severe cases. Specifics: Septal leads: V1 and V2. Anterior leads: V3 and V4. The involvement of both groups defines the "anteroseptal" pattern. Q Waves (Pathological) As the infarction evolves (or if it is age-indeterminate), pathological Q waves will develop in leads V1–V4. A QS complex (a deep negative deflection with no preceding positive R wave) is commonly seen in V1 and V2. Poor R-Wave Progression (PRWP) In a healthy heart, the R wave (the first positive spike of the QRS complex) should get progressively taller as you move from lead V1 to V6. In an anteroseptal MI, the destruction of heart muscle in the anterior wall leads to a loss of these positive forces. Consequently, you will see small or absent R waves in leads V1–V3 (or even up to V4). T-Wave Changes Hyperacute Phase: Immediately after occlusion, you may see tall, peaked "hyperacute" T waves in V1–V4 before ST elevation becomes obvious. Evolving/Subacute Phase: As the MI progresses, the T waves typically become inverted (negative) in the same leads (V1–V4). Reciprocal Changes Unlike inferior or lateral MIs, pure anteroseptal MIs often lack distinct reciprocal changes (ST depression) in the standard limb leads. Why? The electrical "opposite" of the anterior wall is the posterior wall. Standard 12-lead ECGs do not have leads on the patient's back to record this directly. Exception: If the infarction extends to the high lateral wall (leads I and aVL), you may see reciprocal ST depression in the inferior leads (II, III, aVF). Summary Checklist for Anteroseptal MI ST Elevation: Leads V1, V2, V3, V4 Q Waves: Developing in V1–V4 R Waves: Loss of height in V1–V3 (Poor R-wave progression) Reciprocal Changes: usually absent (unless lateral extension is present) Read more: LITFL (ST segment): https://litfl.com/st-segment-ecg-library/ LITFL (MI localization): https://litfl.com/mi-localization-ecg-library/ LITFL (anterior STEMI): https://litfl.com/anterior-myocardial-infarction-ecg-library/ Explanation for left ventricular hypertrophy (LVH) The signs of left ventricular hypertrophy (LVH) on a 12-lead ECG generally fall into two categories: **voltage criteria ** (increased amplitude of electrical signals) and non-voltage criteria (changes in wave shape, timing, or axis). Because the left ventricle is the largest and thickest chamber of the heart, its hypertrophy (thickening) generates stronger electrical forces that take longer to conduct. This results in taller R waves, deeper S waves, and specific repolarization abnormalities. 1. Voltage Criteria These are the most commonly cited signs. While there are many different formulas, the following are the most widely used in clinical practice. A. Sokolow-Lyon Criteria This is the classic "quick check" for LVH. Formula: Amplitude of S wave in V1 + Amplitude of R wave in V5 or V6 (whichever is taller). Threshold: A sum ≥ 35 mm (3.5 mV) indicates LVH. Note: A separate criterion is an R wave in aVL ≥ 11 mm. B. Cornell Voltage Criteria This criterion is often considered more accurate (higher sensitivity) than Sokolow-Lyon because it accounts for gender differences. Formula: Amplitude of S wave in V3 + Amplitude of R wave in aVL. Threshold (Men): > 28 mm. Threshold (Women): > 20 mm. C. Limb Lead Voltage Criteria High voltage in the limb leads alone can also indicate LVH. Lead aVL: R wave ≥ 11 mm. Lead I: R wave ≥ 14 mm. Lead aVR: S wave ≥ 14 mm. 2. Non-Voltage Criteria (Supporting Signs) Voltage criteria alone can sometimes be misleading (e.g., in very thin young patients). The presence of these non-voltage signs significantly increases the likelihood of true pathological hypertrophy. A. "Strain" Pattern (ST-T Wave Abnormalities) This is a sign of repolarization abnormality due to the thickened muscle. Appearance: You will see ST-segment depression and T-wave inversion in the lateral leads (I, aVL, V5, V6). Morphology: The depression is often down-sloping with an asymmetric T-wave inversion (the downslope is gradual, the return to baseline is steep). Discordance: The ST-T changes are usually "discordant" to the QRS complex (if the QRS is predominantly positive, the ST/T is negative). B. Left Axis Deviation (LAD) The QRS axis is shifted to the left (between -30° and -90°). On a standard trace: Positive QRS in Lead I, Negative QRS in leads II and aVF. C. Left Atrial Enlargement (LAE) Because a stiff, hypertrophied ventricle is harder to fill, the left atrium often enlarges. Lead II: A "P-mitrale" pattern (a broad, notched M-shaped P wave). Lead V1: A deep, wide terminal negative portion of the P wave (at least 1mm deep and 1 small box wide). D. Prolonged R-Wave Peak Time (Intrinsicoid Deflection) It takes longer for the electrical impulse to travel through the thickened ventricular wall. Measurement: Time from the onset of the QRS to the peak of the R wave in leads V5 or V6. Threshold: > 50 ms (more than 1.25 small boxes). 3. The Romhilt-Estes Score This is a point-scoring system that combines the above features to estimate the probability of LVH. Definite LVH: 5 or more points. Probable LVH: 4 points. Voltage Criteria (Any of: Limb R or S ≥20mm, V1/V2 S ≥30mm, V5/V6 R ≥30mm): 3 points ST-T Abnormalities (Strain pattern without digoxin): 3 points Left Atrial Enlargement (Terminal P in V1 ≥1mm deep/wide): 3 points Left Axis Deviation (≥ -30°): 2 points QRS Duration (≥ 90 ms): 1 point Delayed Intrinsicoid Deflection (V5/V6 ≥ 50 ms): 1 point Summary Checklist If you suspect LVH, scan the ECG for: Big Voltages: Deep S in V1/V2 + Tall R in V5/V6. Strain: Inverted T waves and depressed ST segments in V5, V6, I, and aVL. Left Axis: Negative QRS in aVF. Atrial Issues: M-shaped or biphasic P waves. Note: ECG has high specificity but low sensitivity for LVH. This means if the signs are present, the patient likely has LVH; however, a normal ECG does not rule out LVH. Echocardiography is the gold standard for confirmation. Read more on LITFL: https://litfl.com/left-ventricular-hypertrophy-lvh-ecg-library/ Explanation for sinus rhythm (SR) Identifying Sinus Rhythm on a standard 12-lead ECG is the foundational skill of ECG interpretation. It essentially means the heart's electrical impulse is originating correctly from the Sinoatrial (SA) node. To confirm sinus rhythm, you must look for specific signs related to the P wave, the rhythm regularity, and the conduction intervals. 1. The P Wave (The most critical sign) The definitive sign of sinus rhythm is the "P wave axis." Because the SA node is located in the top right of the heart, the electrical current should flow down and to the left. Lead II: The P wave must be upright (positive). This is the most important lead to check. Lead aVR: The P wave must be inverted (negative). If the P wave is upright in aVR, the rhythm is likely not sinus (e.g., it may be a low atrial or junctional rhythm). Leads I and aVF: P waves are typically upright. Consistency: The P waves should all look the same (consistent morphology) within a single lead. 2. The Relationship Between P and QRS The SA node should be driving the ventricles. 1:1 Ratio: Every P wave must be followed by a QRS complex, and every QRS complex must be preceded by a P wave. PR Interval: The time between the start of the P wave and the start of the QRS complex should be constant and within normal limits (0.12 to 0.20 seconds, or 3–5 small squares). 3. Rhythm Regularity Regularity: The distance between R waves (R-R interval) and P waves (P-P interval) should be consistent. Note: Minor variation is normal due to breathing (called respiratory sinus arrhythmia), but the rhythm should look visibly regular to the naked eye. 4. Heart Rate While "Sinus Rhythm" describes the origin of the beat, the rate determines the specific diagnosis: Normal Sinus Rhythm (NSR): Rate between 60 and 100 bpm. Sinus Bradycardia: All sinus criteria met, but rate is < 60 bpm. Sinus Tachycardia: All sinus criteria met, but rate is > 100 bpm. Summary Checklist When looking at a 12-lead ECG, you can confidently state "Sinus Rhythm" if: P waves are upright in Lead II. P waves are inverted in aVR. There is a P wave before every QRS. The rhythm is regular. Read more on LITFL: https://litfl.com/normal-sinus-rhythm-ecg-library/

[image: 3024.png] Diagnostic Code Explanations (1) Diagnoses for this EKG: sinus rhythm (SR) Explanation for sinus rhythm (SR) Identifying Sinus Rhythm on a standard 12-lead ECG is the foundational skill of ECG interpretation. It essentially means the heart's electrical impulse is originating correctly from the Sinoatrial (SA) node. To confirm sinus rhythm, you must look for specific signs related to the P wave, the rhythm regularity, and the conduction intervals. 1. The P Wave (The most critical sign) The definitive sign of sinus rhythm is the "P wave axis." Because the SA node is located in the top right of the heart, the electrical current should flow down and to the left. Lead II: The P wave must be upright (positive). This is the most important lead to check. Lead aVR: The P wave must be inverted (negative). If the P wave is upright in aVR, the rhythm is likely not sinus (e.g., it may be a low atrial or junctional rhythm). Leads I and aVF: P waves are typically upright. Consistency: The P waves should all look the same (consistent morphology) within a single lead. 2. The Relationship Between P and QRS The SA node should be driving the ventricles. 1:1 Ratio: Every P wave must be followed by a QRS complex, and every QRS complex must be preceded by a P wave. PR Interval: The time between the start of the P wave and the start of the QRS complex should be constant and within normal limits (0.12 to 0.20 seconds, or 3–5 small squares). 3. Rhythm Regularity Regularity: The distance between R waves (R-R interval) and P waves (P-P interval) should be consistent. Note: Minor variation is normal due to breathing (called respiratory sinus arrhythmia), but the rhythm should look visibly regular to the naked eye. 4. Heart Rate While "Sinus Rhythm" describes the origin of the beat, the rate determines the specific diagnosis: Normal Sinus Rhythm (NSR): Rate between 60 and 100 bpm. Sinus Bradycardia: All sinus criteria met, but rate is < 60 bpm. Sinus Tachycardia: All sinus criteria met, but rate is > 100 bpm. Summary Checklist When looking at a 12-lead ECG, you can confidently state "Sinus Rhythm" if: P waves are upright in Lead II. P waves are inverted in aVR. There is a P wave before every QRS. The rhythm is regular. Read more on LITFL: https://litfl.com/normal-sinus-rhythm-ecg-library/

[image: 14941.png] Diagnostic Code Explanations (5) Diagnoses for this EKG: inferior myocardial infarction (IMI), complete right bundle branch block (CRBBB), left atrial overload/enlargement (LAO/LAE), Q waves present (QWAVE), sinus rhythm (SR) Explanation for inferior myocardial infarction (IMI) An inferior myocardial infarction (MI) is a heart attack affecting the inferior wall of the heart, which rests on the diaphragm. It is most commonly caused by an occlusion of the Right Coronary Artery (RCA), though it can also result from a blockage in the Left Circumflex Artery (LCx). On a standard 12-lead ECG, you can identify an inferior MI by looking for specific changes in the "inferior leads" (II, III, and aVF). 1. Primary Diagnostic Signs The hallmark of an acute inferior MI is ST-segment elevation in the leads that view the bottom of the heart: Lead II Lead III Lead aVF Diagnostic Criteria: Significant ST-elevation (usually ≥1 mm) in at least two of these contiguous leads. 2. Reciprocal Changes Reciprocal changes are "mirror images" of the injury that appear in leads on the opposite side of the heart. These are critical for confirming the diagnosis and ruling out mimics like pericarditis. Lead aVL: This is the most sensitive lead for reciprocal changes in an inferior MI. You will typically see * ST-segment depression* here. Lead I: Often shows ST-depression as well, though usually less pronounced than in aVL. 3. Identifying the Culprit Artery (RCA vs. LCx) While an angiogram is definitive, the ECG can offer clues about which artery is blocked: RCA Occlusion (Most Common): ST elevation in Lead III > Lead II. Reciprocal ST depression in Lead I and aVL. LCx Occlusion: ST elevation in Lead II ≥ Lead III. May have ST elevation in lateral leads (I, aVL, V5, V6) or absence of reciprocal depression in Lead I. 4. Associated "Danger" Signs Inferior MIs are unique because they often extend to other areas of the heart. You must check for these two common complications immediately, as they change treatment: A. Right Ventricular (RV) Infarction Because the RCA supplies the right ventricle in most people, roughly 40% of inferior MIs involve the RV. ECG Clue: Look at lead V1. If you see ST elevation in V1 (or if V1 is isoelectric while V2 is significantly depressed), suspect RV involvement. Action: Perform a Right-Sided ECG. Look for ST elevation in V4R. Clinical Relevance: These patients are very sensitive to preload-reducing medications (like nitrates/nitroglycerin), which can cause severe hypotension. B. Posterior Extension The infarction may extend to the back of the heart (posterior wall). ECG Clue: Look at the anterior leads (V1, V2, V3). Instead of ST elevation, you will see the "mirror image": Horizontal ST depression Tall, dominant R waves (the mirror of a Q wave) Upright, prominent T waves (the mirror of T-wave inversion) Action: Perform a Posterior ECG (leads V7, V8, V9) to confirm ST elevation. 5. Rhythm Disturbances The RCA supplies the AV node in ~90% of people. Consequently, inferior MIs are frequently associated with: Bradycardia (slow heart rate). AV Blocks (First-degree, Wenckebach/Type I, or even Complete Heart Block). Read more: LITFL (ST segment): https://litfl.com/st-segment-ecg-library/ LITFL (MI localization): https://litfl.com/mi-localization-ecg-library/ LITFL (inferior STEMI): https://litfl.com/inferior-stemi-ecg-library/ Explanation for complete right bundle branch block (CRBBB) To diagnose a Complete Right Bundle Branch Block (RBBB) on a standard 12-lead ECG, you must look for specific changes in the QRS duration, the pattern in the right precordial leads (V1, V2), and the pattern in the lateral leads ( I, V6). 1. Diagnostic Criteria (The "Big Three") For a block to be considered "complete," the QRS duration must be ≥ 120 ms (3 small squares). If the QRS is between 100–119 ms but meets the other criteria, it is termed an incomplete RBBB. A. Leads V1 & V2 (The "M" Pattern) The right bundle is blocked, so the right ventricle depolarizes after the left ventricle. This late signal moves toward V1 (which sits over the right ventricle), creating a second positive wave. Morphology: You will see an rSR' pattern (often called "bunny ears" or "M-shaped"). r: The first small positive deflection (septal depolarization). S: A downward deflection (left ventricular depolarization moving away from V1). R': A second, usually taller and wider positive deflection (delayed right ventricular depolarization moving toward V1). Note: Sometimes the S wave is missing, appearing simply as a wide, notched R wave. B. Leads I & V6 (The Slurred S Wave) These leads look at the heart from the left side. As the delayed electrical signal slowly moves to the right (away from these leads), it creates a prolonged negative deflection. Morphology: You will see a wide, slurred S wave. The S wave will be deep and, more importantly, broad (often wider than the preceding R wave or >40 ms). C. Secondary ST-T Wave Changes Because depolarization is abnormal, repolarization (recovery) is also abnormal. This results in discordance, meaning the ST segment and T wave go in the opposite direction of the main QRS vector. In V1 & V2: Since the terminal QRS is positive (the tall R'), you should expect ST depression and T-wave inversion. Clinical Tip: If the T wave is upright in V1 in the presence of a complete RBBB, it may suggest ischemia (a primary T-wave abnormality). Summary QRS Width: Wide (≥ 120 ms) in all leads. P-Wave: Present (originates above ventricles) in all leads. Terminal Wave in V1, V2: Positive R' (rSR' "Bunny Ears"). Terminal Wave in I, V6: Wide, Slurred S Wave. Repolarization in V1, V2: Inverted T waves (Discordant). Visual Mnemonics "MaRRoW": This classic mnemonic helps differentiate Right vs. Left bundle branch blocks. M in V1 R (Right Bundle Branch Block) W in V6 (referring to the wide S wave which creates a W-like ending to the complex). Turn Signal: V1 is the "turn signal" of the heart. If the terminal wave is Up in V1, the problem is on the * Right* (Right Bundle Branch Block). If it is Down, the problem is on the Left (Left Bundle Branch Block). Read more on LITFL: https://litfl.com/right-bundle-branch-block-rbbb-ecg-library/ Explanation for left atrial overload/enlargement (LAO/LAE) Left atrial enlargement (often referred to clinically as left atrial abnormality or overload) produces distinct changes in the P wave. Because the P wave represents atrial depolarization, structural changes in the atria alter how this electrical signal is conducted. Physiologically, the right atrium depolarizes first, followed by the left atrium. In left atrial enlargement (LAE), the left atrial component is prolonged and delayed. This separation of the two atrial signals creates the classic ECG signs seen primarily in Lead II and Lead V1. 1. Lead II: "P Mitrale" In Lead II (and often other inferior leads like III and aVF), the P wave becomes wide and notched. Appearance: The P wave looks like the letter "M" (M for Mitral). Mechanism: The first hump of the "M" represents the normal right atrial depolarization. The second hump represents the delayed and enlarged left atrial depolarization. Criteria: Duration: The total P wave width is > 120 ms (more than 3 small boxes). Morphology: It is bifid (notched), with the space between the two peaks being > 40 ms (1 small box). 2. Lead V1: Biphasic P Wave In Lead V1 (a precordial lead sitting over the right side of the heart), the P wave is typically biphasic (goes up, then down). The second half of the wave represents the left atrium. Appearance: A small initial positive deflection followed by a deep, wide negative deflection. Criteria: The terminal negative portion (the downward part) must be: > 1 mm deep (1 small box vertically) > 40 ms wide (1 small box horizontally) Summary of Diagnostic Criteria Lead II (P Mitrale): Broad, notched P wave with total duration > 120 ms (3 small squares) and notch separation > 40 ms. Lead V1 (Biphasic P wave): Deep terminal negative force where the terminal negative portion is > 1 mm deep and > 40 ms wide. Common Causes This pattern is classically associated with conditions that increase left atrial pressure or volume: Mitral Valve Disease: Specifically Mitral Stenosis (hence the name "P Mitrale") and Mitral Regurgitation. Hypertension: Systemic high blood pressure causes the left ventricle to stiffen, backing pressure up into the left atrium. Left Ventricular Hypertrophy (LVH): Often seen concurrently with LAE. Aortic Stenosis: Increases left ventricular pressure, which eventually impacts the left atrium. Note: While "enlargement" is the traditional term, "Left Atrial Abnormality" is technically more accurate, as these electrical signs can sometimes occur due to conduction delays or pressure overload without gross physical dilation of the chamber. Read more on LITFL: https://litfl.com/left-atrial-enlargement-ecg-library/ Explanation for Q waves present (QWAVE) In a standard 12-lead ECG, a Q wave is defined as the first negative deflection of the QRS complex that is not preceded by any positive deflection (R wave). If there is any positive wave before the dip, that dip is an S wave, not a Q wave. While small Q waves can be normal in certain leads, "pathological" Q waves are a key sign of myocardial infarction ( heart attack) or other structural heart issues. The following breakdown details how to identify Q waves and distinguish normal from abnormal. 1. Identifying a Q Wave Visual Check: Look at the QRS complex. Does it start immediately with a downward line? That is a Q wave. QS Complex: If the entire QRS complex is just one deep downward stroke with no upward R wave at all, this is called a QS complex and is considered a Q wave equivalent. 2. Normal (Physiological) Q Waves Not all Q waves are bad. Normal "septal" Q waves represent the normal depolarization of the interventricular septum ( from left to right). Appearance: Narrow (short duration) and shallow. Duration: Less than 0.03 seconds (less than 1 small box). Amplitude: Less than 25% of the height of the following R wave. Location: Commonly seen in the lateral leads (I, aVL, V5, V6). Note: Deep Q waves can be normal in lead aVR (because it looks at the heart from the opposite angle) and sometimes in lead III (especially if they disappear when the patient takes a deep breath). 3. Pathological (Abnormal) Q Waves Pathological Q waves usually indicate that there is an electrical "hole" in the heart tissue, often scar tissue from a previous myocardial infarction (MI). Since dead tissue cannot conduct electricity, the electrode "looks through" the dead patch and records the electrical forces moving away from it on the opposite side of the heart, resulting in a deep negative deflection. Criteria for Pathological Q Waves: Duration (Width): ≥ 0.04 seconds (1 small box or wider). This is the most specific sign of pathology. Amplitude (Depth): ≥ 2 mm deep OR > 25% of the height of the subsequent R wave. Location: Leads V1–V3: Any Q wave in these leads (except a QS complex in V1) is generally considered abnormal. Contiguity: To be significant, Q waves must appear in two or more contiguous leads (leads that look at the same area of the heart). 4. Locating the Damage (Infarct Localization) The leads where pathological Q waves appear tell you which part of the heart has scarring: Inferior: II, III, aVF (Right Coronary Artery - RCA) Lateral: I, aVL, V5, V6 (Left Circumflex - LCx) Anterior: V3, V4 (Left Anterior Descending - LAD) Septal: V1, V2 (Left Anterior Descending - LAD) Posterior: Tall R wave in V1, V2* (RCA or LCx) (Note: Posterior MI is often indicated by a "reciprocal" change: a tall, wide R wave in V1 or V2, which is essentially an upside-down Q wave.) 5. Other Causes of Q Waves (Mimics) While a prior heart attack is the most common cause, other conditions can create Q waves or Q-wave-like patterns: Lead Placement Errors: Switching limb leads can create fake Q waves. Left Bundle Branch Block (LBBB): Can cause QS complexes in leads V1–V3. Ventricular Hypertrophy (LVH/RVH): Enlarged heart muscle can distort electrical forces, creating deep Q waves in certain leads. Wolff-Parkinson-White (WPW) Syndrome: The delta wave (pre-excitation) can sometimes be negative, mimicking a Q wave (pseudo-Q wave). Pulmonary Embolism: Classic "S1Q3T3" pattern includes a Q wave in lead III. Read more on LITFL: https://litfl.com/q-wave-ecg-library/ Explanation for sinus rhythm (SR) Identifying Sinus Rhythm on a standard 12-lead ECG is the foundational skill of ECG interpretation. It essentially means the heart's electrical impulse is originating correctly from the Sinoatrial (SA) node. To confirm sinus rhythm, you must look for specific signs related to the P wave, the rhythm regularity, and the conduction intervals. 1. The P Wave (The most critical sign) The definitive sign of sinus rhythm is the "P wave axis." Because the SA node is located in the top right of the heart, the electrical current should flow down and to the left. Lead II: The P wave must be upright (positive). This is the most important lead to check. Lead aVR: The P wave must be inverted (negative). If the P wave is upright in aVR, the rhythm is likely not sinus (e.g., it may be a low atrial or junctional rhythm). Leads I and aVF: P waves are typically upright. Consistency: The P waves should all look the same (consistent morphology) within a single lead. 2. The Relationship Between P and QRS The SA node should be driving the ventricles. 1:1 Ratio: Every P wave must be followed by a QRS complex, and every QRS complex must be preceded by a P wave. PR Interval: The time between the start of the P wave and the start of the QRS complex should be constant and within normal limits (0.12 to 0.20 seconds, or 3–5 small squares). 3. Rhythm Regularity Regularity: The distance between R waves (R-R interval) and P waves (P-P interval) should be consistent. Note: Minor variation is normal due to breathing (called respiratory sinus arrhythmia), but the rhythm should look visibly regular to the naked eye. 4. Heart Rate While "Sinus Rhythm" describes the origin of the beat, the rate determines the specific diagnosis: Normal Sinus Rhythm (NSR): Rate between 60 and 100 bpm. Sinus Bradycardia: All sinus criteria met, but rate is < 60 bpm. Sinus Tachycardia: All sinus criteria met, but rate is > 100 bpm. Summary Checklist When looking at a 12-lead ECG, you can confidently state "Sinus Rhythm" if: P waves are upright in Lead II. P waves are inverted in aVR. There is a P wave before every QRS. The rhythm is regular. Read more on LITFL: https://litfl.com/normal-sinus-rhythm-ecg-library/

[image: 4040.png] Diagnostic Code Explanations (1) Diagnoses for this EKG: sinus rhythm (SR) Explanation for sinus rhythm (SR) Identifying Sinus Rhythm on a standard 12-lead ECG is the foundational skill of ECG interpretation. It essentially means the heart's electrical impulse is originating correctly from the Sinoatrial (SA) node. To confirm sinus rhythm, you must look for specific signs related to the P wave, the rhythm regularity, and the conduction intervals. 1. The P Wave (The most critical sign) The definitive sign of sinus rhythm is the "P wave axis." Because the SA node is located in the top right of the heart, the electrical current should flow down and to the left. Lead II: The P wave must be upright (positive). This is the most important lead to check. Lead aVR: The P wave must be inverted (negative). If the P wave is upright in aVR, the rhythm is likely not sinus (e.g., it may be a low atrial or junctional rhythm). Leads I and aVF: P waves are typically upright. Consistency: The P waves should all look the same (consistent morphology) within a single lead. 2. The Relationship Between P and QRS The SA node should be driving the ventricles. 1:1 Ratio: Every P wave must be followed by a QRS complex, and every QRS complex must be preceded by a P wave. PR Interval: The time between the start of the P wave and the start of the QRS complex should be constant and within normal limits (0.12 to 0.20 seconds, or 3–5 small squares). 3. Rhythm Regularity Regularity: The distance between R waves (R-R interval) and P waves (P-P interval) should be consistent. Note: Minor variation is normal due to breathing (called respiratory sinus arrhythmia), but the rhythm should look visibly regular to the naked eye. 4. Heart Rate While "Sinus Rhythm" describes the origin of the beat, the rate determines the specific diagnosis: Normal Sinus Rhythm (NSR): Rate between 60 and 100 bpm. Sinus Bradycardia: All sinus criteria met, but rate is < 60 bpm. Sinus Tachycardia: All sinus criteria met, but rate is > 100 bpm. Summary Checklist When looking at a 12-lead ECG, you can confidently state "Sinus Rhythm" if: P waves are upright in Lead II. P waves are inverted in aVR. There is a P wave before every QRS. The rhythm is regular. Read more on LITFL: https://litfl.com/normal-sinus-rhythm-ecg-library/

[image: 1336.png] Diagnostic Code Explanations (1) Diagnoses for this EKG: sinus rhythm (SR) Explanation for sinus rhythm (SR) Identifying Sinus Rhythm on a standard 12-lead ECG is the foundational skill of ECG interpretation. It essentially means the heart's electrical impulse is originating correctly from the Sinoatrial (SA) node. To confirm sinus rhythm, you must look for specific signs related to the P wave, the rhythm regularity, and the conduction intervals. 1. The P Wave (The most critical sign) The definitive sign of sinus rhythm is the "P wave axis." Because the SA node is located in the top right of the heart, the electrical current should flow down and to the left. Lead II: The P wave must be upright (positive). This is the most important lead to check. Lead aVR: The P wave must be inverted (negative). If the P wave is upright in aVR, the rhythm is likely not sinus (e.g., it may be a low atrial or junctional rhythm). Leads I and aVF: P waves are typically upright. Consistency: The P waves should all look the same (consistent morphology) within a single lead. 2. The Relationship Between P and QRS The SA node should be driving the ventricles. 1:1 Ratio: Every P wave must be followed by a QRS complex, and every QRS complex must be preceded by a P wave. PR Interval: The time between the start of the P wave and the start of the QRS complex should be constant and within normal limits (0.12 to 0.20 seconds, or 3–5 small squares). 3. Rhythm Regularity Regularity: The distance between R waves (R-R interval) and P waves (P-P interval) should be consistent. Note: Minor variation is normal due to breathing (called respiratory sinus arrhythmia), but the rhythm should look visibly regular to the naked eye. 4. Heart Rate While "Sinus Rhythm" describes the origin of the beat, the rate determines the specific diagnosis: Normal Sinus Rhythm (NSR): Rate between 60 and 100 bpm. Sinus Bradycardia: All sinus criteria met, but rate is < 60 bpm. Sinus Tachycardia: All sinus criteria met, but rate is > 100 bpm. Summary Checklist When looking at a 12-lead ECG, you can confidently state "Sinus Rhythm" if: P waves are upright in Lead II. P waves are inverted in aVR. There is a P wave before every QRS. The rhythm is regular. Read more on LITFL: https://litfl.com/normal-sinus-rhythm-ecg-library/

[image: 17887.png] Diagnostic Code Explanations (1) Diagnoses for this EKG: sinus bradycardia (SBRAD) Explanation for sinus bradycardia (SBRAD) On a standard 12-lead ECG, sinus bradycardia is identified by a heart rate of less than 60 beats per minute (bpm) while maintaining all the characteristics of a normal sinus rhythm. To diagnose it, you must confirm that the rhythm originates from the sinus node (the heart's natural pacemaker) and is conducting normally to the ventricles. Here are the specific signs and criteria to look for: 1. Heart Rate Criterion: Ventricular rate is < 60 bpm. Calculation: On standard ECG paper (25 mm/s), the distance between consecutive R-waves will be greater than 5 large squares (since $300 \div 5 = 60$). 2. Rhythm Criterion: The rhythm is regular. Check: The R-R intervals (distance between heartbeats) are consistent. Slight variation (respiratory sinus arrhythmia) is common and acceptable, but the overall pattern should be steady. 3. P-Wave Morphology (Crucial for "Sinus" Origin) To confirm the rhythm is sinus (coming from the sinoatrial node) rather than an atrial or junctional escape rhythm, the P-waves must have a specific vector: Leads I, II, and aVF: P-waves must be upright (positive). Lead aVR: P-waves must be inverted (negative). Consistency: All P-waves should look identical, indicating they originate from the same focus. 4. P-to-QRS Relationship Criterion: There is a 1:1 relationship. Check: Every P-wave is followed by a QRS complex, and every QRS complex is preceded by a P-wave. This rules out Complete Heart Block (3rd-degree AV block), where P-waves and QRS complexes beat independently. 5. Intervals PR Interval: Normal and constant (between 0.12 and 0.20 seconds; 3–5 small squares). This rules out 1st-degree AV block (where the PR is > 0.20s). QRS Duration: Usually normal (< 0.12 seconds; less than 3 small squares), unless there is a pre-existing bundle branch block. Note on Differential Diagnosis: If the rate is slow but the P-waves are inverted in lead II or absent entirely, the rhythm is likely a Junctional Escape Rhythm, not sinus bradycardia. If there are multiple P-waves for a single QRS, you are looking at an **AV Block **. Read more on LITFL: https://litfl.com/sinus-bradycardia-ecg-library/

[image: 15020.png] Diagnostic Code Explanations (4) Diagnoses for this EKG: incomplete right bundle branch block (IRBBB), low amplitude T-waves (LOWT), undefined (INVT), sinus rhythm (SR) Explanation for incomplete right bundle branch block (IRBBB) An Incomplete Right Bundle Branch Block (IRBBB) is a conduction delay where the electrical impulse travels to the right ventricle more slowly than normal, but not slowly enough to be classified as a "complete" block. On a standard 12-lead ECG, the diagnosis is made by looking for specific changes in the QRS duration and the wave shapes in leads V1 and V6. 1. QRS Duration (The Main Criteria) The primary difference between an incomplete and a complete block is the duration of the QRS complex. Incomplete RBBB: The QRS duration is prolonged but remains between 100 and 119 milliseconds (0.10 to < 0.12 seconds). Complete RBBB: The QRS duration is ≥ 120 milliseconds (≥ 0.12 seconds). 2. Signs in the Right Precordial Leads (V1 and V2) The most distinct sign of IRBBB appears in leads V1 and V2 (the leads placed over the right ventricle). RSR' Pattern ("M-Shaped" Complex): You will typically see a three-phase wave described as rSr', rsR', or * *rSR'**. The first wave (r) represents normal activation of the septum. The second wave (s) represents the normal activation of the left ventricle. The third wave (R') is a second upward spike that represents the delayed activation of the right ventricle. R' is often taller than r: The second upward spike (R') is usually larger than the initial one. 3. Signs in the Lateral Leads (V6 and Lead I) While V1 shows the delayed "push" of electricity toward the right, the leads on the left side of the chest (V6 and Lead I) show that same electrical force moving away from them. Slurred S Wave: You will see a wide, "slurred" S wave at the end of the QRS complex. This S wave is wider than the preceding R wave (or > 40ms) and looks "messy" or slow compared to the sharp upstroke of the R wave. Summary of Diagnostic Criteria | Feature | Incomplete RBBB Signs | QRS Duration: 100 – 119 ms (Narrower than complete block) Lead V1 / V2: rSR' pattern (Bunny ears / M-shape) Lead I / V6: Wide, slurred S wave Clinical Significance It is important to note that an incomplete RBBB is frequently a normal variant (benign finding), particularly in children, young adults, and well-conditioned athletes. However, it can occasionally be associated with conditions that strain the right side of the heart, such as an atrial septal defect (ASD) or right ventricular hypertrophy. Read more on LITFL: https://litfl.com/right-bundle-branch-block-rbbb-ecg-library/ Explanation for low amplitude T-waves (LOWT) Low amplitude T waves, often described as flat or flattened T waves, are a subtle but potentially clinically significant finding on a standard 12-lead ECG. They are characterized by a reduction in the voltage (height) of the T wave in leads where it is normally upright and prominent. 1. Visual Signs and Criteria On a standard ECG grid (where 1 small box = 1 mm or 0.1 mV), the signs of low amplitude T waves are: Height Criteria: The most common definition is a T wave amplitude of less than 1 mm (0.1 mV) in leads where the T wave is usually upright. Some definitions encompass any T wave between -1.0 mm and +1.0 mm. Proportionality (Rule of Thumb): A T wave is considered low if it is less than 10% of the height of the R wave in the same lead. Morphology: The wave loses its normal "peaked" or rounded appearance. It appears "smudged" or horizontally stretched, lacking a distinct summit. Leads to Watch: This finding is most significant in leads I, II, and V3–V6, where T waves are normally robust and upright. 2. Differentiating from Normal Variants Low amplitude T waves can be normal in certain contexts: Lead aVL: It is common and often normal to have a flat or even inverted T wave in lead aVL if the QRS is also small or negative. Lead III and V1: T waves are frequently low, flat, or inverted in these leads normally. Body Habitus: Patients with a thick chest wall or obesity may show globally low voltage (low QRS and T waves) due to the damping of the electrical signal. 3. Clinical Causes (The "Why") Because low amplitude T waves are considered a "non-specific" change, they do not point to a single diagnosis. However, they are a hallmark sign for several specific conditions when correlated with the patient's status: A. Hypokalemia (Low Potassium) This is the most classic metabolic cause. Sign: As potassium levels drop, the T wave flattens and decreases in amplitude. Associated Sign: You may see a prominent U wave appearing immediately after the flattened T wave, sometimes creating a "camel hump" appearance. B. Myocardial Ischemia While ischemia often causes T wave inversion (negative waves) or ST depression, the earliest or resolving phases can present as T wave flattening. Sign: A T wave that was previously upright becomes flat. This is particularly concerning if it is a new change compared to a prior ECG. Localization: If the flattening is localized (e.g., only in leads II, III, aVF), it may suggest ischemia in that specific wall of the heart (inferior wall in this example). C. Hormonal & Metabolic Issues Hypothyroidism: Often causes generalized low voltage across the entire ECG (small P waves, QRS complexes, and T waves) and sinus bradycardia (slow heart rate). D. Medications Digoxin: Can cause a "scooped" ST segment depression with a flattened or inverted T wave and a short QT interval. E. Dysautonomia / Hyperventilation Anxiety, hyperventilation, or high sympathetic tone can cause transient T wave flattening or inversion. Summary Checklist for Interpretation When you see low amplitude T waves, check for: Global vs. Local: Is it in all leads (think hypothyroidism, electrolytes, pericardial effusion) or just a few ( think ischemia)? U Waves: Are there U waves present? (Strong sign of Hypokalemia). QRS Voltage: Is the QRS also tiny? (Think damping effects like obesity, COPD, or pericardial effusion). Comparison: Is this new? A T wave that has gone from 4 mm to 0.5 mm is clinically significant even if it isn't " inverted." Read more on LITFL: https://litfl.com/t-wave-ecg-library/ Explanation for undefined (INVT) Sorry! It looks like no explanation is currently available for this diagnosis. Please let us know at support@ekgbattle.com. Explanation for sinus rhythm (SR) Identifying Sinus Rhythm on a standard 12-lead ECG is the foundational skill of ECG interpretation. It essentially means the heart's electrical impulse is originating correctly from the Sinoatrial (SA) node. To confirm sinus rhythm, you must look for specific signs related to the P wave, the rhythm regularity, and the conduction intervals. 1. The P Wave (The most critical sign) The definitive sign of sinus rhythm is the "P wave axis." Because the SA node is located in the top right of the heart, the electrical current should flow down and to the left. Lead II: The P wave must be upright (positive). This is the most important lead to check. Lead aVR: The P wave must be inverted (negative). If the P wave is upright in aVR, the rhythm is likely not sinus (e.g., it may be a low atrial or junctional rhythm). Leads I and aVF: P waves are typically upright. Consistency: The P waves should all look the same (consistent morphology) within a single lead. 2. The Relationship Between P and QRS The SA node should be driving the ventricles. 1:1 Ratio: Every P wave must be followed by a QRS complex, and every QRS complex must be preceded by a P wave. PR Interval: The time between the start of the P wave and the start of the QRS complex should be constant and within normal limits (0.12 to 0.20 seconds, or 3–5 small squares). 3. Rhythm Regularity Regularity: The distance between R waves (R-R interval) and P waves (P-P interval) should be consistent. Note: Minor variation is normal due to breathing (called respiratory sinus arrhythmia), but the rhythm should look visibly regular to the naked eye. 4. Heart Rate While "Sinus Rhythm" describes the origin of the beat, the rate determines the specific diagnosis: Normal Sinus Rhythm (NSR): Rate between 60 and 100 bpm. Sinus Bradycardia: All sinus criteria met, but rate is < 60 bpm. Sinus Tachycardia: All sinus criteria met, but rate is > 100 bpm. Summary Checklist When looking at a 12-lead ECG, you can confidently state "Sinus Rhythm" if: P waves are upright in Lead II. P waves are inverted in aVR. There is a P wave before every QRS. The rhythm is regular. Read more on LITFL: https://litfl.com/normal-sinus-rhythm-ecg-library/

[image: 21736.png] Diagnostic Code Explanations (3) Diagnoses for this EKG: anteroseptal myocardial infarction (ASMI), left anterior fascicular block (LAFB), abnormal QRS (ABQRS) Explanation for anteroseptal myocardial infarction (ASMI) An anteroseptal myocardial infarction (MI) is characterized by ischemia and necrosis affecting the anterior wall of the left ventricle and the interventricular septum. It typically results from an occlusion of the Left Anterior Descending (LAD) coronary artery. On a standard 12-lead ECG, the signs of an anteroseptal MI are primarily observed in the precordial leads V1 through V4. Key ECG Findings ST-Segment Elevation Leads: The hallmark sign is ST-segment elevation in leads V1, V2, V3, and V4. Morphology: The elevation is often significant and may evolve into a "tombstone" pattern in severe cases. Specifics: Septal leads: V1 and V2. Anterior leads: V3 and V4. The involvement of both groups defines the "anteroseptal" pattern. Q Waves (Pathological) As the infarction evolves (or if it is age-indeterminate), pathological Q waves will develop in leads V1–V4. A QS complex (a deep negative deflection with no preceding positive R wave) is commonly seen in V1 and V2. Poor R-Wave Progression (PRWP) In a healthy heart, the R wave (the first positive spike of the QRS complex) should get progressively taller as you move from lead V1 to V6. In an anteroseptal MI, the destruction of heart muscle in the anterior wall leads to a loss of these positive forces. Consequently, you will see small or absent R waves in leads V1–V3 (or even up to V4). T-Wave Changes Hyperacute Phase: Immediately after occlusion, you may see tall, peaked "hyperacute" T waves in V1–V4 before ST elevation becomes obvious. Evolving/Subacute Phase: As the MI progresses, the T waves typically become inverted (negative) in the same leads (V1–V4). Reciprocal Changes Unlike inferior or lateral MIs, pure anteroseptal MIs often lack distinct reciprocal changes (ST depression) in the standard limb leads. Why? The electrical "opposite" of the anterior wall is the posterior wall. Standard 12-lead ECGs do not have leads on the patient's back to record this directly. Exception: If the infarction extends to the high lateral wall (leads I and aVL), you may see reciprocal ST depression in the inferior leads (II, III, aVF). Summary Checklist for Anteroseptal MI ST Elevation: Leads V1, V2, V3, V4 Q Waves: Developing in V1–V4 R Waves: Loss of height in V1–V3 (Poor R-wave progression) Reciprocal Changes: usually absent (unless lateral extension is present) Read more: LITFL (ST segment): https://litfl.com/st-segment-ecg-library/ LITFL (MI localization): https://litfl.com/mi-localization-ecg-library/ LITFL (anterior STEMI): https://litfl.com/anterior-myocardial-infarction-ecg-library/ Explanation for left anterior fascicular block (LAFB) Left anterior fascicular block (LAFB), also known as left anterior hemiblock, is a conduction abnormality where the anterior fascicle of the left bundle branch is blocked. This forces electrical impulses to depolarize the left ventricle via the posterior fascicle, altering the direction of the electrical vector. Diagnosing LAFB relies on specific changes in the QRS axis and morphology. Below are the primary signs seen on a standard 12-lead ECG. 1. Pathological Left Axis Deviation (LAD) This is the hallmark sign of LAFB. Because the anterior fascicle is blocked, the electrical wavefront travels from the posterior/inferior wall towards the anterior/superior wall. This creates a net electrical vector that points upward and to the left. Criterion: The axis is typically between -45° and -90°. Note: While an axis between -30° and -45° suggests possible LAFB, it is less specific and can be a normal variant or due to left ventricular hypertrophy. A shift beyond -45° is generally required for a definitive diagnosis. 2. Lead-Specific QRS Morphology The redirected electrical vector creates a "divergent" pattern between the lateral and inferior leads. Lateral Leads (I and aVL): Pattern: qR complex (small Q wave, tall R wave). Explanation: The initial forces go downwards/rightwards (creating the small 'q'), followed by the major forces going upwards/leftwards (creating the tall 'R'). Inferior Leads (II, III, and aVF): Pattern: rS complex (small R wave, deep S wave). Explanation: The initial forces move briefly towards these leads (small 'r'), but the bulk of the depolarization moves away from them (deep 'S'). Key Check: Lead aVF should be negative. Lead II should also be negative (which confirms the axis is more negative than -30°). 3. Normal or Minimally Prolonged QRS Duration Unlike a complete Left Bundle Branch Block (LBBB), the main conduction trunk is intact, so ventricular depolarization is not significantly delayed. Criterion: QRS duration is usually < 120 ms (typically 80–110 ms). Context: If the QRS is ≥ 120 ms, a complete bundle branch block is likely present (e.g., RBBB + LAFB). 4. Prolonged R-Wave Peak Time in Lead aVL This measures the time from the onset of the QRS to the peak of the R wave (intrinsicoid deflection), representing the time it takes for depolarization to reach the lateral wall. Criterion: R-wave peak time ≥ 45 ms in lead aVL. Comparison: This is often delayed compared to lead V6. Summary of Diagnostic Criteria (Rule of Thumb) To quickly identify LAFB, look for: Left Axis Deviation (Positive Lead I, Negative Lead aVF). Negative Lead II (Deep S wave). Small Q waves in I and aVL. Normal QRS width (< 120 ms). Important Clinical Note: LAFB is a diagnosis of exclusion. You must ensure that the Left Axis Deviation is not caused by other conditions, such as a preexisting inferior myocardial infarction (which causes deep Q waves in II, III, and aVF) or severe Left Ventricular Hypertrophy (LVH). Read more on LITFL: https://litfl.com/left-anterior-fascicular-block-lafb-ecg-library/ Explanation for abnormal QRS (ABQRS) An abnormal QRS complex on a 12-lead ECG is typically identified by deviations in three main categories: Duration ( width), Amplitude (height/voltage), and Morphology (shape/contour). 1. Abnormal Duration (Wide QRS) A normal QRS complex lasts between 0.08 and 0.10 seconds (80–100 ms). A duration of > 0.12 seconds (>120 ms or 3 small squares) is considered abnormally wide. Bundle Branch Blocks (BBB): A blockage in the electrical conduction system causes one ventricle to depolarize later than the other, widening the QRS. Right Bundle Branch Block (RBBB): Characterized by an rSR' ("bunny ears") pattern in leads V1–V2 and a wide, slurred S wave in leads I and V6. Left Bundle Branch Block (LBBB): Characterized by a deep, broad S wave in V1 and a broad, notched, or " M-shaped" R wave in leads I, aVL, V5, and V6. Ventricular Rhythms: Rhythms originating from the ventricles (rather than the atria) do not use the fast conduction system, resulting in a wide QRS. Examples include Premature Ventricular Complexes (PVCs), Ventricular Tachycardia (VT), and Idioventricular rhythms. Hyperkalemia: High potassium levels can slow conduction, leading to a bizarrely wide QRS that may merge with the T wave (sine-wave pattern). Wolff-Parkinson-White (WPW) Syndrome: An accessory pathway allows early activation of the ventricles ( pre-excitation), causing a Delta wave (slurring of the initial upstroke) and a widened QRS. 2. Abnormal Amplitude (Voltage) The height of the QRS complex represents the electrical force generated by the ventricular muscle mass. High Voltage (Hypertrophy): Left Ventricular Hypertrophy (LVH): The muscle wall is thickened, generating stronger electrical forces. A common sign is the Sokolow-Lyon criteria: Depth of S wave in V1 + Height of R wave in V5 or V6 > 35 mm. Right Ventricular Hypertrophy (RVH): Often causes a dominant R wave in lead V1 (height > 7 mm) and a deep S wave in V5 or V6. Low Voltage: Defined as QRS amplitude < 5 mm in all limb leads and < 10 mm in all precordial (chest) leads. Causes: Anything that insulates the heart or dampens the signal, such as pericardial effusion (fluid around the heart), COPD (air trapping), obesity, or hypothyroidism (myxedema). 3. Abnormal Morphology (Shape) Even if the width and height are normal, the shape of the wave can indicate pathology. Pathological Q Waves: Q waves are the first downward deflection of the QRS. While small "septal" Q waves are normal in some leads, * pathological* Q waves indicate dead myocardial tissue (previous Myocardial Infarction). Signs: Duration > 0.04 s (40 ms) or depth > 25% of the following R wave height. Poor R Wave Progression: Normally, the R wave grows larger as you move from lead V1 to V6. If the R wave remains small or absent in leads V1–V3, it is termed "poor progression." Causes: Anterior Myocardial Infarction (old or new), LBBB, or lead misplacement. Fragmented QRS: Presence of additional spikes, notches, or slurs within the QRS complex (not fitting a typical BBB pattern). This often represents myocardial scarring or fibrosis. Electrical Alternans: The height of the QRS complex alternates between beats (large, small, large, small). This is a specific sign of a large pericardial effusion (cardiac tamponade) as the heart swings back and forth in the fluid. Read more: LITFL (interventricular conduction delay): https://litfl.com/intraventricular-conduction-delay-qrs-widening/ LITFL (low QRS voltage): https://litfl.com/low-qrs-voltage-ecg-library/

[image: 16844.png] Diagnostic Code Explanations (1) Diagnoses for this EKG: sinus rhythm (SR) Explanation for sinus rhythm (SR) Identifying Sinus Rhythm on a standard 12-lead ECG is the foundational skill of ECG interpretation. It essentially means the heart's electrical impulse is originating correctly from the Sinoatrial (SA) node. To confirm sinus rhythm, you must look for specific signs related to the P wave, the rhythm regularity, and the conduction intervals. 1. The P Wave (The most critical sign) The definitive sign of sinus rhythm is the "P wave axis." Because the SA node is located in the top right of the heart, the electrical current should flow down and to the left. Lead II: The P wave must be upright (positive). This is the most important lead to check. Lead aVR: The P wave must be inverted (negative). If the P wave is upright in aVR, the rhythm is likely not sinus (e.g., it may be a low atrial or junctional rhythm). Leads I and aVF: P waves are typically upright. Consistency: The P waves should all look the same (consistent morphology) within a single lead. 2. The Relationship Between P and QRS The SA node should be driving the ventricles. 1:1 Ratio: Every P wave must be followed by a QRS complex, and every QRS complex must be preceded by a P wave. PR Interval: The time between the start of the P wave and the start of the QRS complex should be constant and within normal limits (0.12 to 0.20 seconds, or 3–5 small squares). 3. Rhythm Regularity Regularity: The distance between R waves (R-R interval) and P waves (P-P interval) should be consistent. Note: Minor variation is normal due to breathing (called respiratory sinus arrhythmia), but the rhythm should look visibly regular to the naked eye. 4. Heart Rate While "Sinus Rhythm" describes the origin of the beat, the rate determines the specific diagnosis: Normal Sinus Rhythm (NSR): Rate between 60 and 100 bpm. Sinus Bradycardia: All sinus criteria met, but rate is < 60 bpm. Sinus Tachycardia: All sinus criteria met, but rate is > 100 bpm. Summary Checklist When looking at a 12-lead ECG, you can confidently state "Sinus Rhythm" if: P waves are upright in Lead II. P waves are inverted in aVR. There is a P wave before every QRS. The rhythm is regular. Read more on LITFL: https://litfl.com/normal-sinus-rhythm-ecg-library/