[image: 21761.png] Diagnostic Code Explanations (2) Diagnoses for this EKG: complete left bundle branch block (CLBBB), atrial fibrillation (AFIB) Explanation for complete left bundle branch block (CLBBB) A complete Left Bundle Branch Block (LBBB) is characterized by a delay in the electrical activation of the left ventricle. This forces the electrical impulse to travel first to the right ventricle and then slowly across the septum to the left ventricle, creating a distinct pattern on the 12-lead ECG. Here are the key signs and diagnostic criteria for complete LBBB: 1. Prolonged QRS Duration Criterion: The QRS complex width is ≥ 120 ms (0.12 seconds or 3 small squares). Why: The impulse has to travel through the myocardium (muscle-to-muscle) rather than the fast-conducting bundle branch system, significantly slowing down depolarization. 2. Lead Morphology The appearance of the QRS complex changes depending on which part of the heart the lead is "looking" at. Lateral Leads (I, aVL, V5, V6): Broad, monophasic R waves: You will see tall, wide, and entirely positive waves. They often look "notched" or like the letter "M" (especially in V6). Absence of Q waves: Normal small "septal" Q waves are absent in leads I, V5, and V6. (This is because the normal left-to-right septal activation is reversed). Anterior/Septal Leads (V1, V2): Deep S waves: The complex is predominantly negative. You typically see a small initial r-wave followed by a deep, wide S-wave (rS complex) or a completely negative complex (QS complex). "W" Shape: Occasionally, the deep S-wave may be notched, though this is less common than the "M" shape in the lateral leads. 3. Discordance (ST-T Wave Changes) In LBBB, the repolarization (resetting) of the heart is abnormal because the depolarization was abnormal. This leads to "Appropriate Discordance," meaning the ST segments and T waves go in the opposite direction of the main QRS vector. In Lateral Leads (Positive QRS): Expect ST-segment depression and inverted T waves. In Anterior Leads (Negative QRS): Expect ST-segment elevation and upright (positive) T waves. Note: This ST elevation in leads V1–V3 is normal for LBBB and can be easily confused with a STEMI (heart attack). 4. Additional Signs Left Axis Deviation: While not strictly required for the diagnosis, most patients with LBBB will have a left axis deviation. Prolonged R-Wave Peak Time: In leads V5 and V6, the time from the start of the QRS to the peak of the R wave is prolonged (> 60 ms), reflecting the delayed activation of the left ventricle. Poor R-Wave Progression: Because the anterior leads (V1–V3) are dominated by deep S waves, the R waves do not grow in size across the chest as they normally would. Summary Wide QRS (≥ 120 ms). V1: Deep, negative S-wave. V6/Lead I: Tall, broad/notched R-wave with no Q-wave. ST/T waves point opposite to the QRS complex. Read more on LITFL: https://litfl.com/left-bundle-branch-block-lbbb-ecg-library/ 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: 21836.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: 21752.png] Diagnostic Code Explanations (3) Diagnoses for this EKG: anterolateral myocardial infarction (ALMI), abnormal QRS (ABQRS), sinus rhythm (SR) Explanation for anterolateral myocardial infarction (ALMI) An anterolateral myocardial infarction (MI) is characterized by specific changes on a standard 12-lead ECG that reflect damage to both the anterior (front) and lateral (side) walls of the left ventricle. Here are the key signs to look for: 1. Affected Leads The hallmark of an anterolateral MI is ST-segment elevation in a combination of precordial (chest) and limb leads: Anterior Leads: V3 and V4 (representing the anterior wall). Lateral Leads: V5, V6, I, and aVL (representing the lateral wall). Note: You may often see involvement of leads V1 and V2 as well if the infarction extends to the septum ( anteroseptal-lateral), often referred to simply as an extensive anterior MI. 2. Characteristic Waveform Changes ST-Segment Elevation: Look for J-point elevation in the leads mentioned above (V3-V6, I, aVL). This is the primary indicator of acute injury. Hyperacute T Waves: In the very early stages (minutes after onset), you may see tall, broad, and peaked T waves in the anterolateral leads before distinct ST elevation develops. Pathological Q Waves: As the infarction evolves (hours to days), deep and wide Q waves may develop in leads V3-V6, I, and aVL, indicating necrosis (tissue death). Poor R-Wave Progression: Normally, the R wave grows larger as you move from V1 to V6. In an anterolateral MI, the R waves in V3 and V4 may remain very small or disappear entirely. 3. Reciprocal Changes ECG leads that look at the heart from the opposite angle often show "mirror image" changes. For an anterolateral MI, you will typically see: ST-Segment Depression: Most prominent in the inferior leads (II, III, and aVF). Summary of Diagnostic Criteria ST Elevation: V3, V4, V5, V6, I, aVL Reciprocal ST Depression: II, III, aVF Culprit Artery: Usually the Left Anterior Descending (LAD) artery or a large Diagonal branch (D1). Less commonly, it can involve the Left Circumflex (LCx) if it supplies the lateral wall. Visual Tip: If you see ST elevation stretching across the chest leads (V3-V6) and "high lateral" leads (I, aVL), combined with ST depression in the bottom leads (II, III, aVF), the diagnosis is highly likely an anterolateral STEMI. 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/ LITFL (high lateral STEMI): https://litfl.com/high-lateral-stemi-ecg-library/ LITFL (lateral STEMI): https://litfl.com/lateral-stemi-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: 21837.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: 1989.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/