CCT Rhythm Analysis: Ventricular/Blocks 2 — Questions and Answers

Question 1: What are the ECG criteria for left bundle branch block (LBBB)?

QRS ≥ 0.12 seconds; broad monophasic R in lateral leads (I, aVL, V5-V6); rS or QS pattern in right precordial leads (V1-V2); absence of septal Q waves in lateral leads (Correct answer)
QRS ≥ 0.12 seconds; RSR' (rabbit ears) pattern in V1; wide S waves in lateral leads
QRS < 0.12 seconds; tall R waves in V1 and V2
QRS ≥ 0.12 seconds; delta wave; short PR interval

Correct answer: QRS ≥ 0.12 seconds; broad monophasic R in lateral leads (I, aVL, V5-V6); rS or QS pattern in right precordial leads (V1-V2); absence of septal Q waves in lateral leads

LBBB criteria: QRS ≥ 0.12 seconds (complete LBBB), broad, notched or monophasic R in leads I, aVL, V5-V6, with no septal Q waves, and a deep rS or QS complex in V1-V2.

In LBBB, the left bundle branch fails to conduct normally, so the left ventricle is activated late from right-to-left spread of the impulse from the right ventricle. This produces: delayed, abnormal LV activation resulting in broad, monophasic (or notched 'W'/'M'-shaped) R waves in lateral leads (I, aVL, V5, V6); rS or QS complexes in V1-V2 (activation moving away from right-sided leads); QRS duration ≥120 ms; and absence of normal septal Q waves in lateral leads (because septal activation is reversed). LBBB makes interpretation of ST changes and ischemia very challenging — a new LBBB with chest pain is treated as equivalent to STEMI by many guidelines (Sgarbossa criteria are used for ischemia in LBBB).

Question 2: What ECG features differentiate right bundle branch block (RBBB) from left bundle branch block (LBBB)?

RBBB: RSR' (rabbit ears) in V1 with wide S in lateral leads; LBBB: broad monophasic R in lateral leads with QS/rS in V1 (Correct answer)
RBBB: broad R in lateral leads; LBBB: RSR' in V1
Both: identical QRS morphology, distinguished only by axis
RBBB: QRS < 0.10 s; LBBB: QRS > 0.14 s

Correct answer: RBBB: RSR' (rabbit ears) in V1 with wide S in lateral leads; LBBB: broad monophasic R in lateral leads with QS/rS in V1

The classic RBBB pattern is RSR' (M-shaped 'rabbit ears') in V1 with a wide terminal S wave in lateral leads (I, V5, V6). LBBB shows a broad, notched R in lateral leads with a deep negative QS or rS complex in V1.

The key to distinguishing RBBB from LBBB is analyzing V1 (the right-facing lead): RBBB in V1: RSR' pattern — initial r (septal activation moving right), S wave (LV activation moving left, away from V1), R' (delayed RV activation moving right back toward V1). Wide S waves in I, V5, V6 (delayed RV conduction moving away from lateral leads). LBBB in V1: QS or rS pattern — entire activation moves away from V1 because LV activation is reversed. Both have QRS ≥ 0.12 seconds (complete block) or 0.10-0.12 (incomplete). Knowing this distinction is essential for CCT exam and daily ECG interpretation.

Question 3: In third-degree (complete) AV block, what is the key ECG finding that distinguishes it from other forms of AV block?

Complete AV dissociation: P waves and QRS complexes are completely independent with no relationship between them; both have their own regular, independent rates (Correct answer)
PR intervals are progressively lengthening with dropped beats
Non-conducted P waves occur after every 2 conducted P waves (3:2 block)
P waves occur at the same rate as QRS complexes but with very long fixed PR intervals

Correct answer: Complete AV dissociation: P waves and QRS complexes are completely independent with no relationship between them; both have their own regular, independent rates

Third-degree AV block shows complete dissociation between atrial and ventricular activity — P waves march through at the sinus rate while QRS complexes occur at a slower, independent escape rate with no fixed relationship between P waves and QRS complexes.

In third-degree (complete) AV block, no atrial impulses cross the AV node to activate the ventricles. The ventricles are maintained by an escape rhythm from below the block: junctional escape (rate 40-60 bpm, narrow QRS) if the block is at AV nodal level, or ventricular escape (rate 20-40 bpm, wide QRS) if the block is in the His-Purkinje system. On ECG, the diagnostic finding is AV dissociation with P rate > QRS rate and NO fixed PR interval — P waves 'march through' the QRS complexes without capturing. This must be distinguished from isorhythmic dissociation (where rates are similar) and from 2:1 block (where the relationship exists, just with 2:1 ratio).

Question 4: What ECG characteristics define monomorphic ventricular tachycardia (VT)?

Three or more consecutive QRS complexes originating in the ventricles at a rate ≥ 100 bpm with identical QRS morphology from beat to beat (Correct answer)
Three PVCs with different QRS morphologies occurring consecutively
P waves at 300 bpm with regular QRS complexes at 150 bpm
Wide QRS complexes at 40-60 bpm with AV dissociation

Correct answer: Three or more consecutive QRS complexes originating in the ventricles at a rate ≥ 100 bpm with identical QRS morphology from beat to beat

Monomorphic VT is defined as three or more consecutive ventricular beats at ≥100 bpm (typically 140-200 bpm) with uniform (identical) QRS morphology in each lead, indicating a single ventricular origin for each beat.

VT originates below the bundle of His in the ventricular myocardium or Purkinje system. Monomorphic VT (all QRS complexes have the same shape) indicates a single stable reentry circuit or focus. ECG features: wide QRS complexes (>0.12 sec); rate typically 140-200 bpm (but can range from 100 to >250 bpm); AV dissociation (P waves independently firing at a slower rate) when visible; fusion beats (P wave partially captures ventricle during VT, creating an intermediate morphology); capture beats (sinus beat fully captures ventricle — narrow QRS interrupts VT). AV dissociation, fusion, and capture beats are pathognomonic of VT but are not always visible.

Question 5: Left anterior fascicular block (LAFB) produces which characteristic ECG finding?

Left axis deviation (axis between -30 and -90 degrees) with small R in inferior leads, deep S in lateral leads, and normal QRS duration (Correct answer)
Right axis deviation with RBBB morphology
Wide QRS with left bundle branch block pattern
Left axis deviation with Q waves in inferior leads and prolonged QRS

Correct answer: Left axis deviation (axis between -30 and -90 degrees) with small R in inferior leads, deep S in lateral leads, and normal QRS duration

LAFB causes left axis deviation (typically -45° to -90°) due to block of the anterior-superior fascicle of the left bundle, producing a small r followed by a deep S in leads II, III, aVF and a small q followed by a tall R in leads I and aVL, without significant QRS widening.

The left bundle branch divides into anterior and posterior fascicles. LAFB (block of the anterior fascicle) forces activation to travel inferiorly first (via the posterior fascicle) before activating the anterior-superior LV wall, pulling the mean QRS axis superiorly and leftward. ECG criteria: left axis deviation -45° to -90°; small r wave in inferior leads (II, III, aVF) followed by deep S; small q with tall R in I and aVL; QRS duration normal or minimally prolonged (<0.12 sec). LAFB is the most common fascicular block and frequently accompanies RBBB (bifascicular block). Isolated LAFB is usually benign but may indicate underlying conduction system disease.

Question 6: Torsades de Pointes (TdP) is a specific form of polymorphic VT associated with which underlying ECG abnormality?

Prolonged QT interval (congenital or acquired) (Correct answer)
Short QT interval
Right bundle branch block
Pre-excitation (WPW pattern)

Correct answer: Prolonged QT interval (congenital or acquired)

Torsades de Pointes is triggered by early afterdepolarizations in the setting of a prolonged QT interval, and is characterized by the twisting of the QRS polarity around the baseline, resembling 'twisting of the points.'

TdP is a potentially life-threatening polymorphic ventricular tachycardia characterized by QRS complexes that gradually change in amplitude and polarity, appearing to 'twist' around the isoelectric baseline over 5-10 beats (500-600 bpm). It is the clinical manifestation of ventricular arrhythmia arising from a prolonged QT interval — either congenital (Long QT syndromes 1-3) or acquired (hypokalemia, hypomagnesemia, bradycardia, QT-prolonging drugs: sotalol, quinidine, some antipsychotics and antidepressants). TdP often terminates spontaneously but can degenerate to VF. Treatment: IV magnesium sulfate, correcting electrolytes, temporary pacing to accelerate heart rate (shortening the QT), and avoiding causative drugs.

CCT Rhythm Analysis: Ventricular/Blocks 2

What are the ECG criteria for left bundle branch block (LBBB)?