Rhythm Analysis: Ventricular/Blocks Flashcards

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

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.

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

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.

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

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.

What ECG characteristics define monomorphic ventricular tachycardia (VT)?

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.

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

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.

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

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.'