Bundle Branch Blocks and Fascicular Blocks Flashcards

What minimum QRS duration is required to diagnose a complete bundle branch block?

Answer: ≥0.12 seconds

A complete bundle branch block requires a QRS duration of ≥0.12 seconds (3 small squares at standard paper speed) on the ECG.

In right bundle branch block (RBBB), which leads classically display the rSR' ('rabbit ears') pattern?

Answer: Leads V1 and V2

RBBB produces a classic rSR' (or RSR') pattern in the right precordial leads V1 and V2, reflecting delayed right ventricular depolarization traveling toward those leads.

In right bundle branch block, a wide, slurred S wave is characteristically seen in which leads?

Answer: I, aVL, V5, and V6

The terminal slurred S wave in leads I, aVL, V5, and V6 represents the delayed right ventricular depolarization force traveling away from these left-sided lateral leads in RBBB.

Which of the following correctly describes an INCOMPLETE right bundle branch block?

Answer: QRS 0.10–0.11 sec with rSR' in V1

Incomplete RBBB shows the characteristic rSR' morphology in V1 but with a QRS duration between 0.10 and 0.11 seconds, indicating partial rather than complete conduction delay.

The secondary R wave (R') seen in lead V1 during RBBB represents delayed depolarization of which cardiac structure?

Answer: Right ventricle

The R' in V1 during RBBB represents delayed activation of the right ventricle, which depolarizes late after normal left ventricular activation has already occurred.

Which ECG feature best helps distinguish LBBB from RBBB when the QRS complex is wide?

Answer: QRS morphology in leads V1 and V6

In LBBB, V1 shows a broad negative QS or rS complex while V6 shows a broad positive R wave; RBBB shows an rSR' in V1 and a slurred S wave in V6 — opposite patterns.

Aberrant ventricular conduction most commonly produces a morphology resembling which bundle branch block?

Answer: Right bundle branch block

Aberrant conduction most commonly mimics RBBB morphology because the right bundle branch has a longer refractory period than the left and is therefore more susceptible to rate-related block.