CRAT Bundle Branch Blocks and Fascicular Blocks 1 — Questions and Answers
Question 1: What minimum QRS duration is required to diagnose a complete bundle branch block?
- ≥0.08 seconds
- ≥0.10 seconds
- ≥0.12 seconds (Correct answer)
- ≥0.14 seconds
Correct 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.
Question 2: In right bundle branch block (RBBB), which leads classically display the rSR' ('rabbit ears') pattern?
- Leads I and aVL
- Leads II and III
- Leads V1 and V2 (Correct answer)
- Leads V5 and V6
Correct 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.
Question 3: In right bundle branch block, a wide, slurred S wave is characteristically seen in which leads?
- V1 and V2
- V3 and V4
- I, aVL, V5, and V6 (Correct answer)
- II, III, and aVF
Correct 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.
Question 4: Which of the following correctly describes an INCOMPLETE right bundle branch block?
- QRS ≥0.12 sec with rSR' in V1
- QRS 0.10–0.11 sec with rSR' in V1 (Correct answer)
- QRS ≥0.12 sec without rSR' pattern
- QRS <0.10 sec with right axis deviation
Correct 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.
Question 5: The secondary R wave (R') seen in lead V1 during RBBB represents delayed depolarization of which cardiac structure?
- Left ventricle
- Interventricular septum
- Right ventricle (Correct answer)
- AV node
Correct 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.
Question 6: Which ECG feature best helps distinguish LBBB from RBBB when the QRS complex is wide?
- PR interval duration
- QRS morphology in leads V1 and V6 (Correct answer)
- P wave axis
- QT interval duration
Correct 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.
Question 7: Aberrant ventricular conduction most commonly produces a morphology resembling which bundle branch block?
- Left bundle branch block
- Right bundle branch block (Correct answer)
- Left anterior fascicular block
- Bifascicular block
Correct 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.
CRAT Bundle Branch Blocks and Fascicular Blocks 1
What minimum QRS duration is required to diagnose a complete bundle branch block?