Question

C (Right bundle branch block with a first degree AV block)

(Right bundle branch block can be recognized by the following pattern: R-S-R' in leads V1 and/or V2 (looks like bunny ears) and a slurred S-wave in V6 on ECG.)

(When there is a bundle branch block (whether left or right), the two ventricles do not depolarize simultaneously. This results in a wide QRS-complex (>120 ms), as pictured here. To differentiate between the two types of complete bundle branch block (Left vs. Right), examine leads V1, V2, and V6. Right bundle branch block can be recognized by the following pattern: R-S-R' in leads V1 and/or V2 (looks like bunny ears) and a slurred S-wave in V6. In addition, there is a prolongation of the PR interval of roughly 240ms which indicates a first degree AV block as well.)

(A quick way to check for bundle branch block on an ECG is to use the WiLLiaM MaRRoW mnemonic. WiLLiaM has 2 Ls in it, so in LBBB you should look for a W-shaped QRS in lead V1 and a M-shaped QRS in lead V6. The opposite is true in RBBB. This rule works on the principle the side of the heart with the bundle branch block will be depolarised by a slow wave of electrical current conducted through the myocardium from the opposite side of the heart. This slow wave of current causes a wide, positive QRS-complex in the leads which it moves towards. This means in LBBB, the lateral lead (V6) has the positive M-shaped complex.)
a) Narrow QRS complex with prolonged PR interval
b) Presence of a W-shaped QRS complex in lead V1 and an M-shaped QRS complex in lead V6
c) Slurred S-wave in leads V1 and V2 with prolongation of the PR interval
d) Absence of QRS prolongation with prominent R-waves in leads V1 and V2

Answer 1

The ECG pattern indicating a Right bundle branch block (RBBB) combined with a first degree AV block is characterized by a c. slurred S-wave in leads V1 and V2 and a prolonged PR interval.

Step-by-step explanation:

The correct answer to the question about ECG patterns is c) Slurred S-wave in leads V1 and V2 with prolongation of the PR interval, which is indicative of a Right bundle branch block (RBBB) with a first degree AV block. In RBBB, one finds a pattern of R-S-R' (resembling bunny ears) in leads V1 and/or V2 and a slurred S-wave in V6 on the ECG. This occurs because in a bundle branch block, the ventricles do not depolarize at the same time, giving rise to a wide QRS-complex (>120 ms). The PR interval prolongation of roughly 240ms indicates the first degree AV block, which is a delay in conduction between the SA and AV nodes, without missing a beat but with prolonged conduction time.

A first-degree AV block is recognized on an ECG as an abnormally long PR interval. This tells us that there is a delay in conduction between the SA and AV nodes. On the other hand, a right bundle branch block (RBBB) can be recognized by the pattern of R-S-R' in leads V1 and/or V2, which looks like bunny ears, and a slurred S-wave in lead V6. In addition to a bundle branch block, the ECG shows a prolongation of the PR interval, indicating a first-degree AV block as well. So the correct answer is c) Slurred S-wave in leads V1 and V2 with prolongation of the PR interval.