Which QRS morphology is typical of a left bundle branch block on ECG?

• A. Wide QRS, broad/notched R in I, aVL, V5-V6; absent or small Q in those leads; V1 shows predominantly negative QRS
• B. Wide QRS with rsR' pattern in V1
• C. Narrow QRS with positive Q in I
• D. Narrow QRS with terminal S wave in V6

Answer: (A)

Left bundle branch block shows a wide QRS with a distinctive pattern from the activation sequence. The block delays left ventricular activation, so the depolarization wavefront travels through the right ventricle first and then slowly to the left. This shifts the QRS vector to the right and posteriorly, producing a predominantly negative QRS in V1. In the lateral leads I, aVL, V5, and V6 you see broad, tall, and often notched R waves, and there are little or no initial Q waves in those leads. The QRS duration is typically ≥120 ms.

So the described pattern—wide QRS, broad/notched R in I, aVL, V5–V6 with absent or small Q in those leads, and a predominantly negative QRS in V1—is characteristic of left bundle branch block.