Which EKG feature would NOT be expected in a left bundle branch block?

Which EKG feature would NOT be expected in a left bundle branch block?

• A. A widened QRS complex
• B. A tall R wave in V1
• C. Two R peaks in a single QRS complex
• D. Normal duration of PR interval

Answer: (B)

In the context of a left bundle branch block (LBBB), a tall R wave in V1 is not an expected finding. Typically in LBBB, the right ventricle depolarizes first due to the block in the left bundle branch, leading to a characteristic pattern where V1 shows a small R wave initially, often followed by a large S wave. Instead, the QRS complex will be widened, reflecting the delay in the depolarization of the left ventricle.

On the other hand, a widened QRS complex is a hallmark of LBBB, as it indicates that the electrical impulse takes longer to traverse through the ventricular myocardium. Similarly, the presence of two R peaks in a single QRS complex, known as a ‘bifid R wave’, can occur in LBBB, particularly in leads like V5 and V6, due to the delayed depolarization of the left ventricle.

Lastly, in LBBB, the PR interval typically remains normal, as the block occurs during ventricular depolarization rather than affecting the atrioventricular conduction pathway. Therefore, the expectation for a normal duration of the PR interval aligns with the characteristics seen in a patient with LBBB.