Incomplete left bundle branch block

Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]

In incomplete left bundle branch block, the QRS interval is widened, but is insufficiently prolonged to meet the criteria for left bundle branch block.

1. QRS duration between 110 and 119 ms in adults, between 90 and 100 ms in children 8 to 16 years of age, and between 80 and 90 ms in children less than 8 years of age.
2. Presence of left ventricular hypertrophy pattern.
3. R peak time greater than 60 ms in leads V4, V5, and V6.
4. Absence of q wave in leads I, V5, and V6.

Also a narrow initial R wave in V1 has been linked with a reduced response to CRT.