Bundle Branch Blocks and Fascicular Blocks Flashcards

The left bundle branch divides into how many main fascicles?

Answer: Two

The left bundle branch divides into two main fascicles: the left anterior fascicle and the left posterior fascicle, each of which can be independently blocked.

Left anterior fascicular block (LAFB) is characterized by which QRS axis on a 12-lead ECG?

Answer: Left axis deviation (−45° to −90°)

LAFB produces marked left axis deviation (typically −45° to −90°) because the depolarization wave must travel inferiorly via the posterior fascicle before activating the anterior wall superiorly.

Which set of criteria correctly describes left anterior fascicular block (LAFB)?

Answer: Left axis deviation, small Q in I and aVL, small R in II/III/aVF, QRS <0.12 sec

LAFB criteria include left axis deviation (−45° to −90°), small Q waves in I and aVL, small R waves in II, III, and aVF, and a QRS duration less than 0.12 seconds.

The combination of right bundle branch block (RBBB) and left anterior fascicular block (LAFB) is called:

Answer: Bifascicular block

Bifascicular block refers to simultaneous blockage of two of the three fascicles; RBBB + LAFB is the most common combination, involving two-thirds of the trifascicular conduction system.

A patient with bifascicular block (RBBB + LAFB) also has a prolonged PR interval. This pattern is clinically significant because it suggests:

Answer: Disease in all three fascicles with risk of progressing to complete AV block

Bifascicular block with a prolonged PR interval (trifascicular block pattern) indicates disease affecting all three conduction fascicles and carries significant risk of progression to complete third-degree AV block.

Left posterior fascicular block (LPFB) produces which axis deviation, and in which leads are small Q waves characteristically seen?

Answer: Right axis deviation; Q waves in II, III, and aVF

LPFB causes right axis deviation with small Q waves in II, III, and aVF because activation travels initially anteriorly via the anterior fascicle then shifts inferiorly, producing inferior Q waves.

Which condition must be excluded BEFORE diagnosing left posterior fascicular block (LPFB)?

Answer: Right ventricular hypertrophy and lateral wall MI must be ruled out

LPFB is a diagnosis of exclusion because right ventricular hypertrophy, lateral MI, and pulmonary disease can all cause right axis deviation that mimics LPFB's ECG pattern.