From the Guidelines
Bifascicular and trifascicular blocks are identified on ECG by recognizing specific conduction abnormalities in the heart's electrical system, with a bifascicular block involving impairment of two fascicles and a trifascicular block involving all three fascicles. A bifascicular block typically appears as a right bundle branch block (RBBB) plus either left anterior fascicular block (LAFB) or left posterior fascicular block (LPFB), while a trifascicular block involves RBBB with either LAFB or LPFB, plus a prolonged PR interval (>200 ms) indicating AV node or His bundle delay 1. Some key ECG features to look for include:
- RBBB: wide QRS complex (>120 ms), RSR' pattern in V1-V2, and wide S waves in leads I and V6
- LAFB: left axis deviation (-30° to -90°)
- LPFB: right axis deviation (+90° to +120°) These blocks are clinically significant as they may progress to complete heart block, especially in patients with syncope or presyncope, and require careful measurement of intervals and axis determination, with particular attention to QRS morphology in the precordial and limb leads 1. It's also important to note that the presence of bundle branch block on ECG is a predictor for abnormal conduction properties identified at electrophysiological study (EPS), and permanent pacing has been recommended for patients with syncope and HV intervals ≥70 ms or frank infranodal block 1.