Table 1. Criteria for the diagnosis of fascicular blocks (morphological criteria for BBB, Figure 2).

Fascicular blocks

LAFB Qualifying statements  S1) QRS duration < 0.12 s (adults)  S2) QRS axis ≤ –30°  S3) rS pattern in II and III and aVF  S5) R peak time ≥ 0.045 s in aVL  S6) Slurred R downstroke in aVL  S7) Slurred S in V5 or V6 Criteria for LAFB  a) S1 and S2 and S3 and S4 and S5 or  b) S1 and S2 and S3 and S4 and S6 or  c) S1 and S2 and S3 and S4 and S7 Qualifying statement S3 is usually present with criteria a, b, and c above. If there is a QS in lead II, LAFB cannot be differentiated from inferior MI.

LPFB Qualifying statements  S1) QRS duration < 0.12 s  S2) 180° > QRS axis > 90°  S3) qR pattern in III and aVF with Q duration ≤ 0.04 sec  S4) Absence of other causes of right axis deviation Criteria for LBBB Criteria for LPFB  a) S1 and S2 and S3 a) S1 and S2 and S3 and S4

With modifications from Vereckei, et al.[6] BBB: bundle branch block; LAFB: left anterior fascicular block; LBBB: left bundle branch block; LPFB: left posterior fascicular block; MI: myocardial infarction.