LBBB: State-of-the-Art Criteria

►Implication for health policy/practice/research/medical education: This report demonstrates new criteria for patient selection in CRT era. We started diagnosing Bundle Branch Block about 100 years ago on dog models (1). However, about 40 years passed until we could diagnose Left Bundle Branch Block (LBBB) correctly on ECG (2). Today, we have conventional criteria for diagnosing LBBB, including QRS duration>120 msec, QS or rS in lead V1, Monophasic R wave with no Q wave in lead V6 and I3, ACC/AHA/HRS added notched R wave in lead I,aVL, V5, and V6, and occasional RS pattern in V5 and V6 (3). In case rate dependent LBBB develops, you can see the disappearance of the q wave in V6, then initial slurring of R wave and delayed increased intrinsicoid deflection. More complete LBBB causes notched plateau after initial peaked R wave (4). LBB has anterior fascicle, posterior fascicle, and sometimes a septal fascicle (5). Blocking the left bundle may cause septal force to disappear; therefore, no initial R wave can be detected in V1 or Q wave in I, V5, and V6, but that is not always the case (5). Sometimes, septal MI causes initial Q in the lateral leads and initial R wave in V1. Also, Grants and Doge found initial septal force in 40% of their cases with LBBB (6). Accordingly, initial septal force should not be considered as a diagnostic criterion for LBBB. Widening of QRS may occur in LBBB as well as many other conditions, such as LVH, RVH, and IVCD. Sometimes, LBBB also causes minimally increased width in QRS named incomplete LBBB. Wilson compared dogs and humans and suggested 120 msec. as the cutoff point for diagnosing LBB (2); however, this may need revision based on the findings of the study by Selvester and Salmon (7). They showed that when LBB is blocked, 40 msec. are required for septal depolarization, then 50 msec to reach the posterolateral wall, and finally 50 msec to complete posterolateral wall activations. Moreover, they suggested 140 msec. for males and 130 msec. for females for diagnosis of LBBB. The most consistent finding in LBBB patients seems to be mid QRS notching or slurring which is best seen in I, aVl, V5, and V6 (3). This mid QRS notching shows two vectors that are in the relatively same direction but one is minimally delayed. The first vector shows depolarization of endocardium of the left ventricle, …