Intermittent left bundle branch block: anatomic substrate as reflected in the electrocardiogram during normal conduction.

Examination of ECGs in patients with intermittent left bundle branch block (LBBB) may provide insight into mechanisms and cause of LBBB. In this study, we obtained ECG files of patients with intermittent LBBB by mail solicitation of cardiologists. The group suitable for analysis included 275 patients in whom both LBBB was documented and at least one 12-lead ECG was available which demonstrated absence of LBBB (ALBBB) after the first LBBB ECG. ALBBB ECGs revealed normal QRS in 151 patients (55%), and abnormal QRS in 124 (45%). These 124 had one or more of the following: left ventricular hypertrophy 53 patients (19% of total group), myocardial infarction (MI) 53 patients (19% of total group) and conduction disturbance (QRS>0.10 sec and/or axis of -30° to -90°) 52 patients (19% of total group). Sixty-three MIs were localized in the 53 patients; 31 (49%) were anteroseptal, seven (11%) were anterior, 17 (27%) were inferior and eight (13%) were lateral (p < 0.10). The 52 patients with conduction defects had the following: left anterior hemiblock 32 patients (62% of conduction defects, 12% of total group), incomplete LBBB 19 patients (31% of conduction defects, 7% of total group), right bundle branch block one patient (2% of conduction defect, 0.1% of total group). More than 50% of the patients with intermittent LBBB did not have abnormal QRS, suggesting an underlying cause of LBBB. If underlying infarction is present, it is most commonly anteroseptal, implying disease of the left anterior descending coronary artery. The site of most intermittent LBBB appears predivisional (His or main left bundle), since preexisting leftsided unifascicular block is infrequent.