Electrocardiographic determinants of axis during left bundle branch block: Study in patients with intermittent left bundle branch block

The frontal plane QRS axis of electrocardiograms during "normal" conduction and left bundle branch block were compared in 231 patients with intermittent left bundle branch block. The QRS axis during left bundle branch block ranged from +85 ° to -70 ° (mean ± standard deviation -7 ± 34 °), 79 patients (34 percent) having left axis deviation (QRS axis -30 ° or less). The QRS axis during "normal" conduction ranged from +90 ° to -75 ° (19 ± 34 °), 26 patients (11 percent) having left axis deviation. Regression analysis of the QRS axis during left bundle branch block versus "normal" conduction revealed the correlation coefficient (r) = 0.25 (p < 0.001). Of 26 patients with left axis deviation during normal conduction, 18 (70 percent) had left axis deviation during left bundle branch block (p < 0.001). Of 79 patients with left axis deviation during left bundle branch block, 61 (77 percent) did not have underlying left axis deviation during "normal" conduction. There was no significant association of axis during left bundle branch block with myocardial infarction or left ventricular hypertrophy diagnosed during "normal" conduction. In summary, although a small but significant correlation was found between the QRS axis during left bundle branch block and "normal" conduction, underlying left anterior hemiblock does not account for the presence of left axis deviation during left bundle branch block in most patients.