The mechanisms responsible for intermittent
bundle branch block are still under debate. The role of the time-dependent behavior of the slow
calcium channel has recently been emphasized. To test this hypothesis and ascertain the possible involvement of the fast
sodium channel, the effects of the slow
calcium channel blocker verapamil and the fast
sodium channel blocker procainamide were compared in 10 patients with intermittent
bundle branch block. All 10 patients showed
bundle branch block during spontaneous sinus rhythm. Maneuvers to slow cardiac rate (that is, carotid sinus
massage, Valsalva maneuver) were performed to identify normal conduction as well as phase 4
bundle branch block. Thus, the ranges of diastolic intervals (RR) resulting in phase 3 (
tachycardia-dependent)
bundle branch block, phase 4 (
bradycardia-dependent)
bundle branch block and normal conduction were measured in two control studies performed before
intravenous administration of
verapamil (control 1) and
procainamide (control 2) and at the peak effect of both drugs. In the control studies, all 10 patients showed phase 3
bundle branch block, whereas phase 4
bundle branch block occurred in only 4 patients. The ranges of phase 3
bundle branch block, phase 4
bundle branch block and normal conduction were very similar in control studies 1 and 2. The phase 3
bundle branch block range was slightly shortened by
verapamil (983 +/- 83.5 ms in control 1; 930 +/- 69.4 ms at the peak effect of
verapamil), whereas phase 4
bundle branch block remained unchanged. In contrast, conduction was systematically worsened by
procainamide.(ABSTRACT TRUNCATED AT 250 WORDS)