Parasympathetic and sympathetic alterations of Mobitz type II heart block.

This study examined the effects of changes in parasympathetic and sympathetic tone on the cycle length at which Mobitz type II second degree atrioventricular (AV) block occurred. Four patients who had electrocardiographic evidence of type II AV block and confirmation of block in the His-Purkinje system during electrophysiologic study were evaluated. These patients received intravenous atropine (1.0 to 2.4 mg), propranolol (0.15 mg/kg body weight) or isoproterenol (1 and 2 micrograms/min) alone or in combination. In two of three patients receiving propranolol, the atrial pacing cycle length at which 1:1 His-Purkinje conduction occurred was prolonged relative to control (from 360 to 470 ms and 440 to 590 ms, respectively). In contrast, atropine in the presence of beta-adrenergic blockade shortened the cycle length at which 1:1 His-Purkinje conduction occurred in three of four patients receiving the drug (470 to 390, 630 to 570 and 590 to 560 ms, respectively). Isoproterenol also improved His-Purkinje conduction in the one patient receiving this drug. No agent affected the duration of the HV interval during spontaneous sinus rhythm or right atrial pacing. Thus, drugs that alter autonomic tone influence abnormal His-Purkinje conduction minimally during sinus rhythm but, importantly, may modulate the atrial pacing cycle length at which type II AV block occurs.