The effects of synchronized direct current countershock on ventricular automaticity, intraventricular conduction, and atrioventricular conduction were studied before and after ouabain administration and after diphenylhydantoin. At the level of electrical energy that induced ventricular arrhythmias, averaging 159 watt-seconds, countershock transiently increased ventricular automaticity and prolonged both intraventricular and atrioventricular conduction time. Immediately after ouabain-induced ventricular tachycardia returned to regular sinus rhythm, underlying ventricular automaticity and intraventricular and atrioventricular conduction times were increased. An average of 23 watt-seconds produced overt and persistent ventricular tachycardias due to enhanced automaticity or re-entry. Atrioventricular and inrraventricular conduction times were also prolonged, and transient complete heart block occurred in two experiments. Diphenylhydantoin administration at this time depressed ventricular automaticity, caused a speeding of atrioventricular and intraventricular conduction times and increased the electrical energy necessary to produce ventricular tachycardia to 355 watt-seconds.ADDITIONAL KEY WORDS intraventricular conduction heart block ventricular automaticity atrioventricular conduction re-entry• Synchronized direct current countershock is an accepted procedure for the termination of a number of cardiac arrhythmias. However, despite its widespread use, the electrophysiological effects of direct current countershock on the heart have only recently received attention (1). This is particularly important in understanding why some patients taking digitalis develop serious postshock arrhythmias (2-5). Although it has been demonstrated experimentally that digitalis administration greatly increases the sensitivity to postshock arrhythmias (6), the electrophysio- Accepted for publication March 20, 1968. logical mechanism for this sensitivity phenomenon remains unknown. In addition, a recent experimental study in our laboratory demonstrated that diphenylhydantoin greatly increases the amount of electrical energy required to produce an arrhythmia by countershock after digitalis sensitization (7). The electrophysiological effects of these interventions were explored in the present study by ascertaining the effects of direct current countershock on ventricular automaticity, intraventricular conduction and atrioventricular conduction before and after ouabain sensitization and after diphenylhydantoin desensitization. MethodTwenty-three adult mongrel dogs were anesthetized with intravenous sodium pentobarbital (30 mg/kg) after an overnight fast. A polyethylene catheter was inserted into the femoral
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