Adrenergic effects on internal cardiac defibrillation threshold.

Autonomic neural tone modulates arrhythmias and could affect the efficacy of an implantable defibrillator if defibrillation threshold is also altered by changes in neural activity. We determined the effects of alpha- and beta-adrenoceptor agonists and antagonists on the energy requirement for defibrillation using a sequential-pulse technique in anesthetized pigs. The doses for each drug were selected based on the results of dose-response curves. The mean defibrillation threshold was 10.2 +/- 0.65 J (mean +/- SE) in control and 10.0 +/- 0.84, 9.4 +/- 0.87 and 8.9 +/- 0.89 J during phenylephrine infusions of 0.7, 1.35, and 2.0-4.0 micrograms X kg-1 X min-1 [n = 8, P = not significant (NS)]. Phenylephrine at all infusion rates increased the ventricular fibrillation threshold, indicating that effects on the ventricular fibrillation threshold may occur independent of changes in defibrillation threshold. No significant change was observed in the defibrillation threshold before and after administration of isoproterenol (6.5 +/- 0.72 and 6.7 +/- 0.93 J, n = 8, P = NS). Similarly, no change in defibrillation thresholds was observed after 1.5-2.0 mg/kg phentolamine (8.5 +/- 0.85 and 7.9 +/- 0.93 J, n = 8, P = NS) or 3.0-6.0 mg/kg atenolol (10.0 +/- 1.7 and 10.3 +/- 2.6 J, n = 8, P = NS). However, when defibrillation threshold was determined using a single-pulse method, isoproterenol infusion produced a significant decrease (17.3 +/- 1.5 vs. 14.6 +/- 1.9 J, n = 7, P less than 0.01).(ABSTRACT TRUNCATED AT 250 WORDS)