EFFECTS OF AUTONOMIC MANIPULATION ON VENTRICULAR-FIBRILLATION AND INTERNAL CARDIAC DEFIBRILLATION THRESHOLDS IN PIGS

Autonomic tone may contribute to cardiac arrhythmogenesis and influenc e the efficacy of implantable defibrillators. Fifty-two anesthetized p igs were randomized to: (1) methacholine (n = 12); (2) nitroprusside ( n = 12); (3) phenylephrine (n = 12); (4) carbachol (n = 8); and (5) sa line (n = 8). Ventricular fibrillation threshold (VFT) and triplicate defibrillation thresholds (DFT) were obtained before and during each i ntervention. Mean (+/- SE) VFT was increased with: methacholine (76 +/ - 10.6 V vs 39 +/- 7.1 V, P < 0.001); phenylephrine (68 +/- 10.5 V vs 38 +/- 6.2 V, P < 0.001); and carbachol (106 +/- 11.5 V vs 30 +/- 6.5 V, P < 0.0001). Nitroprusside and saline failed to alter VFT. Mean (+/ - SE) DFT was decreased with: methacholine (7.7 +/- 0.8 J vs 9.7 +/- 0 .8 J, P < 0.001); phenylephrine (9.8 +/- 0.9 J vs 11.3 +/- 1.0 J, P < 0.05); and carbachol (9.2 +/- 0.7 J vs 12.2 +/- 0.8 J, P < 0.0001), re maining unchanged following nitroprusside and saline infusion. Thus, m odulation of autonomic tone modified arrhythmia susceptibility and the energy necessary for defibrillation, increased parasympathetic tone, increased VFT, and decreased DFT. Evaluation of autonomic balance, par ticularly parasympathetic tone, may be useful with She implantation of automatic defibrillators.