Defibrillating shocks delivered to the heart impair efferent sympathetic responsiveness

Functional studies indicate that sympathetic efferents are located in the superficial subepicardium and vagal efferents are located in the subendocardium. It is possible that electrical shocks applied directly to the heart might affect the function of these autonomic nerves.

Low- (< or = 1 J), medium- (6 to 16 J), or high- (30 to 35 J) energy truncated monophasic exponential shocks, synchronized to the R wave during sinus rhythm, were delivered over implantable patches sutured inside the pericardium in anesthetized open-chest dogs. Shortening of ventricular effective refractory period (ERP), produced by bilateral ansae subclaviae stimulation (SS), was measured before and after shock delivery. High-energy shocks shifted the SS frequency-ERP response curves downward and to the right (P < .001) for sites beneath and apical to the patches; ERP shortening at basal sites remained unchanged. Such sympathetic attenuation occurred with shocks > 10 J but not with shocks < or = 10 J, was noted 15 minutes after the shock, and showed incomplete return to control values at 3 hours. Neither low- nor high-energy shocks affected norepinephrine dose-ERP response curves, indicating normal myocardial responsiveness. Low- and high-energy shocks did not attenuate bilateral cervical vagal stimulation-induced ERP prolongation. High-energy shocks delivered over patches sutured to the outside of the pericardium showed no effects on sympathetic response, suggesting a protective effect of the pericardium against shock-induced sympathetic attenuation.

DC shocks > 10 J delivered directly to the epicardium attenuated efferent sympathetic neural function. Such changes may affect electrophysiological, as well as hemodynamic, responses to sympathetic neural stimulation after cardioversion-defibrillation.