β-Adrenergic modulation of direct defibrillation energy in anesthetized dog heart

Catecholamines facilitate ventricular defibrillation in animals. We examined the effects of β-adrenergic stimulation and blockade on ventricular defibrillation threshold in anesthetized dogs. Calibrated shocks were delivered between epicardial and superior vena caval electrodes, and defibrillation threshold was measured before and after administration of isoproterenol and propranolol. Eight dogs (group 1) received isoproterenol before propranolol. Nine dogs (group 2) received propranolol before isoproterenol. In group 1, the minimum energy required to defibrillate before isoproterenol was 10.6 ± 1.7 (SE) J and decreased to 5.9 ± 1.3 with isoproterenol (P < 0.001). In group 2, the minimum energy required to defibrillate was 8.3 ± 2.4 J before propranolol and increased to 10.7 ± 2.2 after propranolol (P < 0.001). In group 1, propranolol after isoproterenol increased defibrillation threshold (P < 0.07), whereas in group 2 isoproterenol after propranolol produced no significant change in defibrillation threshold. Thus β-stimulation decreased defibrillation threshold significantly in the anesthetized dog heart, an effect that was blocked by propranolol. Conversely, propranolol increased defibrillation threshold, an effect that occurred despite prior β-stimulation, probably because of the short half-life of isoproterenol.