The effects of psychological stress and vagal stimulation with morphine on vulnerability to ventricular fibrillation (VF) in the conscious dog.

Ventricular vulnerability to fibrillation was assessed in 12 conscious dogs in aversive and nonaversive environments using the repetitive extrasystole (RE) threshold method. In the average environment, RE threshold was 45 per cent lower than in the nonaversive setting and heart rate and blood pressure were significantly elevated. This decrease in RE threshold occurred within 10 minutes of exposing the animals to stress. In contrast, the recovery in RE threshold in the nonaversive setting occurred over a 40 minute period. When morphine sulfate (MS) 0.25 mg./Kg was administered to dogs in the aversive environment, the RE threshold was significantly increased. Cholinergic blockade of vagal efferent activity with atropine (0.2 mg./Kg) annulled partially the effect of MS on RE threshold MS was without effect in the nonaversive environment. It is concluded that MS exerts a significant protective effect on increased ventricular vulnerability associated with psychological stress. This effect is mediated by the vagotonic and sedative actions of morphine.