Effect of posterior hypothalamic stimulation on ventricular fibrillation threshold

The effect of posterior hypothalamic stimulation on cardiac susceptibility to ventricular fibrillation (VF) was studied in 34 dogs. VF threshold was determined by inducing a sequence of early extrasystoles (R/T pulsing). Hypothalamic stimulation was associated with sinus tachycardia, systemic hypertension, and a 40% reduction in VF threshold. The effects of hypothalamic stimulation on the VF threshold persisted when heart rate acceleration and the pressor response were prevented. Cervical vagotomy and bilateral adrenalectomy were likewise without effect on fibrillation threshold changes. However, the decrease in threshold was abolished by beta-adrenergic blockade. It is concluded that the reduction in VF threshold associated with hypothalamic stimulation derives from the direct action of sympathetic nerves upon the myocardium, rather than from secondary hemodynamic effects.