Norepinephrine release by ventricular stimulation: effect on fibrillation thresholds.

In adult dogs anesthetized with pentobarbital sodium, ventricular fibrillation thresholds determined by scanning the vulnerable period with a train of 14 pulses (4 ms, 100 Hz) were significantly less (P less than 0.01) than fibrillation thresholds determined by scanning the vulnerable period with a single 10-ms stimulus. Following beta-adrenergic blockade with propranolol, the relative increase in the fibrillation thresholds measured with trains was about seven times greater than the relative increase in the single-stimulus threshold (P less than 0.01). In a group of animals subjected to chronic cardiac denervation, fibrillation thresholds measured with trains were about three times greater than in a group of shamthoracotomized animals (P less than 0.01). Furthermore, the train fibrillation thresholds of the catecholamine-depleted hearts were not significantly different from the train thresholds of beta-blocked innervated hearts. These data, coupled with the finding that trains of stimuli produced a marked augmentation in local myocardial contractile force, suggest that locally released norepinephrine during a fibrillation-threshold determination may exert a profound influence on the level of current necessary to evoke fibrillation.