Repetitive extrasystole as an index of vulnerability to ventricular fibrillation

The assessment of ventricular vulnerability by inducing ventricular fibrillation (VF) presents limitations when neural activity is being investigated, especially in the unanesthetized animal. As repetitive extrasystoles (RE) have been observed to precede the occurrence of VF, it was relevant to determine whether the RE threshold provides a reliable index of cardiac susceptivility to fibrillation. The RE and VF threshold relationships were studied in 32 chloralose-anesthetized dogs during left stellate ganglion stimulation, vagus nerve stimulation, and beta-adrenergic blockage with practolol. The vulnerable period was scanned at 1-ms intervals and at 2-mA increments with a single, 2-ms, constant-current cathodal stimulus; RE and multiple RE were induced reproducibly when 66% and 82%, respectively, of the fibrillatory current was administered. The nadirs for RE and multiple RE were coincident in the cardiac cycle with the vulnerable-period threshold for VF. Stellate and vagal stimulation and beta-adrenergic blockade resulted in comparable changes in RE and VF thresholds and produced equivalent shifts in the cardiac cycle of the RE and VF vulnerable-period nadirs. These observations suggest that RE and VF phenomena share a common electrophysiologic basis and that the RE threshold can be used as an end point for measuring ventricular vulnerability to VF.