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The role of electrophysiologic parameters on the induction of ventricular fibrillation (VF) by continuous 50 cycle per second (cps) electrical stimulation was studied in 21 open chest dogs. The current strength of the 50 cps stimulation required to induce VF when applied to the ventricle for 2 seconds was defined as the ventricular fibrillation threshold (VFT). The intravenous injection of antiarrhythmic drugs raised the VFT in a dose dependent manner. The changes in VFT were associated with a rise in excitation threshold. The slopes of the regression equations relating the excitation threshold to VFT were almost identical, that is, 2.8 with lidocaine, 3.4 with procainamide and 3.2 with disopyramide. Prolongation of refractory period increased the cycle length of ventricular excitations just prior to VF but was not correlated with the changes in VF. Localized myocardial ischemia induced by coronary ligation also resulted in the elevation of VFT. The slope of the regression equation between excitation threshold and VFT was 1.9 which was slightly lower than that observed at the administration of the drugs. The fact that the VFT was mainly attributed to the changes in excitation threshold at the site where the test stimulus was applied would limit the usefulness of the 50 cps continuous stimulation method for the evaluation of vulnerability to VF.

Ventricular fibrillation threshold measured by continuous 50 cps stimulation for the evaluation of the antifibrillatory effect of the drugs.