Effects of encainide (MJ9067) on the ventricular fibrillation threshold in anesthetized dogs.

The right ventricular epicardial ventricular fibrillation threshold (VFT) was determined during paced supraventricular rhythm using 100 Hz trains of stimuli at 15 min intervals in dogs before and during the intravenous administration of encainide, a new antiarrhythmic drug. With each VFT determination, simultaneous blood samples were obtained for determination of drug concentration. In 6 control dogs, VFT determined every 15 min during a 210 administered as a 90 min intravenous infusion at three successive rates (0.01, 0.02 and 0.04 mg/kg/min) for 30 min each. VFT measured at 5 and 20 min of each infusion increased from a mean control of 11.5 +/- 1.5 (+/-SE) to 20.2 +/- 2.2 mA (p less than 0.01) after 20 min of the third infusion. The maximal effect occurred during the second infusion with plasma concentration of 594 +/- 46 ng/ml and then reached a plateau. In group II (n = 6), encainide was administered in four successive sequences, each one including a bolus loading intravenous dose followed by a 45 min intravenous infusion. VFT measured at 30 and 45 min of each infusion when the encainide plasma concentration was close to a steady state increased significantly (p less than 0.01) after the second infusion from 11.8 +/- 2 to 27.3 +/- 4 mA. Two dogs in group II developed transient complete atrioventricular block at an encainide plasma concentration of greater than 800 ng/ml. These results show that the new antiarrhythmic drug encainide increases the VFT in anesthetized dogs.