Change of a beneficial effect into an untoward effect by ischaemia: effect of quinidine-like drugs on vulnerability to ventricular fibrillation

The effects of three quinidine-like drugs, disopyramide, lidocaine and flecainide were investigated in anaesthetized, open-chest pigs on vulnerability to ventricular fibrillation under normal conditions and under myocardial ischaemia conditions. Vulnerability to fibrillation was evaluated by electrical ventricular fibrillation threshold (VFT), measured with 100 ms duration diastolic impulses the intensity of which was increased by steps of 1.0 or 0.5 mA. Impulses were delivered at the rate of 180 beats · min −1. The ventricles were subjected to pacing at the same rate before the VFT determination, particularly throughout periods of ischaemia of increasing duration (30, 60, 90, 120, 150 s), separated by appropriate intervals for reproducibility of the results. Monophasic action potential (MAP) duration and conduction time were monitored in the ischaemic area under pacing. Ischaemia was obtained by complete occlusion of the left anterior descending coronary artery near its origin. The three drugs were i.v. administered in clinical dose range (1.00 mg · kg −1 plus 0.04 mg · kg −1 · min −1). In the absence of ischaemia, they increased almost equally VFT (from about 7 to 10 mA), despite 25% prolongation of conduction time. But, none of them was able to impede the increasingly marked fall of VFT caused by ischaemia: at 30 s, they had already lost any capacity for raising VFT and, beyond this time, they even aggravated its fall which led to spontaneous fibrillation when VFT approached 0 mA. The faster fall of VFT shortened time to onset of fibrillation ( 20 24 fibrillations for the three drugs at 150 s as against 12 24 in control period), the ischaemia-induced reduction of MAP duration (by 20%) being also hastened and slowing of conduction enhanced, given the addition of the depressant effects of ischaemia and drugs on conduction. Consequently, the antifibrillatory properties normally manifested by the studied drugs are first suppressed, then inverted by ischaemia.