CHANGE OF A BENEFICIAL EFFECT INTO AN UNTOWARD EFFECT BY ISCHEMIA - 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 vul nerability to ventricular fibrillation under normal conditions and und er myocardial ischaemia conditions. Vulnerability to fibrillation was evaluated by electrical ventricular fibrillation threshold (VFT), meas ured with 100 ms duration diastolic impulses the intensity of which wa s 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 pe riods of ischaemia of increasing duration (30, 60, 90, 120, 150 s), se parated by appropriate intervals for reproducibility of the results. M onophasic action potential (MAP) duration and conduction time were mon itored in the ischaemic area under pacing. Ischaemia was obtained by c omplete occlusion of the left anterior descending coronary artery near its origin. The three drugs were i.v. administered in clinical dose r ange (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 m A), despite 25% prolongation of conduction time. But, none of them was able to impede the increasingly marked fall of VFT caused by ischaemi a: at 30 s, they had already lost any capacity for raising VFT and, be yond this time, they even aggravated its fall which led to spontaneous fibrillation when VFT approached 0 mA. The faster fall of VFT shorten ed time to onset of fibrillation (20/24 fibrillations for the three dr ugs at 150 s as against 12/24 in control period), the ischaemia-induce d reduction of MAP duration (by 20%) being also hastened and slowing o f 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 suppres sed, then inverted by ischaemia.