Ventricular fibrillation threshold in canine hearts with quinidine induced prolonged QT interval: effects of atrial pacing, isoproterenol, and lignocaine

Ventricular fibrillation threshold was determined using a train of pulses, 4 ms in duration, delivered at 10 ms intervals, in 20 anaesthetised dogs receiving toxic doses of quinidine sulphate (30 mg X kg-1) injected intravenously over 5 min to prolong the QT interval. The effects of atrial pacing, isoproterenol, and lignocaine on ventricular fibrillation threshold were investigated. Quinidine administration lowered the mean(SD) threshold from 14.7(5.1) to 8.8(3.6) mA (n = 20, p less than 0.001). Atrial pacing shortened the mean(SD) basic cycle length from 652 to 400 ms and the QT interval (378(28) vs 334(19) ms, n = 8, p less than 0.01) but did not alter the mean(SD) threshold (7.0(2.3) vs 7.5(1.5) mA). Isoproterenol infusion to maintain basic cycle length at 421(23) ms shortened the mean(SD) QT interval to 291(14) ms (p less than 0.001) and increased the mean(SD) threshold to 13.6(5.9) mA (n = 8, p less than 0.01). Lignocaine (plasma concentration 1.36 micrograms X ml-1) increased the mean(SD) threshold from 10.9(3.4) to 16.7(6.7) mA (n = 7, p less than 0.02) without affecting basic cycle length and QT interval. Thus atrial pacing shortened the QT interval slightly but did not make the canine heart with a quinidine induced prolonged QT interval less vulnerable to electrically induced ventricular fibrillation, whereas both isoproterenol and lignocaine did, although they affected basic cycle length and QT interval differently.