We studied the effect of six class I antiarrhythmic drugs, i.e., quinidine (5 pg/mI), disopyramide (1 0 g/ml), procainamide (30 zg/ml), flecainide (4,g/ml), Iidocaine (4,g/mI) and mexiletine (4,g/ml), on the durations of the basic action potential (APD) at a cycle length of 500 ms and on the premature APD (APD) elicited at progressively increasing diastolic intervals (Dl) in canine Pur-kinje fibers. The difference between APD1 elicited at diastolic intervals of 100 msec and the earliest APDI elicited at the onset of effective refractory period was defined as the range of APD. In control this range was 98 ± 1.8 ms (n = 59). Disopyramide and procainamide did not change the range significantly but the other four drugs decreased it significantly (P <.01) as follows: quinidine by 50.2%, lidocaine by 60.2%, mexiletine by 61.6% and flecainide by 61.4%. The following four factors contributed to this decrease in range of APD: shorter duration of APDb
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