Abstract

Both conduction time (CT) and effective refractory period (ERP), absolute and relative to action potential duration (APD), are major determinants of re-entry arrhythmia circuits. We compared the effects of 3 commonly used class I antiarrhythmic agents, lidocaine, mexiletine and quinidine, and of the combination of the latter 2, on APD, ERP, ERP/APD ratio and interventricular CT in 26 in vivo canine hearts. To assess also the frequency dependence of these effects, each measurement was made at multiple steady-state cycle lengths ranging from 600 to 250 msec. A modified contact electrode technique was used to measure both APD and ERP simultaneously and at the same left ventricular site. Interventricular CT was measured as the interval from the stimulus of right ventricular paced beats to the upstroke of the ensuing left ventricular action potential. Lidocaine did not change APD, ERP and ERP/APD ratio significantly at any basic cycle length. In contrast, both mexiletine and quinidine increased the ERP/APD ratio, with progressively greater effects toward shorter cycle lengths. The quinidine/mexiletine combination increased the ERP/APD ratio significantly more than either drug alone (cycle length 350 msec: 8.3 +/- 2.2% quinidine; 17.6 +/- 7.0% mexiletine; 35.3 +/- 9.6% mexiletine/quinidine combination, P less than .01 vs. quinidine, P less than .05 vs. mexiletine). CT increased only with quinidine but not with mexiletine or lidocaine. Combination of mexiletine and quinidine caused no further slowing of conduction as compared to quinidine alone. Thus, although both ERP/APD ratio and CT are related to sodium channel conductance, drug effect on one parameter does not necessarily imply quantitatively similar effects on the other.(ABSTRACT TRUNCATED AT 250 WORDS)