Electrophysiological, rate dependent, and autonomic effects of the class III antiarrhythmic almokalant after myocardial infarction in the pig.

Ventricular arrhythmias remain a major problem, in particular in patients with left ventricular dysfunction or heart failure. In this group of patients, Class I drugs were shown to be ineffective, and they even increased mortality during chronic treatment. New antiarrhythmic agents should preferably not only have pure antiarrhythmic effects, but should also be free from adverse autonomic properties. In the present study, the electrophysiological, rate dependent and autonomic effects of intravenously administered almokalant, a new Class III antiarrhythmic drug, were investigated in nine pigs surviving a myocardial infarction. The ventricular effective refractory period (VERP) increased after almokalant (loading dose: 0.05 mumol.kg-1.min-1, continuous infusion: 0.0025 mumol.kg-1.min-1) from 292 +/- 25 to 308 +/- 13 ms (pacing cycle length [PCL] 500 ms + 1 extrasystole [ES]), from 249 +/- 19 to 261 +/- 16 ms (PCL 400 ms +1ES), and from 209 +/- 18 to 219 +/- 18 ms (PCL 300 ms +1ES). The VERPs increased most after three ES at PCL 400 ms: from 167 +/- 27 to 186 +/- 29 ms (P < 0.05) and at PCL 300 ms: from 150 +/- 29 to 174 +/- 27 ms (P < 0.05). The ventricular monophasic action potential durations (MAPD) were similarly prolonged and the ratio VERP/MAPD did not change. Prolongation of MAPD after almokalant remained present at short pacing cycle lengths. Before almokalant infusion, sustained monomorphic ventricular tachycardia (VT) was inducible in two pigs, and nonsustained VT in a third animal. After almokalant, only one pig remained inducible. Two weeks after myocardial infarction, heart rate variability and baroreflex sensitivity were reduced. Furthermore, subsequent electrophysiological testing transiently reduced these parameters of autonomic activity. During almokalant however, no changes in autonomic functions were observed after programmed stimulation. Heart rate variability decreased after myocardial infarction from 6.3 +/- 2.5 ms to 5.4 +/- 4.2 ms (P = NS). After programmed stimulation, it further decreased to 2.8 +/- 2.0 ms (P = 0.028). Almokalant infusion prevented autonomic deterioration: 3.3 +/- 2.2 ms before stimulation and 3.3 +/- 1.3 after stimulation (P = NS). In postinfarct pigs, almokalant prolongs VERP and MAPD at shorter pacing cycle lengths. The results indicate absence of reverse rate dependence and of adverse autonomic changes.