Mechanism of Antiarrhythmic Efficacy of Propafenone as Assessed by Programmed Electrical Stimulation in Conscious Dogs

Programmed electrical stimulation (PES) was performed in 18 conscious, chronically instrumented mongrel dogs 6-21 days after a 4-h occlusion of the left anterior descending coronary artery (LAD). At baseline, 8 of 10 animals responded with sustained ventricular tachycardia (SVT) and 2 of 10 responded with ventricular fibrillation (VF). Cumulative administration of 2 and 4 mg/kg propafenone intravenously (i.v.) prevented induction of the baseline arrhythmia in 7 of 10 (p less than 0.05) and 5 of 10 (p = 0.1) animals, respectively. Cumulative administration of two doses of saline to 8 control animals with SVT inducible at baseline did not affect subsequent inducibility. QRS duration was only slightly prolonged after 2 mg/kg propafenone (+3.5 +/- 1.1 ms, p less than 0.05). Administration of 4 mg/kg was associated with a further increase in QRS duration (+8.0 +/- 2.3 ms, p less than 0.01) and a decrease in sinus cycle length (-60.0 +/- 17.2 ms, p less than 0.05). Propafenone consistently and significantly prolonged ventricular refractoriness only in responders. Furthermore, at both dosages, there was a negative correlation between drug-induced increases in ventricular refractoriness and baseline refractoriness (r = -0.72, p = 0.02; r = -0.81, p = 0.005 with 2 mg/kg and 4 mg/kg propafenone, respectively). Thus, the lesser antiarrhythmic efficacy of 4 mg/kg as compared with 2 mg/kg may result from excessive increases in intraventricular conduction time and/or unfavorable hemodynamic effects of this dose. Furthermore, our study confirms an association of the antiarrhythmic efficacy of propafenone with increases in ventricular refractoriness. In addition, the present investigation demonstrated that such increases in refractoriness are most likely to occur at short baseline values.