The effect of bretylium and clofilium on dispersion of refractoriness and vulnerability to ventricular fibrillation in the ischemic feline heart

Abstract

Bretylium has been shown to have a pronounced antifibrillatory effect. The purpose of this study was to examine the effects of bretylium on changes in vulnerability to ventricular fibrillation (VF) and refractoriness which occur during acute myocardial infarction. Right ventricular VF thresholds and effective refractory periods (ERP) at six left ventricular sites were measured before and serially after left anterior descending coronary occlusion in chloralose-anesthetized cats. In eight untreated animals, there was a decrease in VF thresholds of 73% ( p < 0.01) immediately after occlusion and dispersion of refractoriness (DR) (maximum difference in ERP between normal and ischemic left ventricular sites) increased from 18 ± 4 to 50 ± 6 msec ( p < 0.01). Five of eight animals manifested spontaneous VF within the first minutes of occlusion but none had nonsustained VF. Pretreatment with bretylium (10 to 20 mg/kg intravenously) increased resting ERP from 181 ± 9 to 201 ± 9 msec ( p < 0.05) and VF threshold from 32 ± 5 to 85 ± 7 mA ( p < 0.001). Bretylium also prevented spontaneous VF in all eight animals and abolished occlusion-related changes in VF and DR. Fourteen animals were similarly studied using clofilium, a bretylium congener which is devoid of sympatholytic effect (no effect on blood pressure response to bilateral carotid artery occlusion). Clofilium increased resting ERP and VF thresholds at both low (0.5 mg/kg intravenously) and high doses (5 mg/kg intravenously). High- but not low-dose clofilium blunted the fall in VF threshold after coronary occlusion. In addition, DR correlated with VF threshold changes at both doses. Neither regimen prevented spontaneous VF but nonsustained VF (lasting 1 to 15 seconds) was consistently observed during testing at subthreshold currents in both bretylium and clofilium-treated animals. Class III drugs appear to prevent VF during coronary occlusion not simply by prolonging baseline refractory periods but by attenuating DR between normal and ischemic zones. This latter measurement may therefore serve as an indirect marker of vulnerability to VF. These effects are partially independent of, but may be enhanced by, a sympatholytic action.