Effects of ranitidine and cimetidine on ventricular fibrillation threshold and dispersion of refractory period in early myocardial ischemia

The effects of two H2-receptor antagonists, ranitidine (Ran) and cimetidine (Cim), on the ventricular fibrillation threshold (VFT) and dispersion of the refractory period in animals with early acute myocardial ischemia induced by coronary artery ligation (CAL) were studied. The measurement of VFT was obtained in anaesthetized rats. The control group showed a decrease in VFT from 6.7 +/- 0.4 to 3.1 +/- 0.7 V 5 min after CAL, while Ran (15 mg/kg iv) and Cim (40 mg/kg iv) increased VFT from 2.8 +/- 0.7 to 5.4 +/- 1.7 V and from 3.1 +/- 0.8 to 8.1 +/- 2.7 V, respectively. By means of suction electrodes, FRP in different ischemic zones were recorded in anaesthetized rabbits, and differences between FRP in different zones were taken as the dispersion of FRP. After CAL, the control group exhibited a prolongation of FRP in central ischemic zone and a shortening of FRP in boundary zone, i.e., a dispersion of FRP was increased. Both Ran (5 mg/kg iv) and Cim (25 mg/kg iv) markedly decreased the extent of dispersion of FRP. It is concluded that the anti-arrhythmic effects of Ran and Cim may be attributable to increases in VFT and decreases in dispersion of FRP.