Comparison of the electropharmacological effects of verapamil and propranolol in the halothane-anesthetized in vivo canine model under monophasic action potential monitoring.

The cardiovascular profile of verapamil was assessed in the halothane-anesthetized canine model and compared with that of propranolol. Verapamil was infused at the rates of 1, 3 and 10 microg x kg(-1) x min(-1) (n=6), whereas propranolol was administered at a fixed rate of 10 microg x kg(-1) x min(-1) (n=6). Each infusion was performed over 30 min, and the parameters were assessed for 20-30 min after the start of each infusion. Verapamil in a dose of 10 microg x kg(-1) x min(-1) significantly suppressed atrio-ventricular (AV) node conduction and slightly decreased the mean blood pressure, but no significant change was detected in the left ventricular end-diastolic pressure, maximum upstroke velocity of the left ventricular pressure, sinus automaticity, double product, cardiac output, intraventricular conduction, and ventricular repolarization phase and refractoriness. Propranolol suppressed AV node conduction to an extent similar to that of verapamil, but it also inhibited intraventricular conduction, sinus automaticity and ventricular contraction, increased the ventricular refractoriness, and decreased the double product and cardiac output, without any significant change in the other variables measured. These results suggest that verapamil can selectively affect the AV node, and that the greater part of the suppressive action of propranolol on the multiple cardiovascular performance is through a beta-blocking action and direct membrane effect, although the halothane inhalation itself might have modified each of the drug's effects. The abbreviation of the relative refractory period of the ventricle by propranolol may show its potential utility for re-entry type ventricular tachycardia.