CARDIAC CONDUCTION INTERACTIONS OF PROPRANOLOL AND VERAPAMIL WITH HALOTHANE IN PENTOBARBITONE-ANAESTHETIZED DOGS

We have studied the effects of propranolol 0.25 mg kg^–1and verapamil 0.075 mg kg^–1 on cardiac conduction andrefractoriness in 21 dogs anaesthetized with pentobarbitone30 mg kg^–1 using His bundle electrocardiography and programmedstimulation. After baseline studies under pentobarbitone andhalothane (1.3 MAC) anaesthesia, the dogs were allocated randomlyto two groups: group 1 received verapamil followed by propranolol;group 2 received pro-pranolol followed by verapamil; the drugswere given in a continuous infusion over 10 min. The atrial—His(AH) interval, the atrioventricular node effective (AVERP),and functional (AVFRP) refractory periods, were prolonged byverapamil in both groups, but not the His-ventricle (HV) intervalor the ventricular effective refractory period (VERP). AVFRPand VERP were prolonged by propranolol in both groups. Correctedsinus node recovery times were normal after each drug. Heartrate and the rate required to produce Wenckebach were decreasedby each drug. The combination of verapamil and propranolol duringhalothane anaesthesia in dogs has significant cardiac conductioneffects; however, no spontaneous AV block occurred during thestudy.