Cardiac conduction interactions of propranolol and verapamil with halothane in pentobarbitone-anaesthetized dogs

We have studied the effects of propranolol 0.25 mg kg-1 and verapamil 0.075 mg kg-1 on cardiac conduction and refractoriness in 21 dogs anaesthetized with pentobarbitone 30 mg kg-1 using His bundle electrocardiography and programmed stimulation. After baseline studies under pentobarbitone and halothane (1.3 MAC) anaesthesia, the dogs were allocated randomly to two groups: group 1 received verapamil followed by propranolol; group 2 received propranolol followed by verapamil; the drugs were 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 by verapamil in both groups, but not the His-ventricle (HV) interval or the ventricular effective refractory period (VERP). AVFRP and VERP were prolonged by propranolol in both groups. Corrected sinus node recovery times were normal after each drug. Heart rate and the rate required to produce Wenckebach were decreased by each drug. The combination of verapamil and propranolol during halothane anaesthesia in dogs has significant cardiac conduction effects; however, no spontaneous AV block occurred during the study.