Effect of Verapamil on Sinoatrial and Atrioventricular Conduction Time in Dogs

The effect of verapamil on sinoatrial and atrioventricural conduction were studied on 11 dogs under the urethane-chloralose anesthesia.The sinoatrial conduction time determinded by using the premature atrial stimulation method and represened as antegrade plus retrograde conduction time was significantly prolonged following the intravenous administration of 0.05mg/kg of verapamil. However, the sinoatrial conduction time was almost unchanged when larger doses of verapamil were administrated. On the other hand, the sinus cycle length and A-H time were prolonged as to the verapamil dose was increased, while H-V time was unchanged. The systolic and diastolic pressures were depressed as to the verapamil dose was increased.The present rather curious observation that the sinoatrial conduction time was not affected by larger dose of verapamil was suspected to be resulted from the prolongation of sinus cycle length and/ or the increased reflex sympathetic tone.