Cardiac arrhythmias may occur in as many as 62 per cent of all patients undergoing anesthesia. They interfere with the mechanical performance of the heart. Until recently, it was generally held that anesthetic-induced changes in pacemaker function (automaticity) were, in most cases, responsible for these arrhythmias. With the recent application of electrophysiological techniques to the study of A-V conduction disorders (His bundle recording techniques), the role of altered A-V conduction in cardiac arrhythmogenesis has become more fully appreciated. Information gained from these studies has been effectively applied by clinical cardiologists to the management of cardiac arrhythmias and conduction disturbances. Comparable data for anesthetic effects on A-V conduction, however, are not available. More importantly, the question of whether anesthetic-induced changes in A-V conduction are responsible for arrhythmias remains to be answered. Utilizing his bundle recording techniques, we propose to address this question on studies of anesthetic effects on A-V conduction. Halothane, enflurane and cyclopropane effects on A-V conduction parameters will be evaluated at comparable and clinically relevant levels of anesthesia in dogs. These agents are known to differ (in humans) in their ability to cause arrhythmias related to myocardial-catecholamine sensitization. Differences in effects on A-V conduction among agents will be related to: 1) Ventricular arrhythmias provoked by epinephrine infusions, and 2) supraventricular arrhythmias produced by critically timed atrial extrasystoles. Because arrhythmia mechanisms in humans and dogs are believed to be similar, our observations will enable anesthesiologists to manage more effectively arrhythmias and conduction disturbances in anesthetized patients.