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Isolated canine papillary muscles with attached false tendons were perfused with Tyrode's solution containing lidocaine. Transmembrane action potentials of ventricular muscle fibers and Purkinje fibers were recorded with glass microelectrodes. Repolarization of Purkinje fibers was accelerated by lidocaine but that of ventricular muscle fibers was unaffected. The maximum rate of rise of the action potential of Purkinje fibers decreased only at 50.0 mg/liter lidocaine; that of ventricular muscle fibers was unchanged by any concentration. The curve relating rate of rise of premature responses to level of membrane potential for Purkinje fibers was unchanged with 5.0 mg/liter but shifted down and to the right with 10.0 and 50.0 mg/liter. The effective refractory period of Purkinje fibers shortened at 5.0 and lengthened at 50.0 mg/liter. The level of membrane potential needed to elicit premature propagated responses in Purkinje fibers with a standardized test stimulus increased and the earliest responses obtained in the presence of lidocaine were relatively large. Duration of the effective refractory period of ventricular muscle fibers was unchanged with 5.0 mg/liter but lengthened progressively with 10.0 and 50.0 mg/liter. Lidocaine impaired the ability of both fibers to respond to rapid frequencies of stimulation, slowed the rate of inherent diastolic depolarization of driven Purkinje fibers, decreased the rate of discharge of spontaneously beating preparations, and minimized the increase in rate and magnitude of diastolic depolarization caused normally by epinephrine. The possible role of these effects of lidocaine in regard to its antiarrhythmic actions was discussed.

Electrophysiological Actions of Lidocaine on Canine Ventricular Muscle and Purkinje Fibers