EFFECTS OF HALOTHANE AND QUINIDINE ON INTRACARDIAC CONDUCTION AND QTC INTERVAL IN PENTOBARBITAL-ANESTHETIZED DOGS

To confirm in vitro data that halothane and quinidine depressed cardiac conduction and prolonged action potential (AP) duration, the electrocardiogram and His bundle electrogram were recorded in dogs during basal pentobarbital anesthesia, after 1% halothane or quinidine (2.38 +/- 0.22 mug/mL serum concentration [mean +/- SEM]), or both. Purkinje fibers from a second dog were superfused with blood from the intact (support) dog, and APs were recorded. In the intact dogs, 1% halothane caused no changes in the electrocardiogram or His bundle electrogram. Quinidine prolonged QRS duration, QT interval, and rate corrected QT (P < 0.05). Ventricular conduction (HS interval) slowed, and atrial effective refractory period increased (P < 0.05). Quinidine combined with halothane widened QRS, QT, and rate-corrected QT, prolonged the HS interval, and increased the vulnerability of the atrioventricular node to conduction block. Three of 20 dogs developed torsades de pointes-type ventricular tachycardia during simultaneous quinidine and halothane administration. In cross-superfused Purkinje fibers, the AP duration to 50% repolarization was shortened, and conduction time was prolonged by 1% halothane (both P < 0.05). Quinidine decreased AP amplitude, prolonged AP duration to 90% repolarization, and slowed conduction (P < 0.05). Quinidine combined with halothane decreased AP amplitude, and prolonged both AP duration to 90% repolarization and conduction (P < 0.05). When 1% halothane and therapeutic concentrations of quinidine are administered in dogs, depressed conduction and an acquired long QT syndrome with malignant ventricular arrhythmias may develop.