QUINIDINE-INDUCED TACHYCARDIA IN DOGS WITH BUNDLE BRANCH BLOCK

¹ Department of Pharmacology, University of Oregon Medical School; Surgical Research Service, Good Samaritan Hospital, Portland, Oregon

Quinidine injected into a dog with naturally occurring right bundle branch block unexpectedly produced an arrhythmia closely resembling ventricular tachycardia. The same tachycardia also appeared when quinidine was administered to dogs with surgically produced right or left bundle branch block. These arrhythmias proved to be supraventricular in origin when "hidden" P waves were revealed from direct atrial leads. The appearance of ventricular tachycardia was simulated by the broad QRS wave which occurred as a result of complete bundle branch block and quinidine's slowing of myocardial conduction. The striking tachycardia appeared only with small doses of quinidine (within the range used in therapy). It was observed that larger amounts caused a slowing of heart rate both in animals with block and in those with normal intraventricular conduction systems. This tachycardia which followed injections of quinidine during bundle branch block was essentially the same as that produced by epinephrine in normal animals, and quinidine was shown to have adrenergic blocking action. These results in dogs support the clinical warnings against the use of quinidine in patients with intraventricular conduction disturbances.