A QUANTITATIVE STUDY OF THE EFFECT OF DIGITALIS ON THE HEART OF THE CAT

¹ From the Department of Internal Medicine, Washington University Medical School, St. Louis

A series of ten experiments is reported in which the tincture of digitalis was administered intravenously to cats in a series of doses while records of the heart beat were being obtained by electrocardiograms. An analysis of these records shows that a change in the form of the ventricular complex, the inversion of the T wave, is the first constant sign of the digitalis action. This occurs approximately when 25 per cent of the minimal lethal dose has been given.

Prolongation of the conduction time between auricles and ventricles first occurs with doses which vary considerably in different animals. The doses causing this effect average approximately 50 per cent of the minimal lethal dose. Idio-ventricular complexes occur in the records when approximately 70 per cent of the minimal lethal dose has been given, and constant auriculo-ventricular dissociation occurs with about 80 per cent.

The heart rate of the cat is slowed by the intravenous injection of digitalis. This occurs gradually and becomes definitely apparent when about 25 per cent of the minimal lethal dose has been given. Maximum auricular slowing usually occurs with about 70 per cent of the mimimal lethal dose, and the minimal rate is fairly constant, regardless of the initial rate. Auricular acceleration occurs with further administration of the drug, but during the latter part of the experiments the independently beating ventricles show greater acceleration. The drug almost invariably produces finally ventricular fibrillation, cardiac death resulting.

The experiments indicate that the action which digitalis exerts on the cat's heart is altered when the vagi are cut. The inversion of the T wave occurs practically with the same dosage and to the same extent as it does when the vagi are intact, but the auriculo-ventricular conduction and the heart rate are almost unaffected by digitalis intravenously injected to the lethal dose after the vagi have been cut. These findings indicate that the drug exerts its action on the heart of the cat both directly and through its effects on the vagus center.

The direct action on the heart as shown by the change in the T wave of the electrocardiogram is the first definite effect to appear when a constant percentage of the lethal dose of the drug is injected at intervals intravenously.

The moderate slowing of the heart rate which followed the injection of digitalis in the vagotomized cats may be an indication of the stimulation of the peripheral end of the vagi.

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