The effect of digitalis upon the refractory period of the ventricular muscle.

Abstract

By a method similar to that used by Lewis and Drury we have tested the effect of digitalis upon the refractory period of the ventricular muscle of the dog. Lewis and Drury found that strophanthin increased the refractory period of the auricular muscle. We were somewhat surprised when we were unable to demonstrate that digitalis had a similar effect upon the refractory period of the ventricle, the more so, because digitalis has been shown by Cohn and Jameson to produce striking changes in the T-wave of the electrocardiogram.

The drug was given intravenously. After control tests of the refractory period were made, one and five-tenths to two cubic centimeters of the tincture diluted ten times with physiological saline solution were given. Ten to fifteen minutes later the refractory period was redetermined. A second dose was then given and this procedure was repeated until the experiment was ended by ventricular fibrillation, which usually occurred upon stimulation after three to four doses. In no experiment did a conspicuous increase in the refractory period occur.

Since the length of the refractory period is greatly influenced by the heart rate, it was, of course, necessary while the tests were being made to maintain artificially a constant rate considerably above the natural level. Between tests the heart was allowed to beat naturally. It was found that when the heart rate was suddenly raised, the refractory period did not at once drop to the level that it afterwards reached and maintained. In our first experiments the influence of the previous rate of beating upon the determination made immediately after a change in heart rate, was not realized. Confusing variations in the refractory period were, therefore, encountered.