We might sum up the theoretically valuable effects of digitalis in a failing circulation as follows:

1. On the heart: (a) Slowing, (b) Increased contractility, (c) Increased tonicity, (d) Improved nutrition, (e) In auricular fibrillation, slowing and steadying of the ventricular rhythm.

2. On the blood - improved oxidation from improved pulmonary blood-flow.

3. In venous accumulation - the removal of edema and dropsy.

Respiratory System

Therapeutic doses have little direct influence on respiration, but they may stimulate the respiratory center through the improvement in the cerebral circulation; or may help the lungs through removal of congestion or edema. Poisonous doses stimulate the respiratory center so that the respiration becomes strong and deep. With the fall in arterial pressure in the late stages of poisoning the respiratory center fails.

Nervous System

The brain may be affected through its increased blood-supply. There is no direct action except upon the centers of the medulla. The chief constantly acting medullary centers are the vagus, the vasoconstrictor, and the respiratory, and in this sequence these are stimulated by the drug. If poisonous doses are administered, these centers are eventually depressed. Other centers sometimes affected by digitalis are the heat-regulating, so that temperature in fever tends to be lowered, the vomiting, and the convulsive, which may be the cause of convulsions in the late stages of poisoning. The nerve-endings which are stimulated are those of the vagus and vasoconstrictor nerves.

Elimination

The active principles are excreted partly by the kidneys and partly by the intestines. Their excretion is slow, so that continued administration of large doses may give rise to cumulative poisoning. And the administration of a full intravenous dose of one of the active principles of the group during or following shortly after a course of digitalis by mouth has, in a number of instances, resulted fatally. This last statement is particularly true of strophanthin, which has been the principle of choice for intravenous use.