1. Two different types of effects are seen in the mammalian heart under the action of the digitalis series.
A. In the normal heart digitalis causes slowing through stimulation of the inhibitory centre in the medulla, and if this is eliminated, the dominant feature is an increase in the excitability which leads to the inception of independent rhythm in the auricle and ventricle, and later to discharges from different points of the same chamber, which culminate in fibrillation.
B. In the excised mammalian heart exhausted by prolonged perfusion with Ringer's solution, digitalis causes slowing and block which closely simulates that induced by inhibition in the normal heart, but which is not prevented by atropine and therefore arises from direct action on the heart. The pace-maker emits fewer impulses and these are still further reduced owing to impaired conduction to the auricle from the Keith-Flack node and to the ventricle from the auricle. The auriculo-ventricular block becomes further developed, the ventricular beats become slower and finally the chamber ceases in more or less complete systole without fibrillation.
2. In auricular fibrillation in man, digitalis slows the pulse and this is independent of the inhibitory mechanism, for it is not prevented by atropine. In the mammalian heart in situ when auricular fibrillation is induced, strophanthin finally arrests the irregularity of the ventricle, by impairing conduction, but the pulse is not slowed owing to the accelerated idioventricular rhythm induced by the drug. In auricular fibrillation induced in the perfused heart, strophanthin renders the ventricle regular through impaired auriculo-ventricular conduction and at the same time slows its rate.
3. The type A, inhibitory slowing, occurs in man and animals with normal cardiac rhythm. The type B, slowing through direct cardiac action, occurs in most cases of auricular fibrillation in man, in rare cases of normal rhythm in man, in the perfused mammalian heart, whether the normal rhythm is preserved or the auricle is fibrillating, and in the frog.
4. The primary cause of the type B in man and animals is the malnutrition of the heart; the auricular fibrillation merely favours its appearance by accentuating the fundamental cardiac malnutrition.