This section is from the book "Materia Medica: Pharmacology: Therapeutics Prescription Writing For Students and Practitioners", by Walter A. Bastedo. Also available from Amazon: Materia Medica: Pharmacology: Therapeutics: Prescription Writing for Students and Practitioners.
This is believed to be the normal controller or pacemaker of the rate of the heart. From it impulses are given to the auricles at more or less regular intervals of time, and normally at the rate of about 72 in a minute. In response to these impulses the auricles contract together and are followed in about one-fifth of a second by contraction of the ventricles together. A rhythm essentially under the control of sinus impulses is known as "normal rhythm."
One effect of the administration of digitalis is to inhibit or retard the projection of impulses by the sinus node, the result being slowing in the rate of the whole heart. There may be a sino-auricular heart-block. The same type of slowing may be produced by stimulation of a vagus nerve as is observed in man by pressure on the vagus nerve in the neck, or in animals by electric stimulation of the peripheral segment of a cut vagus. Whether the slowing results from electric or mechanical vagus stimulation or from digitalis, it is abolished by atropine, which paralyzes the vagus nerve-endings in the heart. Thus we have evidence that digitalis slowing may be identical with that from vagus stimulation. (See "effect on auriculoventricular bundle.")
Again, in an animal with vagus nerves cut, or in an isolated heart, i.e., a heart severed from all its connection with the centers, the digitalis slowing is very slight. This is evidence that the essential slowing from digitalis does not come from action on the sinus node directly, but from action on the vagus centers. In other words, the effects are vagus effects, and they are not to any great extent produced when the heart is severed from connection with the vagus centers. Therefore we have the evidence that, in a heart with normal rhythm, digitalis may slow the rate by stimulating the vagus centers. There is probably also a slight stimulating effect on the ends of the vagus nerves, but this is not important.
In therapeutics this type of slowing is not usually obtained; but it may be a toxic manifestation if the slowing becomes so marked that the heart does not beat frequently enough to maintain an efficient circulation. As a matter of fact, except in auricular fibrillation or auricular flutter, any marked degree of slowing is not a usual effect from the therapeutic use of digitalis, hence absence of slowing must not be taken as an indication of the drug's inefficiency.
Fever and old age have been said to counteract the vagus effects of the drug, but in 105 pneumonia cases Cohn and Jamie-son found that digitalis acted with full effect even though the fever was high. Jamieson proved the same in other acute infections in cats. The author has seen pronounced effects in many old people.
In some cases it is possible that digitalis causes complete physiologic standstill of both auricle and ventricle for a moment, as is seen upon electric simulation of a vagus nerve, but this has not been reported as a digitalis effect in man.