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.
In cases with auricular fibrillation already established from disease the combined effects on irritability and conduction are strikingly to be observed after digitalis. The therapeutic effect of the drug in auricular fibrillation is not to overcome the fibrillation, so far as we know, but essentially to impair conductivity. It thus checks the passage of the frequent small and irregular auricular impulses, which in this condition serve only to nag the ventricle and make its action disorderly. In other words, it establishes a degree of heart-block. The effect is partly due to vagus stimulation, and pressure on the vagus in the neck will sometimes momentarily produce a similar result, while atropine will prevent it. It is probably also, in some instances, due to a direct action of the digitalis on the junctional tissues (Cushny). The block may become complete, with regularity of the ventricular beats and a much slowed rate, and this is an undesirable effect.
Fig. 20. - Coupled rhythm developing in a case of auricular fibrillation. This is an exceedingly common effect. It resulted after five days of powdered digitalis, 2 grains three times a day.
Fig. 21. - Phasic arhythmia developing in a case of auricular fibrillation. This followed digalen, 10 minims every four hours, for one day, and digipuratum, 1 1/2 grains three times daily for two days. Upper line shows respiration, which is not synchronous with the phases of quickening and slowing of the pulse.
But more frequently in auricular fibrillation digitalis results in a condition in which, owing to an area of excitability in the muscle, each beat that occurs in response to an auricular stimulus is followed quickly by another beat which originates in the ventricle. Thus the beats appear in pairs or couples, and make "coupled rhythm" In this the distance between the members of a couple is fairly constant, while that between the couples may vary considerably; and the second beat of the pair may or may not be palpable at the wrist. What is probably an early stage of coupled rhythm is an alternation of single beats with coupled beats. A serious stage of it is present when the distance between the couples is short, so that the ventricle beats very rapidly. Coupled rhythm is a common digitalis manifestation in auricular fibrillation, but is sometimes also present with normal rhythm, every second beat being a premature one.
Another digitalis effect in auricular fibrillation is "phasic arhythmia" which corresponds in general character with that arising from the sinus, but, so far as known, has its origin not at the sinus, but in the ventricle. Cohn has discovered that in some cases vagus fibers pass directly to the ventricle, and it may be that phasic arhythmia occurs only in such cases and is a vagus effect.