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 incipient or partial heart-block digitalis is contraindicated, for it tends to increase the degree of block. In complete block it has been recommended by Bachmann and others on the ground that it tends to bring the auricular and ventricular rates more nearly together, by slowing the rate of the auricle and increasing that of the ventricle; but in the only one of my cases in which it had any effect (see case report under Auriculoventricular Bundle, p. 173) it brought the auricle and ventricle to the same rate, but in "reversed rhythm," the auricle following the ventricle instead of preceding it; and this was harmful.
In this weakened state digitalis may at times be of some value, but its effects are problematic, and at least in some cases are harmful. Especially is this true of the myocarditis cases with coronary sclerosis.
As this is a peculiar action of the heart, coming on with great suddenness and ceasing just as abruptly, and lasting from a fraction of a minute even to months, it is difficult to say whether any drug given is effective or not. Some cases cease soon after the commencement of digitalis and some do not. Where the beats arise at the sinus node or in the auricle, digitalis might be expected to be of value by retarding conduction, but when the beats arise in the ventricle, it can only be harmful.
In auricular flutter, a condition Characterized by an extremely rapid auricular contraction, rate above 300, usually with ventricle beating at the same rate or half the rate, digitalis may change the flutter to fibrillation, and this seems to act by submerging the original fast rhythm and eventually restoring the rhythm to normal. Even if it does not do this, digitalis will be of value by establishing some degree of block (Thomas Lewis).
Though a few cases have been reported of the disappearance of premature contractions during the administration of digitalis, it is certain that in most cases digitalis has a decided tendency to increase these indications of irritability.
It is in auricular fibrillation, above all other cases, in which there is an almost ideal effect from digitalis; in fact, the results of digitalis are dramatic. Lewis says that "in hospital practice, of those with obvious cardiac failure at least 60 per cent have auricular fibrillation." Large doses should be given at the outset, and if the fibrillation is permanent, should be followed by smaller doses once or twice a week or once a day, for months, or even throughout the life of the patient. The action of the drug is not to overcome the fibrillation, though a slowing in the rate of fibrillation has been noted (Cushny); but, so far as we know, it is to impair the conductivity of the auriculoventricular bundle, i. e., to establish a partial heart-block. The result is that impulses from the auricle get through to the ventricle only at longer intervals, and, as a consequence, the ventricle becomes more nearly regular, is less rapid, and has greatly increased power. The production of complete block, shown by the regularity of the pulse, should be avoided; if it occurs, it is an indication for immediate reduction of the dose.
In a case of auricular fibrillation, if the condition is immediately serious, an intravenous injection of digipuratum, 1 1/2 grains (0.1 gm.), or of strophanthin 1/130 to 1/65 grain (0.0005-0.001 gm.), may be employed. But usually it suffices to give 15-30 minims (1-2 c.c.) of the tincture three or four times a day, or a corresponding amount of the powdered leaves, i. e., 1 1/2-3 grains (0.1-0.2 gm.), or of the infusion, i. e., 1 1/2-3 drams (6-12 c.c.).
It is to be noted that frequently the infusion is given in larger proportional dosage than other preparations. Doses of 1/2 ounce (15 c.c.) are not unusual, and this dose is made from the same amount of digitalis as 36 minims (2.4 c.c.) of the tincture. Yet such a dose of the tincture is seldom employed. This is perhaps the reason why some thoughtlessly consider the infusion the better preparation.
The table on p. 191, giving the effects of digitalis as recorded by Mackenzie in a case of mitral stenosis with auricular fibrillation, is topical. The B. P. tincture was used by Mackenzie. Its equivalent in U. S. P. tincture is expressed in the table.
In cases in which great excitability shows by varying periods of auricular fibrillation, paroxysmal tachycardia, and premature ventricular beats, digitalis is much less certain than in simple auricular fibrillation. For only such beats as have their origin in the auricle, and consequently are affected by depression of conductivity, will be favorably modified by digitalis; while those arising in the ventricle itself may be made worse by the increase of excitability. I have seen several of these cases. In some, digitalis gave good results; in others it did no apparent good or harm.
In the cases in which the heart is beating in normal rhythm and is regular, but rapid and weak, it is quite customary to employ digitalis with the dual purpose of slowing the heart and strengthening its beat. And it is in these cases, in which we desire and might expect so much, that we often meet with disappointment. At times the drug seems utterly lacking in power to check the rate or to add to the strength of the heart, even though, as shown by the development of toxic effects, the digitalis is given beyond the physiologic limit. This may be due either to an affection of the muscle caused by failure of nutrition or the toxins of the disease, or to reflexes of which we do not know the nature.
In this condition the question may arise as to the advisability of employing digitalis. As the doses administered in therapeutics do not have a strong tendency to raise arterial pressure, high pressure is not of itself a contraindication to the employment of the drug. The author has seen a number of cases with tension between 200 and 260, in which the pressure fell during digitalis administration.