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.
The rhythm serves merely to determine the functional condition. The most met with rhythms, with their probable significance as judged by rate, are as follows:
1. Ventricle regular in frequency -
Tinct. Digitalis, U. S. P.
Oz. of Urine.
37 1/2 minims
112 1/2 "
112 1/2 "
112 1/2 "
112 1/2 "
Better; no vomiting
Breathing much easier
(b) Pulse below 55 - heart-block? - avoid digitalis.
(c) Pulse above 140 - paroxysmal tachycardia, auricular flutter - try digitalis.
(d) Pulse alternating weaker and stronger beats - pulsus alternans - try digitalis.
2. Ventricle showing regular waxing and waning of the rate independently of respiration - sinus arhythmia - avoid digitalis.
3. Ventricle showing premature or abortive beats - avoid digitalis.
4. Ventricle beating in couples - avoid digitalis.
5. Ventricle regularly intermittent - partial heart-block - avoid digitalis.
(a) In Simple Muscular Inability Without Valvular Lesion.
Simple dilatation. In this the muscle has lost its tone and become abnormally relaxed, and its contraction is weak; in addition, there may be a systolic leakage through the mitral valves, not due to valvular disease, but to the dilatation of the mitral orifice and the loss of tone of the papillary muscles. Digitalis tends to make the systole stronger and more complete, and, by restoring the tone, prevents the abnormal diastolic relaxation and weakness. At the same time the mitral ring contracts to normal again and the papillary muscles are toned, so that the relative insufficiency of the mitral valves disappears. The result is an efficient circulation. In the moderate dilatation of acute febrile diseases digitalis may be ineffective because of the toxic action of the bacterial products.
Chronic myocarditis and fatty degeneration. In these a portion of the muscle substance is changed and replaced by non-contractile tissue (connective tissue in myocarditis; fat in fatty degeneration), so that the drug has less muscle substance to stimulate by direct action. In some of these cases, too, there is impairment of the coronary circulation by coronary sclerosis; and in some the slowing of the heart takes place without a corresponding increase in ventricular strength, so that the output is actually lessened instead of increased. Because of these things, therefore, digitalis may be contraindicated, or at least must be used with caution.
In acute toxic myocarditis, as in the infectious febrile diseases, digitalis may fail either to slow or to strengthen the heart. In most cases, however, it is effective.
The common valvular defects are those of the left heart, and they either make a valve inefficient so as to permit backward leakage or regurgitation, or cause a narrowing or stenosis of the valvular orifice so as to obstruct the onward passage of the blood. The common valvular lesions which allow regurgitation of blood are mitral insufficiency and aortic insufficiency. The common lesions which cause obstruction to the passage of blood are mitral stenosis and aortic stenosis.
In mitral insufficiency there is a systolic regurgitation of blood from the ventricle into the auricle through the insufficient mitral valve. This leakage is ordinarily compensated for by enlargement of the ventricular cavity and hypertrophy of the heart muscle. When the muscle fails, there is a condition of flabby heart wall and papillary muscles, with relaxed mitral orifice, resembling that in simple dilatation, but with a permanent mitral leak. In this condition digitalis may prove valuable.
In aortic insufficiency there is a diastolic regurgitation from the aorta through the insufficient aortic valves back into the ventricle. In this condition the left ventricle is usually very large and its capacity enormously increased. In the arteriosclerotic type the aorta is impaired, there is usually more or less myocarditis and general arteriosclerosis, and the failure of the sclerosed coronaries to meet the needs of the very large heart is probable. Hence digitalis should be used with caution. In the endocarditic type the dilatation and hypertrophy of the ventricle through the natural compensatory changes are regularly very marked, the heart is enormous, and there is a very great output of blood at each systole. This factor and the prompt leakage are enough to make a great difference between the systolic and diastolic aortic pressures, hence a sudden great distention of the aorta in systole, a matter of importance if there is aortic disease. In such a case the prolongation of diastole by digitalis does not seem to make any serious difference so far as the leakage is concerned (Stewart), and it allows a longer time for the additional coronary blood-supply needed by the greatly hyper-trophied wall of the heart.
The peripheral pressure, however, is not influenced so much by the size of the leak as by reflexes through the depressor nerve which in man runs afferently in the vagus from the heart or from the adjoining portion of the aorta. When the intra-aortic pressure is abnormally high, this nerve carries impulses which result in a reflex dilatation of the peripheral arterioles. So in aortic insufficiency, either because of the very high aortic systolic pressure or the sudden overdilatation of the aorta from the great output at a single beat, depressor impulses are set going; and there is immediately a reflex dilatation of the arterioles, which causes greatly lessened peripheral resistance and low diastolic pressure. Whether or not digitalis, through its effect upon the vasoconstrictor mechanism, may counteract this depressor reflex, which is protective by letting off at the periphery the excessive pressure caused by the great output in systole, is a question. If it does so, it may be harmful.