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.
Tonicity of muscle is its property of maintaining, during its resting period, a state of partial contraction or incomplete relaxation, i. e., a state of tone, which keeps it in readiness to respond promptly when a stimulus comes. In a hollow organ like the heart the tone gives it resistance to a bursting pressure during the period when the organ is not actively contracting. It is measured by the degree of relaxation in diastole. Contractility is the power to contract against resistance. It is measured by the size of the heart at the end of systole. Tonicity differs from contractility, which has to do with the active contraction, and from irritability, which deals with sensitiveness to stimuli.
In a heart whose contractility and tonicity are below the normal, the ventricular chambers are dilated and weak, so that in diastole the muscle is stretched beyond the normal by the venous inflow, and in systole contracts feebly. The result is a decreased output of blood.
If we take two concentric spheres and let one represent the capacity of the heart during the resting period of diastole, and the other the capacity at the end of systole, we might represent the normal and the weak heart, as in the illustration, the diminished excursion of the muscle in the latter lowering the output. Digitalis, by increasing the tone and contractility, tends to bring the heart muscle back to normal, and so increases the output. Its site of action in producing this effect may be determined by administering a large dose of atropine to a laboratory animal to eliminate vagus effects, and a dose of apocodeine
Weak and dilated to cut off the accelerators. All influences through the nervous system are thus removed, but digitalis still results in striking increase in contractility and tonicity. It must, therefore, stimulate the muscle itself. It gives these effects with decided force in the laboratory, and probably to some extent in therapeutics.
Fig. 7. - D, Capacity at end of diastole; S, capacity at end of systole.
Fig. 8. - Diagram to illustrate "ventricular extrasystole." As, Auricular systoles; Vs, ventricular systoles. At x the ventricle beats spontaneously. This beat is followed by a refractory period, during which the regular auricular impulse is ineffective, and the ventricle does not beat until the next auricular impulse. The auricle beats regularly throughout.
The right ventricle, though its muscular wall is normally much thinner, is stimulated as much proportionally as the left.
The papillary muscles are also strengthened and toned, a matter of special importance in a weak, dilated heart. For these muscles must contract coincidentally with the ventricle, or of tincture of nux vomica three times a day. It ceased within two days of stopping the medicine. (Top line, apex; lower, radial pulse.) they will allow the valves to bulge into the auricle during systole and make a relative insufficiency, i. e., a leakage backward. As a matter of fact, the normal ventricular contraction begins in the papillary muscles.
Fig. 9. - Ventricular extrasystoles developing in a heart with normal rhythm and moderate dilatation. This resulted from 10 minims (0.7 c.c.) of tincture of digitalis and 20 minims (1.3 c.c.)
An effect on the electrocardiograph record regularly obtained after digitalis is attributed by Cohn to a probable action on contractility (see page 178, Fig. 22).
2. Irritability or excitability is the susceptibility to stimuli. Normally, it does not determine the rate of the heart, for the normal pacemaker is the sinus node. But an increase of irritability beyond the normal tends to result in spontaneous muscular contractions that do not have their origin in the sinus node. The effects of these are harmful. They may be produced by digitalis. Excessive irritability may be confined to a small area and yet be the cause of abnormal beats, "normally inactive points in the heart taking on the power of originating stimuli" (Cushny).
Fig. 10. - From same case as Fig. 9. Every fourth beat is premature. Top line, jugular; middle, apex; lower, radial.
Overirritability or overexcitability may show in auricular or ventricular premature beats, in paroxysms of tachycardia, in auricular flutter, in auricular fibrillation, or in ventricular fibrillation. In some excitable hearts there are alternations of premature beats, paroxysmal tachycardia, and auricular fibrillation.
One of the earliest indications of excessive irritability is the so-called extrasystole, a premature or interpolated beat which has its origin elsewhere than at the sinus node. The site of origin may be the auricle, the result being a premature auricular beat, usually followed by a corresponding premature ventricular beat in response to the auricular stimulus. But much more commonly the premature beat has its origin in the ventricle, the ventricle alone giving a premature beat, while the auricular rhythm is not affected. A premature beat may appear at regular intervals or irregularly, and frequently or infrequently. It may follow the normal beats so that the ventricle beats in couples. It may show in the radial pulse or it may not, but it is an irregularity of the heart and not an intermittence. In susceptible hearts it may sometimes accompany or follow holding the breath. It is one of the most commonly observed of the toxic manifestations of digitalis. (b) In auricular fibrillation the auricular muscle is in a state of such excitability that muscle groups here and there contract independently, i. e., the fibers quiver or fibrillate, instead of contracting coordinately to make an auricular beat. The fibrillations occur at the rate of several hundred per minute, and their effect upon the ventricle is to make it beat in a rapid, irregular, and disorderly manner. In a pulse-tracing of this condition unmodified by drugs - (a) No two sections are alike, the radial pulse being irregular and disorderly; (b) the height of the pulse wave-has no definite relation to the length of the preceding pause; and (c) the jugular tracing shows absence of the normal auricular waves, and in some instances numerous small fibrillation waves. Auricular fibrillation may exist without serious symptoms, but it is usually serious, is one of the most frequent causes of lack of compensation, and may be the precursor of ventricular fibrillation and death.
Auricular fibrillation and complete heart-block developing in a case of cirrhosis of liver, with weak heart, but with normal rhythm. Digipuratum, 1 1/2 grains three times a day, was given from April 17th to 20th, when tracing showed auricular fibrillation and complete heart-block, rate 42. The drug was stopped, and two days later tracing 12 showed auricular fibrillation alone, rate about 135. Tracing 13 taken the next day showed return to normal rhythm, rate 100. Similar phenomena followed the administration of digitalis a month later.
Fig. 14. - Extrasystoles and auricular flutter. Case with auricular fibrillation. Digitalis, 1 1/2 grains four times a day for four days, resulted in alternating periods of halving of the pulse-rate due to extrasystoles (ventricle 140, pulse 70), and very rapid, almost regular pulse at the same rate as the ventricle 186, and half the rate of the auricle 372 (auricular flutter).
(c) In paroxysmal tachycardia the heart is regular or nearly so, but very rapid, the rate usually being over 150. The beats may have their origin in the auricle, in the ventricle, or at the auriculoventricular node. If the tachycardial beats originate in the auricle it is known as "auricular flutter." If the beats originate at the auriculoventricular node, there is true nodal rhythm, and the auricle and ventricle receive their stimulus at the same time, and consequently beat simultaneously. If the beats originate in the ventricle, there may be a reversed or retrograde rhythm, the excitable ventricle beating prematurely and imposing its rhythm upon an auricle in a similar state of excitability. The ventricle may pass into a state of fibrillation, which almost invariably means immediate death.
(d) Ventricular fibrillation is the usual terminal effect of digitalis poisoning in mammal experiments (Cushny). It corresponds in mammals with the continuous systole in coldblooded animals. It usually leaves the mammal heart in a state of diastolic relaxation, but Eckler (1912) reports that after death from digitalis, strophanthus, and ouabain, 12 out of 62 mammal hearts were found in systolic contraction.
Figs. 15 and 16. - Complete heart-block. Developing after digipuratum, 1 1/2 grains three times a day for nine days. Fig. 16 shows return to normal rhythm after the digitalis effect had worn off. This block was suspected when a pulse that had been beating between 106 and 116 for several days suddenly changed to a rate between 60 and 70. The auricle was not slowed.