The diastole of the heart is partly an active, partly a passive act. The cavities behave as would rubber balls, and their distension after contraction is partly due to their elasticity. The heart is a suction- as well as a force-pump. In the ventricles, for example, after systole the active dilatation draws blood from the auricle—must do so, in fact, in the very process of dilating—and then the auricular systole completes the process, fully accomplishing the diastole. Dilatation occurs during this period, and results from distension beyond the limits of the contractile power of the wall. More blood is contained in the cavity than the muscle of the wall can control—i.e. expel—but if the organ is healthy, hypertrophy ensues and the chamber accommodates itself to the altered condition. It is the heightened pressure during diastole which is dangerous; during systole the pressure may be extreme, and yet no dilatation may ensue, as in aortic stenosis, in which condition the size of the chamber may remain normal, and yet the walls hypertrophy to meet the greatly-increased resistance to the outflow of the blood during the systole. In the auricles, however, the increased tension during contraction may be accompanied with considerable dilatation, as in mitral stenosis.
ETIOLOGY.—There are two important causes in the production of dilatation: increased pressure within the cavities, and impaired resistance due to disease of the muscular substance of the heart. They may act singly, but are often combined. Weakened walls may yield under normal distending force, or normal walls may yield under a heightened blood-pressure, or both factors may prevail.
1. Increased endocardiac pressure—which results, as before stated, either from an augmented quantity of blood to be moved or an obstacle to be overcome—is the most frequent cause of dilatation. It does not necessarily cause it. Simple hypertrophy may be the result, as in the early period of aortic stenosis and in the hypertrophy of the left ventricle in Bright's disease.
Most of the important causes of increased endocardiac pressure have already been considered under Hypertrophy, but we may refer to one or two more particularly.
The size of the cardiac chambers is variable in conditions of health. With slow action of the heart the dilatation during diastole must be much more full and complete than with rapid action. Physiologically, the limits of dilatation have been reached when the chamber cannot be emptied during the systole. We find this as an acute, transient condition in severe exertion—during, for example, the ascent of a steep mountain. There may be great distension of the right heart, as shown by the increased epigastric pulsation, and even increase in the cardiac dulness. The safety-valve action of the tricuspid valves may here come into play, and by permitting regurgitation into the auricle relieve the lungs. Rest causes it to pass off, but if it has been extreme, the heart may suffer a strain from which it may recover slowly, or, indeed, the person may never again be able to undertake severe exertion. In the process of training the getting wind, as it is called, is largely a gradual increase in the capability of the heart, particularly the right chambers. A degree of exertion can be safely maintained in full training which would be quite impossible under other circumstances, because by a gradual process of what we may call physical education the heart has strengthened its reserve force—widened enormously its limits of physiological work. Endurance in prolonged contests is measured by the capabilities of the heart, and its essence consists in being able to meet the continuous tendency to overstep the limit of dilatation.
We have no definite information as to the nature of the change in the heart which occurs in the process of training, but it must be in the direction of increased vigor, muscular and nervous. The large hearts often noted in athletes may be due, as already mentioned, to the prolonged use of their muscles; but probably no one can become a great runner or oarsman who has not naturally a large and capable heart. Master McGrath, the celebrated greyhound, and Eclipse, the race-horse, both famous for endurance rather than speed, had very large hearts.
Over-training and heart-strain are closely connected with this question of excessive dilatation during severe muscular effort. Both mean the same thing in many cases. A man, perhaps not in very good condition, calls upon his heart for much extra work during a race or the ascent of a very steep mountain, and is seized with cardiac pain and a feeling of distension in the epigastrium, and the rapid breathing continues an unusual time, but the symptoms pass off after a night's quiet. An attempt to repeat the exercise is followed by another attack, or indeed an attack of cardiac dyspnoea may come on while he is at rest.62 For months such a man may be unfitted for severe exertion, or may be permanently incapacitated. He has overstrained his heart and has become broken-winded. We see the same thing sometimes in horses. What exactly has taken place in these hearts we cannot say, but their reserve force is lost, and with it the power of meeting the demands exacted in maintaining the circulation during severe exertion.63 The heart-shock of Latham64 includes cases of this nature—sudden cardiac breakdown during exertion and not due to rupture of a valve. It seems probable that some cases of sudden death in men and animals during long-continued violent efforts, as in a race, are due to over-distension and paralysis of the heart.
62 In St. George's Hospital Reports, 1872, Clifford Albutt gives his own experience.
63 H. C. Wood tells me he believes that wind in athletes is in large part a question of vagus control, and that he has noticed in races of dogs used in hunting and other violent exercise the vagi are more sensitive and powerful than in sedentary breeds. He thinks that a similar difference exists between tame and wild rabbits.
64 Diseases of the Heart, New Sydenham Soc. ed.