Left Ventricle.—This chamber is much more frequently affected than any other, and may be involved alone or as part of a general enlargement of the organ. The more important causes are as follows:
(1) Aortic Stenosis.—To send the normal charge of blood through a narrowed orifice the muscle must contract with increased force, and to accomplish the work the walls increase in thickness. There may be simple hypertrophy without dilatation of the chamber, but in the later stages this inevitably supervenes.
(2) Aortic Regurgitation.—Curling and foreshortening of the aortic cusps permits of a backward flow into the ventricle during its diastole, with the production of dilatation and increased pressure, to overcome which the walls thicken—eccentric hypertrophy. This is one of the most common causes, and leads to enormous enlargement of the heart.
(3) Mitral Insufficiency.—In extreme grades of mitral stenosis the left ventricle is usually small, but when the curtains are curled and the patent auriculo-ventricular orifice large, there may be very great hypertrophy. Free regurgitation is always accompanied by considerable eccentric hypertrophy, due to the distension of the chamber by the extra quantity of blood forced in at each auricular systole.
(4) Pericardial adhesions, particularly when in addition to union of the layers the parietal membrane is firmly united to the pleura or to the sternum, may cause hypertrophy of the left ventricle alone, but more commonly of the whole heart.
(5) Abnormal Conditions of the Aorta.—(a) Atheroma, with or without dilatation of the arch, is a cause of hypertrophy, for the heart has to compensate for the loss of arterial elasticity, an important factor in the onward movement of the blood during the diastole; and, again, there is increased resistance in the wider tube. (b) Great narrowing, as in the congenital coarctation just beyond the ductus arteriosus, which may produce colossal hypertrophy. Pressure upon the large vessels in the thorax by tumors may act in the same way. (c) Aneurism of the aorta is not often accompanied by hypertrophy unless the valves are affected. Theoretically, it might be expected, as a large saccular dilatation would certainly appear to be a cause of increased resistance, but in uncomplicated cases the experience of most observers appears to accord with that of Stokes,42 who states that we usually find a small heart. Occasionally, however, there is marked hypertrophy even without valvular disease.
42 Loc. cit.
(6) Kidney disease, acute and chronic, is very frequently accompanied with hypertrophy of the left ventricle. Indeed, simple hypertrophy is more often met with in chronic Bright's disease than under any other conditions. Increased blood-pressure in the smaller arteries throughout the body is now very generally acknowledged to be the immediate cause. But how this is brought about is a question not yet satisfactorily determined.
We have to deal with two sets of cases. There is the cardiac hypertrophy accompanying acute or subacute nephritis, particularly the scarlatinal. Here there are no chronic arterial changes, and the increased arterial tension appears to be due to contraction of the smaller arteries under the influence of retained excreta, which may act through the vaso-motor centre, as Ludwig observes, or possibly directly upon the unstriped fibres of the tunica media of the arteries. Bright's original explanation still holds good, I think, when he says that the altered quality of the blood "so affects the minute and capillary circulation as to render greater action necessary to send the blood through the distant subdivisions of the vascular system."43
43 Guy's Hospital Reports, 1836.