The right auricle hypertrophies when there is greatly increased blood-pressure in the lesser circulation, whether due to mitral stenosis or pulmonary lesions, and incompetency at the tricuspid orifice. Stenosis of the auriculo-ventricular orifice is a less frequent cause. The dilatation is always excessive.
MORBID ANATOMY.—In general hypertrophy the entire organ is increased in size and weight; more commonly we find the condition limited to two or three chambers or to one side. The estimation of slight grades of enlargement is difficult, but where the increase is marked the process is simple enough. The volume of the heart varies in different individuals according to their age and size. The normal heart is about the size of the closed fist, and, as Virchow suggests, a fair estimate can be made by comparing the two together. By careful weighing we get much more accurate information. The heart of an average-sized man weighs about 9 oz., of a woman about 8 oz. In great hypertrophy the organ may weigh three or four times the normal amount. A heart which weighs over 12 oz. in a man, and over 10 oz. in a woman, may be considered hypertrophied. Hearts weighing from 16 to 20 oz. are not uncommonly met with. Weights above 25 oz. are rare. The heaviest hearts on record are described by Beverley Robinson of New York,52 53 oz.; Dulles of Philadelphia, 48 oz.; and there are several cases described in the Transactions of the London Pathological Society of the organ weighing as much as 46 oz.
52 New York Medical Record, 1883.
Next to weighing, careful measurement of the thickness of the walls is the best means of determining hypertrophy. When there is great dilatation of a chamber the walls, though actually thick, may look proportionately thin; and on the other hand, when rigor mortis is present the cavity may be very small and the walls appear enormously thick. In this case measurements should not be made until the heart has been soaked in water and thoroughly relaxed. The normal thickness of the left ventricle is about half an inch (12 or 13 millimeters), being thicker toward the base. It is well to measure in two or three places, not including the papillary muscles. A thickness of 10 lines or over (20 to 25 mm.) indicates hypertrophy. It is rare to meet with the wall thicker than 1 inch (25 mm.), even in very great hypertrophy. The right ventricle is thinner than the left, and has an average diameter of from 2 to 3 lines (4 to 7 mm.). A thickness of from 6 to 9 lines (13 to 20 mm.) may be met with in great hypertrophy. It is very rare to see a diameter of more than three-quarters of an inch, but cases are reported of a thickness of over an inch. The left auricle has a normal thickness of about a line and a half (3 mm.), which in considerable hypertrophy may be nearly doubled. The wall of the right auricle is even thinner than the left, rarely exceeding 1 line in diameter. In hypertrophy the sinus does not present a marked increase in thickness, but the appendix, particularly the musculi pectinati, may be greatly developed and measure from 2 to 3 lines in diameter.
The shape of the heart is much affected by the degree of hypertrophy in different cavities. Great enlargement of the ventricles broadens the apex, and the conical shape is lost. In the enormous hypertrophy and dilatation of aortic insufficiency the increased breadth and rotundity of the apex becomes very marked. When the right ventricle is chiefly affected, it occupies a large share of the apex, and the transverse diameter of the organ is increased. When due to mitral stenosis the contrast between the large broad right ventricle extending well to the apex and the small left chamber is very striking.
When not degenerated the muscle-tissue of an hypertrophied heart is of a deep-red color, firm, and usually cuts with slightly increased resistance. The right ventricle often has a peculiarly hard, leathery feel, which was noticed by Rokitansky. In simple hypertrophy of the left ventricle the papillary muscles and columnæ carneæ may be increased in size, but the former often appear flattened in great eccentric enlargement. The trabeculæ are usually much more developed in the right ventricle and in the appendix of the right auricle than in the left chambers. Very often the tissue looks pale, and may be soft from the occurrence of fatty degeneration.
The histological characters of the changes in hypertrophy have been much studied, particularly with a view of determining the question of numerical increase. Hepp53 described an increase in the thickness; but most recent observers regard the hypertrophy as due to numerical increase, resulting from the development of new fibres, either by the splitting of the old ones (Rindfleisch) or their growth from interfibrillar nuclei.54 Wilks and Moxon55 and Gowers56 find that the fibres are not increased in size. Letulle57 thinks that there is a process of progressive hyper-nutrition of the fibres.
53 Henle's Zeitschrift, 1854.
54 Zielonko, Virchow's Archiv, lxii.
55 Pathological Anatomy, London, 1875.