The distribution of fibrosis in the left ventricle in congenital aortic stenosis and coarctation of the aorta.

Left ventricular fibrosis occurs in congenital aortic stenosis. To investigate whether fibrosis is greatest in the subendocardial region, where there is presumably decreased blood flow relative to the subepicardial region, as shown in animal experiments, and whether the distribution of fibrosis in aortic stenosis differs from that in another afterload lesion with less limitation of myocardial blood supply, coarctation of the aorta, hearts from 10 children who died with congenital aortic stenosis and nine children who died with aortic coarctation were studied. No gross scarring was seen in the free left ventricular wall in any specimen. Sections of free left ventricular wall were taken at the level of the papillary muscle tips and stained with Masson trichrome, which colors fibrous tissue blue and muscle red. The slide was projected (magnified X 25) onto a grid of points 1 cm apart. The image was divided into three approximately equal sections, an inner, middle, and outer section. Points falling on muscle and fibrous tissue were counted and expressed as a percentage of all points falling on tissue. To examine microscopic interstitial fibrous tissue not associated with macroscopic scarring, photomicrographs were taken in each section of the heart (avoiding macroscopic areas of scarring), coded and quantitated by a similar counting technique. There was significantly more fibrosis in the inner third of the left ventricular wall in the hearts with aortic stenosis than in the middle or outer thirds (inner 23.4 ± 8.9%; middle 15.1 ± 5.8%; outer 8.3 ± 2.2%). A similar but less striking increase in inner wall fibrosis was seen in coarctation of the aorta (inner 13.0 + 8.6%; middle 10.1 ± 5.8%; outer 6.2 ± 2.6%). There was significantly more fibrosis in the inner and middle layers of the hearts with aortic stenosis than in those with coarctation, but no difference in the outer layers between the two lesions. The amount of interstitial fibrous tissue was not significantly different in the three layers in aortic stenosis (inner 12.5 ± 5.3%; middle 13.4 ± 6.7%; outer 8.9 ± 4.2%) or in coarctation of the aorta (inner 9.3 ± 4.6%; middle 10.6 ± 4.1%; outer 6.5 ± 2.9%). There was significantly more fibrosis in the subendocardial than in the subepicardial region of the left ventricle in aortic stenosis, with similar distribution but lesser amounts of fibrosis in coarctation of the aorta. One possible explanation for this increased fibrosis might be decreased subendocardial blood flow, as seen in experimental aortic stenosis. The lesser degrees of fibrosis in coarctation of the aorta might result from the lesser reduction in myocardial blood supply due to the higher diastolic pressure and longer diastolic filling period compared with aortic stenosis.