Pulmonary vascular changes were studied in 100 cases of isolated mitral stenosis; these included 90 patients in whom lung biopsies were obtained at valvotomy and 10 patients who came to necropsy. Medial thickness of the pulmonary arteries was measured in each case and in 12 cases was correlated with the haemodynamic data. Most patients were young, 78 being 30 years of age or less and 42 under 20 years or less. Males predominated 2:1. All patients with mitral stenosis showed varying degrees of vascular and other associated parenchymal changes. The most conspicuous were those observed in the muscular branches of the pulmonary artery in which the media was thickened in all cases, moderately in 44 and considerably in 28 cases. Dilation lesions representing grade 4 lesions of hypertensive pulmonary vascular disease (Heath and Edwards, 1958), hitherto not described in mitral stenosis, were observed in 4 cases. The intima was found to be frequently abnormal, showing oedema, fibrosis, and, more importantly, variable degrees of muscularization, often suggesting the incipient formation of a second media. Arteries and arterioles were often occluded by thrombi in various stages of organization, and the freshly formed channels tended to acquire a muscular lining. Arterioles were muscularized in all cases, and in many there was a pronounced intimal proliferation. Other changes included medial hypertrophy in the veins and and occasional muscularization and dilatation of the lymphatics. A notable feature was hypertrophy of the musculature of the bronchiolo-alvelar system seen in a majority of cases. The alveolar walls showed variable degrees of thickening and fibrosis, intimal proliferation of alveolar capillaries, and "epithelialization" of alveoli. Haemosiderosis was present in 70 cases. On the whole the more severe changes were observed more often in the younger subjects, further supporting the observation that rheumatic mitral stenosis in India commonly affects the juvenile age groups and is characterized by association with severe pulmonary hypertension. Medial hypertrophy was proportional to the level of pulmonary artery pressure.
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