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blood flow per unit volume of lung is several times greater in the lower zones of the lungs than in the upper (1, 2). Dollery and West (3) have shown that in patients with mitral stenosis this pattern is often reversed and that the upper zones may have several times the blood flow per unit volume of the lower zones. Two explanations for this finding have been suggested. West, Dollery, and Heard (4) have shown that a similar reversal of flow pattern can be produced in an isolated dog lung by sustained elevation of the pulmonary vein pressure, causing perivascular edema in the dependent portion. They suggested that a similar process may cause the increased vascular resistance in the lower zones in patients with mitral stenosis. Another theory proposes that vascular engorgement and edema in the lung bases cause a local loss of compliance or an increase in airway resistance, and the resultant underventilation and hypoxia could cause vasoconstriction in these regions (5). This is consistent with the observations that oxygen breathing can reduce vascular resistance in some cases of mitral stenosis (6-8) and that infusion of acetylcholine into the pulmonary artery may simultaneously lower pulmonary arterial pressure and reduce arterial oxygen saturation (5, 9, 10). The fall in arterial saturation was attributed to the relief of hypoxic vasoconstriction in underventilated lung units (9).

Regional Lung Function in Patients with Mitral Stenosis Studied with Xenon133 during Air and Oxygen Breathing*