The Diffusing Characteristics and Pressure-Volume Relationships of the Pulmonary Capillary Bed in Mitral Valve Disease*

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Abstract
Pulmonary diffusing capacity for carbon monoxide, alveolar membrane diffusing capacity, and pulmonary capillary blood volume were investigated under resting, steady-state conditions in 18 normal subjects and 18 patients with mitral valve disease. In patients with mitral valve disease, pulmonary diffusing capacity was signficantly less than normal, due mainly to impairment of membrane diffusion. This impairment of membrane diffusion may be due in part to reduction in surface area available for diffusion and, perhaps more important, to thickening of the alveolar-capillary membrane. Increasing pulmonary vascular pressures and pulmonary vascular resistance were found to be significantly related to diminution in the pulmonary capillary blood volume. This reduction in capillary blood volume, as a result of either constriction or partial obliteration of the pulmonary capillary bed, may be an important factor contributing to the elevation of pulmonary vascular resistance found in mitral valve disease.