Direct measurement of pulmonary microvascular distensibility

Pulmonary vascular distensibility has an important influence on pulmonary hemodynamics. Although many measurements of distensibility have been made on large pulmonary vessels, there is less information on microvascular distensibility. We have measured the distensibility of the smallest (<70- μm-diam) precapillary arterioles and postcapillary venules. Isolated dog lobes, at 2.5 cmH₂O transpulmonary pressure, were perfused at low flows, which caused the arteriovenous pressure gradient to be very small and thereby permitted accurate estimation of microvascular pressure. As microvascular pressure was systematically varied between 0 and 30 mmHg, subpleural microvascular diameters were determined from computer-enhanced images obtained by videomicroscopy. Arteriolar and venular distensibilities were not different from each other. The microvascular pressure-diameter relationship was alinear with distensibility coefficients of 1-3% mmHg^-1, values that are of the same order of magnitude as previously measured distensibilities of 100- to 1,000-μm-diam canine pulmonary vessels.