Estimation of effective pulmonary capillary pressure in intact lungs

Abstract

A method was described for estimating pulmonary capillary pressure in intact dog lungs. The vascular pressure transients after occlusion of a small pulmonary artery or vein with Swan-Ganz catheters were analyzed for the inflection point between rapid and slow components. This transition point was assumed to represent the beginning of the discharge of blood stored in pulmonary microvessels and defined as the effective capillary pressure. For the arterial pressure decay curves, the fast and slow components had rate constants of 0.77 and 0.25/s, respectively. Comparable inflection point capillary pressures were obtained with either digitized curves displayed in semilogarithmic fashion or direct visual inspection of the recorder chart of the same arterial pressure tracings. When the capillary pressures were compared, using wedge transients in the pulmonary artery (Pc,a), with pressures obtained from pulmonary vein (Pc,v) transients, the following regression equation was obtained: Pc,a = -1.7 + 0.96 Pc,v (r = 0.85, n = 11). Estimates of Pc,a indicated that 40% of the pulmonary vascular resistance was postcapillary, as previously determined using isogravimetric capillary pressures. Although Pc,a and Pc,v are functions of microvascular capacitance and vascular resistance, the effective capillary pressures obtained using wedge pressure tracings were well correlated with isogravimetric measurements of capillary pressure and should provide a useful method for evaluating the capillary filtration pressure in the lungs of intact animals or patients.