Compliance of the Human Pulmonary Arterial System in Disease

Pulmonary arterial and pulmonary arterial “wedge” pressures were measured in 35 patients during cardiac catheterization. Pulmonary arterial compliance (C) was calculated from the diastolic pulmonary arterial pressure profile, left atrial pressure, and pulmonary vascular resistance (R). In the pulmonary normotensive group, compliance averaged 2.87 ml/mm Hg; in patients with severe pulmonary hypertension, values as low as 0.7 ml/mm Hg were found. In 16 of these patients, pulmonary capillary blood flow was measured by N2O body plethysmography. In pulmonary normotensive patients the pulsatility of capillary flow, i.e., the ratio of peak-to-mean flow rates, was 2:1; advanced pulmonary hypertension was associated with values as low as 1.2:1, suggesting marked damping of the flow pulse. The pulsatility index correlated well with the RC time constant of the pulmonary arterial system, so that damping of the pulmonary capillary flow pulse was associated with a prolonged time constant. This suggested that in severe pulmonary hypertension the pulmonary arterial walls are stiff and indistensible. A method was developed for estimating mean pulmonary arterial and left atrial pressure from the pulmonary capillary flow pulse, and the relationships between compliance and pressure and pulsatility index and RC time constant. N2O-plethysmography can thus provide a reliable and repeatable, noninvasive method for investigation of the pulmonary vascular bed.