The Pulmonary Capillary Bed in Various Forms of Pulmonary Hypertension

Pulmonary diffusing capacity for CO (DL), membrane diffusing capacity (DM), and pulmonary capillary blood volume (Vc) were measured in 44 patients with various forms and severity of pulmonary vascular disease, and these measurements were compared with hemodynamic data obtained at cardiac catheterization, with clinical disability, and, in 27 patients, with the results of corrective cardiac surgery. In 11 of these patients, DL, DM, and Vc were remeasured up to 8 months after cardiac surgery.In general, the diffusing characteristics and pressure-volume relationships in this larger group confirmed the previously reported findings in mitral valve disease.1 DM was impaired in almost all cases regardless of etiology or severity. Most of the 11 patients restudied showed no change in DL, DM, or Vc up to 8 months after surgery. Reduction in Vc correlated well both with clinical disability and with surgical results.Two patients with primary pulmonary hypertension and three of the four patients with equally severe pulmonary hypertension due to mitral valve disease had high Vc relative to pulmonary vascular resistance. If the measurements of Vc are not falsely high, then it appears that the capillary bed makes relatively and absolutely less contribution to the total rise in pulmonary vascular resistance in these patients. This could be explained by intense precapillary vasoconstriction in the presence of a relatively normal pulmonary capillary bed.