Alveolar-capillary block in patients with AIDS and Pneumocystis carinii pneumonia.

To determine the value of subdividing diffusing capacity for carbon monoxide (DL) in diagnosing and monitoring the course of Pneumocystis carinii pneumonia (PCP), we measured DL, membrane diffusing capacity (DM), and pulmonary capillary blood volume (VC) in 20 control subjects, 20 patients with a low DL (less than 75% predicted) and newly diagnosed PCP, and 16 patients with a low DL in most of whom PCP had been suspected and excluded. Ten patients with PCP were restudied approximately 60 days after treatment. When clinically indicated, lung biopsies were obtained for histologic examination. Compared with mean values in control subjects (DL = 92%, DM = 101%, and VC = 35 ml/m2), all values were decreased (p less than 0.01) in patients with PCP (DL = 58%, DM = 33%, and VC = 26 ml/m2) and in those without PCP (DL = 61%, DM = 56%, and VC = 22 ml/m2). Values of DM were significantly less (p less than 0.05) in patients with, than in those without, PCP. Analysis of lung biopsies by light and electron microscopy showed overlapping morphologic abnormalities in the 2 groups of patients. In the 10 patients with PCP restudied after successful treatment, the mean DL increased from 60 to 80% (p less than 0.0005), the DM increased from 35 to 108% (p less than 0.006), and the VC did not change. These results suggest that in contrast to most disorders in which DL is decreased, PCP causes reversible alveolar-capillary block.