Effect of raised alveolar pressure on leukocyte retention in the human lung

To determine whether an increase in alveolar pressure delays the passage of leukocytes (WBCs) through the lung by compressing the lung capillaries, we measured the concentration of WBC across the lung in response to a forced expiratory maneuver. In 20 human subjects, blood was sampled from catheters placed in the pulmonary artery (PA) and left ventricle (LV) before, during, and after a forced expiratory maneuver held for greater than or equal to 20 s against an occluded airway. Pressures were recorded at the mouth and from both catheters. A significant fall in LV WBC (P less than 0.01) but not in PA WBC occurred during or immediately after the maneuver in 18 subjects, with a mean maximum decrease of 26 +/- 12% (SD) from base line (range 9-46%). Between 1 and 3 min after the maneuver, there was an increase in LV and PA WBC (P less than 0.01) above base line. The neutrophil and lymphocyte counts showed similar changes, but erythrocyte and platelet counts remained unchanged. The degree of fall in LV WBC correlated closely (r = 0.68, P less than 0.01) with the changes from lung zone 3 to zone 2 and 1 conditions, as determined from the pressure changes. We conclude that WBCs are retained in the lung during a forced expiratory maneuver because of alveolar capillary compression.