Effect of alveolar pressure on single-breath CO diffusing capacity at mid-lung volume

Abstract

The present study examines whether changes in the alveolar pressure (P a) affect the single breath diffusing capacity for carbon monoxide (D l CO) more strongly at mid-lung volume than at total lung capacity (TLC) in normal subjects. D l CO was measured at 60%, 80% and 100% of TLC, while P a was kept at +30, 0, or −30 cm H 2O by the subject's effort during the measurement of D l CO at each lung volume. The capillary blood volume (Vc) and the membrane diffusing capacity (Dm) were also determined. D l CO at zero P a was found to be higher at 100% TLC than at lower lung volumes. At P a = +30 cm H 2O D l CO at 100%, 80%, and 60% TLC decreased by 8%, 13%, and 13%, respectively, and the decreases in Vc were 2%, 10%, and 21%, respectively. However, negative P a did not cause any significant changes in D l CO or Vc at any lung volume. Also, Dm di not change at any P a. We conclude that D l CO is more affected by a positive P a at mid-lung volume than at a high lung volume, probably due to a greater decrease in Vc.