Supine change in arterial oxygenation in patients with chronic obstructive pulmonary disease

In normal subjects, recumbent PaO2 differs from upright PaO2 as a function of the relationship between FRC and closing volume. However, in patients with lung disease, the factors controlling PaO2 change. Furthermore, the distribution of airway closure does not strictly reflect gravitational gradients in pleural pressure. Hence, we evaluated the effects of recumbency on PaO2 in patients with advanced COPD. We conducted pulmonary function tests and measured PaO2, AaPO2, change from upright to supine FRC, VE, cardiac output (QT), and mixed venous oxygen saturation (SvO2). Eight patients with COPD in whom PaO2 decreased in the supine position (Group I) were compared with 8 patients with COPD in whom PaO2 improved in the recumbent position (Group II); the only anthropometric difference between the groups was greater body weight in Group II (p less than 0.05). Pulmonary function was comparable in the 2 groups. In both groups, FRC decreased with recumbency, but supine FRC and delta FRC did not differ between groups nor correlate significantly with PaO2, AaPO2, or venous admixture (QVA/QT). Supine SVO2 increased in both groups, but supine QT increased significantly only in Group I. In Group II, recumbent QVA/QT did not change significantly, whereas recumbent QVA/QT increased (p less than 0.02) in Group I. The cause for increased QVA/QT in Group I was reduced VE plus increased QT while supine; in contrast, by maintaining a higher supine VE and by not increasing QT significantly, an increase in QVA/QT was prevented in Group II subjects, an increase that would otherwise have overwhelmed the benefits of increased recumbent SVO2 on PaO2.(ABSTRACT TRUNCATED AT 250 WORDS)