Postural variations in dead space and CO2 gradients breathing air and O2

Abstract

Effects of postural changes on anatomic and physiologic dead space and arterial-alveolar CO₂gradients were studied in 11 healthy, adult subjects breathing air and O₂. Results indicate that, on moving from the supine to the sitting position, Vads and Vpds increased by corresponding amounts (42 and 37 ml) with no increase in alveolar dead space or volume of lung which is nonperfused. Arterial-alveolar CO₂ gradients were unaffected by posture, but more than doubled with O₂ breathing, suggesting that O₂ may relax the pulmonary vascular bed and diminish perfusion of highest lung segments. Isoproterenol aerosol (0.5%) produced significant bronchodilatation (27 ml increase in Vads), but only small and inconsistent increases in alveolar dead space and CO₂ gradients. The PDS/Vt ratio in these subjects while sitting, breathing air, averaged 31 ± 6%, which is higher than the normally accepted value of 30%. As a result, the upper normal limit for PDS/Vt has been increased to 40% in our laboratories. Submitted on January 22, 1962