The detailed mechanisms of oxygen-induced hypercapnia were examined in 22 patients during an acute exacerbation of chronic obstructive pulmonary disease. Ventilation, cardiac output, and the distribution of ventilation-perfusion (V A/Q ) ratios were measured using the multiple inert gas elimination technique breathing air and then 100% oxygen through a nose mask. Twelve patients were classified as retainers (R) when Pa(CO(2)) rose by more than 3 mm Hg (8.3 +/- 5.6; mean +/- SD) after breathing 100% oxygen for at least 20 min. The other 10 patients showed a change in Pa(CO(2)) of -1.3 +/- 2.2 mm Hg breathing oxygen and were classified as nonretainers (NR). Ventilation fell significantly from 9.0 +/- 1.5 to 7.2 +/- 1.2 L/min in the R group breathing oxygen (p = 0.007), whereas there was no change in ventilation in the NR group (9.8 +/- 1.8 to 9.9 +/- 1.8 L/min). The dispersion of V A/Q ratios as measured by log SD of blood flow (log SD Q) increased significantly in both R (0.96 +/- 0. 17 to 1.13 +/- 0.17) and NR (0.77 +/- 0.20 to 1.04 +/- 0.23, p < 0.05) groups breathing oxygen, whereas log SD of ventilation (log SD Q ) increased only in the R group (0.97 +/- 0.24 to 1.20 +/- 0.46, p < 0.05). This study suggests that an overall reduction in ventilation characterizes oxygen-induced hypercapnia, as an increased dispersion of blood flow from release of hypoxic vasoconstriction occurred to a significant and similar degree in both groups. The significant increase in wasted ventilation (alveolar dead space) in the R group only may be secondary to the higher carbon dioxide tension, perhaps related to bronchodilatation.

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