Interpretation of the alveolar-arterial oxygen difference in patients with hypercapnia

According to conventional wisdom the difference between alveolar and arterial O2 tensions, the AaPO2, should distinguish between hypoxemia caused by alveolar hypoventilation and hypoxemia caused by alveolar hypoventilation complicated by other abnormalities of gas exchange. To test this concept we have calculated the AaPO2 from arterial blood gas measurements, breathing air, in 23 patients with hypercapnia, hypoxemia, and advanced obstructive lung disease (mean FEV1 = 0.88 L). We found that AaPO2 varied inversely with PaCO2 (r = -0.83, p less than 0.001). In five of these patients with the most severely elevated PaCO2 (range, 59 to 81 mm Hg) the AaPO2 was within normal limits. We also calculated the difference between the O2 contents of "ideal" pulmonary capillary blood and arterial blood and expressed this as the venous admixture (QVA/QT) based on an assumed arteriovenous content difference of 4.5 ml/dl. In contrast to the AaPO2, the QVA/QT, was abnormal in all patients (mean = 41 +/- 8%). We conclude that the AaPO2 may be an unreliable indicator of abnormal gas exchange in the presence of alveolar hypoventilation. This finding can be explained by substantial changes in the position of the alveolar and arterial points on the oxygen dissociation curve for hemoglobin in the presence of alveolar hypoxia secondary to hypoventilation.