Alveolar-to-arterial-to-venous anesthetic partial pressure differences in humans

To determine the correlation between the partial pressures of anesthetics in venous and arterial blood (Pv and Pa), and to assess whether this correlation was better than that between the partial pressure of anesthetic in alveolar gas (PA) and Pa, isoflurane (n = 4) or halothane (n = 4) was administered to eight patients undergoing surgery, and Pv, PA, and Pa were measured. PA correlated with Pa better than did Pv (R = 0.960 vs. 0.878), and there was less variability in the data. Differences between Pv and Pa increased as the relative blood flow to the hand decreased [indicated by an increasing arterial-to-venous (a-v) O2 content difference]. The difference between PA and Pa was approximately 20% of the difference between inspired gas (PI) and Pa. The differences between PA and Pa appear to be due primarily to contamination of alveolar gas by physiologic dead space gas.