Ratio of venous-to-arterial PCO to arteriovenous oxygen content difference during regional ischemic or hypoxic hypoxia.

The purpose of the study was to evaluate the behavior of the venous-to-arterial CO tension difference (ΔPCO) over the arterial-to-venous oxygen content difference (ΔO) ratio (ΔPCO/ΔO) and the difference between venous-to-arterial CO content calculated with the Douglas' equation (ΔCCO) over ΔO ratio (ΔCCO/ΔO) and their abilities to reflect the occurrence of anaerobic metabolism in two experimental models of tissue hypoxia: ischemic hypoxia (IH) and hypoxic hypoxia (HH). We also aimed to assess the influence of metabolic acidosis and Haldane effects on the PCO/CO content relationship. In a vascularly isolated, innervated dog hindlimb perfused with a pump-membrane oxygenator system, the oxygen delivery (DO) was lowered in a stepwise manner to decrease it beyond critical DO (DO) by lowering either arterial PO (HH-model) or flow (IH-model). Twelve anesthetized and mechanically ventilated dogs were studied, 6 in each model. Limb DO, oxygen consumption ([Formula: see text]), ΔPCO/ΔO, and ΔCCO/ΔO were obtained every 15 min. Beyond DO, [Formula: see text] decreased, indicating dysoxia. ΔPCO/ΔO, and ΔCCO/ΔO increased significantly only after reaching DO in both models. At DO, ΔPCO/ΔO was significantly higher in the HH-model than in the IH-model (1.82 ± 0.09 vs. 1.39 ± 0.06, p = 0.002). At DO, ΔCCO/ΔO was not significantly different between the two groups (0.87 ± 0.05 for IH vs. 1.01 ± 0.06 for HH, p = 0.09). Below DO, we observed a discrepancy between the behavior of the two indices. In both models, ΔPCO/ΔO continued to increase significantly (higher in the HH-model), whereas ΔCCO/ΔO tended to decrease to become not significantly different from its baseline in the IH-model. Metabolic acidosis significantly influenced the PCO/CO content relationship, but not the Haldane effect. ΔPCO/ΔO was able to depict the occurrence of anaerobic metabolism in both tissue hypoxia models. However, at very low DO values, ΔPCO/ΔO did not only reflect the ongoing anaerobic metabolism; it was confounded by the effects of metabolic acidosis on the CO-hemoglobin dissociation curve, and then it should be interpreted with caution.