Ratio of venous-to-arterial PCO2 to arteriovenous oxygen content difference during regional ischemic or hypoxic hypoxia
Jihad Mallat, B. Vallet
Abstract
Abstract The purpose of the study was to evaluate the behavior of the venous-to-arterial CO 2 tension difference (ΔPCO 2 ) over the arterial-to-venous oxygen content difference (ΔO 2 ) ratio (ΔPCO 2 /ΔO 2 ) and the difference between venous-to-arterial CO 2 content calculated with the Douglas’ equation (ΔCCO 2D ) over ΔO 2 ratio (ΔCCO 2D /ΔO 2 ) 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 2 /CO 2 content relationship. In a vascularly isolated, innervated dog hindlimb perfused with a pump-membrane oxygenator system, the oxygen delivery (DO 2 ) was lowered in a stepwise manner to decrease it beyond critical DO 2 (DO 2crit ) by lowering either arterial PO 2 (HH-model) or flow (IH-model). Twelve anesthetized and mechanically ventilated dogs were studied, 6 in each model. Limb DO 2 , oxygen consumption ( $${\dot{\text{V}}\text{O}}_{2}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msub> <mml:mrow> <mml:mover> <mml:mtext>V</mml:mtext> <mml:mo>˙</mml:mo> </mml:mover> <mml:mtext>O</mml:mtext> </mml:mrow> <mml:mn>2</mml:mn> </mml:msub> </mml:math> ), ΔPCO 2 /ΔO 2 , and ΔCCO 2D /ΔO 2 were obtained every 15 min. Beyond DO 2crit , $${\dot{\text{V}}\text{O}}_{2}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msub> <mml:mrow> <mml:mover> <mml:mtext>V</mml:mtext> <mml:mo>˙</mml:mo> </mml:mover> <mml:mtext>O</mml:mtext> </mml:mrow> <mml:mn>2</mml:mn> </mml:msub> </mml:math> decreased, indicating dysoxia. ΔPCO 2 /ΔO 2 , and ΔCCO 2D /ΔO 2 increased significantly only after reaching DO 2crit in both models. At DO 2crit , ΔPCO 2 /ΔO 2 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 2crit , ΔCCO 2D /ΔO 2 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 2crit , we observed a discrepancy between the behavior of the two indices. In both models, ΔPCO 2 /ΔO 2 continued to increase significantly (higher in the HH-model), whereas ΔCCO 2D /ΔO 2 tended to decrease to become not significantly different from its baseline in the IH-model. Metabolic acidosis significantly influenced the PCO 2 /CO 2 content relationship, but not the Haldane effect. ΔPCO 2 /ΔO 2 was able to depict the occurrence of anaerobic metabolism in both tissue hypoxia models. However, at very low DO 2 values, ΔPCO 2 /ΔO 2 did not only reflect the ongoing anaerobic metabolism; it was confounded by the effects of metabolic acidosis on the CO 2 –hemoglobin dissociation curve, and then it should be interpreted with caution.