The main finding of this study was that Pv–aCO2/Ca–vO2 failed to properly reflect RQ in hemodilution. It increased before the appearance of the dependency of VO2 on DO2. Its correlation with RQ was moderate, but it showed a strong association with Hb levels. Indeed, Pv–aCO2/Ca–vO2 was more explained by Hb levels than by anaerobic metabolism. Changes in the dissociation of CO2 from Hb mostly account for these results.
Several studies have tried to link Pv–aCO2/Ca–vO2 with some events suggestive of anaerobic metabolism such as hyperlactatemia , decreased lactate clearance [5, 6], increased VO2 in response to fluid challenge [7, 8], and worse outcome . Since RQ was not measured in those studies, it was not clear whether Pv–aCO2/Ca–vO2 effectively reflected the presence of anaerobic metabolism or was only the result of factors that could increase that ratio in the absence of anaerobic metabolism. In fact, Pv–aCO2/Ca–vO2 is not a straightforward variable. Although related to RQ, it might be hypothetically increased by several factors beyond anaerobic metabolism. Many of the changes in Pv–aCO2/Ca–vO2 might be ascribed to modifications of the CO2-Hb dissociation curve. Haldane effect, metabolic acidosis, and anemia can increase PCO2 for a given CCO2 . In addition, taking into account the curvilinear characteristics of the dissociation curve, the effects are even greater at higher PCO2. When the slope of the dissociation curve flattens, substantial increases in Pv–aCO2 may actually represent negligible increases in Cv–aCO2. Therefore, high oxygen venous saturation , hyperlactatemia , and hemodilution  can increase Pv–aCO2 even though Cv–aCO2 remains unchanged.
In line with the previous discussion, our results showed that isovolemic anemia disproportionally increased Pv–aCO2/Ca–vO2, compared to hemorrhage. Furthermore, this ratio was elevated before the beginning of oxygen supply dependency. Progressive hemodilution was associated with opposing effects on Pv–aCO2 and Cv–aCO2: Pv–aCO2 increased and Cv–aCO2 decreased. Previous studies showed that decreasing hemoglobin levels results in widened Pv–aCO2 for a given Cv–aCO2 . In a similar model of progressive hemodilution, the contrasting effects of low Hb levels on Pv–aCO2 and Cv–aCO2 were also noticed . Therefore, increased Pv–aCO2 is a predictable consequence of anemia.
Another expected consequence from hemodilution is the decrease in Ca–vO2 . Increases in oxygen extraction always occur in response to reductions in DO2, irrespective of the mechanism of oxygen supply limitation. The impact of the increase in oxygen extraction on Ca–vO2, however, depends on cardiac output. According to Fick’s principle, Ca–vO2 should widen in conditions of low cardiac output and decreased in states of reduced DO2 with increased cardiac output, if VO2 remains constant. Our study also confirmed this assumption.
As a result of the opposite effects of hemodilution on Pv–aCO2 and Ca–vO2, the ratio between both variables markedly augmented in the absence of anaerobic metabolism. The increase in Pv–aCO2/Ca–vO2 was even higher during the oxygen supply dependency, due to the interplay of the aforementioned factors and the ongoing anaerobic CO2 production.
Considering the coefficient of determination of the regression (R
2 = 0.41), RQ only explains a minor part of the Pv–aCO2/Ca–vO2 variability. As supported by the results of the multiple linear regression model, Pv–aCO2/Ca–vO2 is a complex variable that has several determinants. Although Hb was the main contributor to the prediction of Pv–aCO2/Ca–vO2, it was also influenced by RQ and by the changes in the dissociation of CO2 from hemoglobin induced by metabolic acidosis and Haldane effect. These effects were magnified at the flattened portion of the CO2Hb dissociation curve as shown by the impact of mixed venous PCO2 in the model.
A study has proposed a Pv–aCO2/Ca–vO2 cutoff of 1.4 for the identification of anaerobic metabolism . This suggestion, however, should be carefully interpreted. The development of anaerobic metabolism is identified by acute increases in RQ, not by isolated values [1,2,3]. Actually, the normal range of RQ is 0.67–1.30  depending also on other factors such as energy source  and overfeeding . In our experiments, values of Pv–aCO2/Ca–vO2 during oxygen supply dependency were considerably higher (10.0 ± 2.7 and 2.5 ± 0.4 in hemodilution and hemorrhage groups, respectively).
Our findings do not challenge the value of Pv–aCO2/Ca–vO2 as an outcome predictor of critically ill patients, which was previously described . The composite characteristics of Pv–aCO2/Ca–vO2, however, suggest that the prognostic ability might be mainly related to the interaction of several mechanisms, not only to anaerobic metabolism.
Our study has certain drawbacks. Secondary analyses pose inherent limitations that have been subject to critiques . In addition, part of our analysis was based on calculations of CCO2, not in actual measurements . This last procedure is complex and cumbersome and is not available in our laboratory. Accordingly, we calculated CCO2 from Fick’s principle. We prefer this method, because the different algorithms for computing CCO2 from blood gases and Hb are frequently misleading and can produce negative Cv–aCO2 values. Finally, the experimental model of hemorrhage and hemodilution does not address the applicability of our results to septic conditions.