Calculation is unsuitable for determination of O2-consumption (VO2) in case of O2-supply-dependency

When O2-delivery to tissues is critically reduced, O2-consumption becomes dependent on O2-delivery and starts to decline, which reflects tissue hypoxia. In order to timely detect tissue hypoxia prior to organ damage, O2-consumption may be calculated or measured from respiratory gases. We have assessed reproducibility of calculated and measured O2-consumption-data and their agreement during O2-supply-dependency.

Data of 31 anesthetized, ventilated pigs were analysed retrospectively. Animals had undergone either controlled hemorrhage ("shock") or isovolemic exchange of blood with colloids (extreme hemodilution, "HD") until O2-consumption had become dependent on O2-delivery. O2-consumption was calculated from the Fick equation and measured simultaneously with a DELTATRAC II metabolic monitor. Repeatability was determined for (1) calculated and (2) for measured.VO2 -values and (3) for input variables of the Fick equation (i.e. cardiac index (CI) and arteriovenous O2-content difference (CaO2-CvO2)). Bias between calculated and measured data and precision of calculation were assessed from paired O2-consumption-values obtained before and after induction of O2-supply-dependency via hemorrhage or extreme hemodilution.

Repeatability of the reversed Fick method was inferior to repeatability of measurement (27 vs 15%) due to error propagation from CI and (CaO2-CvO2). Between-method-bias at baseline ("BL") was 3%, and changed in case of O2-supply-dependency (shock -15%; HD -31%, both p<0.05 vs BL), precision of the reversed Fick method deteriorated (BL 32%; shock 60%; HD 60%) due to variability of CI (CV: 16%; shock 27%; HD 41%).

In anesthetized pigs calculated and measured O2-consumption values are in agreement, while in presence of O2-supply-dependency the reversed Fick method (1) grossly underestimates true O2-consumption and (2) precision deteriorates not allowing to verify or reject the presence of tissue hypoxia.