Intraoperative calorimetry in aortic bifurcation reconstruction

: Oxygen uptake (VO2) and carbon dioxide elimination (VCO2) can be measured with an indirect calorimeter, this method is well established in routine monitoring of ICU patients to evaluate metabolic state as a reflection of stress. In various experimental studies it was demonstrated that anaesthetics can influence whole-body metabolism. The purpose of this study was to examine whether indirect calorimetry can be used intraoperatively during routine anaesthesia and whether presumable changes in metabolism can be detected immediately. Abdominal aortic cross-clamping changes circulation, nutritional supply of the lower extremities and thus VO2 and VCO2. We therefore used this operation for our study. METHOD. Eleven patients, mean age 64 years, undergoing reconstruction of the aortic bifurcation, were studied. After premedication with piritramid and atropine, total intravenous anaesthesia (TIVA) was performed with fentanyl and midazolam after an induction with thiopental. Patients were ventilated with a Servo-Ventilator 900 D and a constant FiO2 of 0.5, without N2O. Routine monitoring consisted of ECG, pulsoximetry, CVP and continuous AP. VO2 and VCO2 were measured with a Deltatrac (Datex), and data were registered every minute. For statistical evaluation we used a Wilcoxon-Ranksum test for matched pairs, p < 0.05 was considered significant. Data from specific time (5 min after intubation, 5 min before clamping; 5, 10 and 15 min after clamping, before declamping and 5 and 10 min after declamping and at the end of surgery) were calculated. In addition to absolute values, we compared the measured VO2 and VCO2 to baseline (5 min before clamping = MP2). RESULTS. Mean operating time was 139 min +/- 37; aortic cross-clamping time for the first extremity was 38 min and 55 min for the second. As expected, there was a significant decrease in VO2 (90% of baseline) and VCO2 (75% of baseline) during aortic cross-clamping. After declamping VO2 again rose to 110% of baseline, or to 103% for the second limb. VCO2 increased to only 90% and 82%, respectively. At the end of surgery VO2 reached baseline, whereas VCO2 remains at 83%. The respiratory quotient VCO2: VO2 was markedly reduced from 0.95 +/- 0.156 to 0.73 +/- 0.06 during surgery. The Deltatrac showed every change in VO2 without delay; changes in VCO2 seem to occur somewhat retarded. DISCUSSION. Aortic cross-clamping leads to a marked decrease in VO2 and VCO2 reflecting the temporary reduction in whole-body metabolism. Declamping results in a compensatory rise, especially in VO2. VCO2 seems to increase less after declamping, perhaps due to the CO2 pool of the organism or to a change in metabolism from carbohydrate to mainly fat oxidation. The results of this study demonstrate that indirect calorimetry can easily be performed during anaesthesia and surgery. Preconditions are a non-rebreathing system without airleak, constant FiO2 < 0.6 and no use of nitrous oxide.