Accuracy of feedback-controlled oxygen delivery into a closed anaesthesia circuit for measurement of oxygen consumption

Abstract

Background. Oxygen consumption (VO2) is rarely measured during anaesthesia, probably because of technical difficulties. Theoretically, oxygen delivery into a closed anaesthesia circuit (VO2-PF; PhysioFlex™ Draeger Medical Company, Germany) should measure VO2. We aimed to measure VO2-PF in vitro and in vivo. Methods. Three sets of experiments were performed. VO2-PF was assessed with five values of VO2 (0-300 ml min-1) simulated by a calibrated lung model (VO2-Model) at five values of FlO2 (0.25-0.85). The time taken for VO2-PF to respond to changes in ̇VO2-Model gave a measure of dynamic performance. In six healthy anaesthetized dogs we compared VO2-PF with VO2 measured by the Fick method (VO2-Fick) during ventilation with nine values of F1O2 (0.21-1.00). VO2-PF and VO2-Fick were also compared in three dogs when ̇VO2 was changed pharmacologically [102 (SD 14), 121 (17) and 200 (57) ml min-1]. In patients during surgery, we measured VO2-PF and ̇VO2-Fick simultaneously after induction of anaesthesia (n=21) and during surgery (n=17) (FlO2 0.3-0.5). Results. Compared with VO2-Model, VO2-PF values varied from time to time so that averaging over 10 min is recommended. Furthermore, at an FlO2 >0.8, VO2-PF always overestimated VO2. With FlO2 <0.8, averaged VO2-PF corresponded to VO2-Model and adapted rapidly to changes. Averaged VO2-PF also corresponded to VO2-Fick in dogs at F1O2 <0.8. VO2 measured by the two methods gave similar results when ̇VO2 was changed pharmacologically. In contrast, VO2-PF systematically overestimated VO2-Fick in patients by 52 (SD 40) ml min-1 and this bias increased with smaller arteriovenous differences in oxygen content. Conclusion. VO2-PF measures VO2 adequately within specific conditions.