Comparison of closed circuit and Fick-derived oxygen consumption in patients undergoing simultaneous aortocaval occlusion

Agreement between continuously measured oxygen consumption during quantitative closed system anaesthesia and intermittently Fick-derived calculated oxygen consumption was assessed in 11 patients undergoing simultaneous occlusion of the aorta and inferior vena cava for hypoxic treatment of pancreatic cancer. All patients were mechanically ventilated using a quantitative closed system anaesthesia machine (PhysioFlex®) and had pulmonary and radial artery catheters inserted. During the varying haemodynamic conditions that accompany this procedure, 73 paired measurements were obtained. A significant correlation between Fick-derived and closed system-derived oxygen consumption was found (r = 0.78, p = 0.006). Linear regression showed that Fick-derived measure = [(1.19 × closed system derived measure) - 72], with the overall closed circuit-derived values being higher. However, the level of agreement between the two techniques was poor. Bland-Altman analysis found that the bias was 36 ml.min^-1, precision 39 ml.min^-1, difference between 95% limits of agreement 153 ml.min^-1. Therefore, we conclude that the two measurement techniques are not interchangeable in a clinical setting.