Breath-by-breath analysis of oxygen uptake using the Datex Ultima

The Datex Capnomac Ultima monitor combines a sidestream rapid gas analyser with a spirometer. In theory the integral of simultaneous flow and oxygen concentration during inspiration is the inspired and expired volume of oxygen. From the difference between these two amounts, oxygen consumption (VO2) can be estimated. VO2 was measured in patients under general anaesthesia and compared with that obtained simultaneously using a Datex Deltatrac metabolic monitor. Two techniques to compensate for the delay in rise time of the oxygen sensor were evaluated. The first method steepened the oxygen curve exponentially, and the second used a linear statistical correction. The linear correlation coefficients for VO2 between the Ultima and the Deltatrac were 0.87 and 0.78 for the two methods. The 10th to 90th centile range for error was -67.4-59.8 ml min-1 for the exponential method and -53.2-56.1 ml min-1 for the statistical method. The Ultima may be used with moderate accuracy to measure oxygen uptake during anaesthesia.