[Errors in the determination of oxygen consumption using the analysis of respiratory gases]

Analytical investigation of errors showed that the noninvasive determination of oxygen consumption (VO2) from the analysis of respiratory gases fails in giving as reliable results as expected in view of the small measuring uncertainties of internationally available analyzers. Absolute errors of delta FO2 = +/- 0.002 for oxygen fractions and delta FCO2 = +/- 0.001 for carbon dioxide fractions already cause the clinically just tolerated error limits of +/- 20% deviation from the true VO2 value to be exceeded under normal conditions. The approximation formula VO2 = VE (FIO2 - FEO2) gives the most reliable results for low inspiratory oxygen fractions (FIO2). It leads, however, systematically rising with FIO2, to an underestimation of VO2. Assuming 0.7 less than or equal to R less than or equal to 0.9 (R = respiratory quotient), the maximum relative error of VO2 can be limited to 21% for air (FIO2 = 0.21) and 30% for FIO2 = 1 by an appropriate correction factor K using the equation VO2 = VE (K.FIO2 - FEO2). For VO2 greater than or equal to 250 ml/min with VE = 10 l/min, K = 1.006 is suggested.