Derived Oxygen Saturations are not Clinically Useful for the Calculation of Oxygen Consumption

In critically ill patients, oxygen consumption (VO₂) and delivery (Do₂) are used to determine optimal haemodynamic management and to grade severity of illness. Vo₂ may be measured by indirect calorimetry with metabolic gas monitoring systems or derived using the reverse Fick principle. Oxygen saturation (Sao₂) may be measured directly by co-oximetry or derived by equations for incorporation into reverse Fick equations. A prospective study comparing Vo₂ measured by these methods was performed in 20 critically ill patients. The mean Vo₂ measured by the metabolic gas monitoring system (308 ± 63.9 ml/min) was significantly greater than that measured by reverse Fick using measured Sao₂ (284 ± 72.0 ml/min) (P < 0.01). This difference may be due to intrapulmonary Vo₂. When SaO₂ was, calculated from three logarithmic equations and incorporated into the reverse Fick equations, calculated Vo₂‘s were significantly greater (P < 0.001) than those measured by indirect calorimetry. Correlation was poor and wide limits of agreement (-118 to + 350 ml/min) were demonstrated. Vo₂ should ideally be measured by indirect calorimetry in the critically ill, or if reverse Fick is used, SaO₂ should be measured by co-oximetry as the use of equations for clinical measurement of SaO₂ is clinically suspect.