A mathematical analysis of indirect calorimetry measurements in acutely ill patients

Bedside indirect calorimetry is generally accepted as reliable for evaluating energy expenditure (EE) and oxidation of carbohydrate (dCH) and fat (dF) in critically ill patients. Daily measurement of nitrogen excretion is usually included in the calculation of EE, dCH, and dF but it is difficult to perform in the clinical setting and is not necessary for accurate determination of EE. In this paper new equations, which are suitable for use with critically ill patients, are given for calculating EE, dCH, and dF without N-excretion measurements. With these equations the maximum errors for EE, dCH, and dF were 56, 179, and 306 kcal/d, respectively. The mean errors in 38 measurements of 26 critically ill patients were 27, 86, and 143 kcal/d, respectively. Measurement of EE by these equations is very accurate and suitable for both research and clinical use. Although the calculation of dCH and dF is much less precise, it can still give an indication of fuel utilization.