Longitudinal prediction of metabolic rate in critically ill patients.

Background: Indirect calorimetry is the criterion method for assessment of energy expenditure in critically ill patients but is decidedly uncommon. Thus, calculation methods proliferate. Even if indirect calorimetry is available, it usually is not repeated more than weekly on the same patient, creating potential for error. The purpose of the current study was to quantify estimation errors against indirect calorimetry measurements in critically ill patients over time.

Methods: In mechanically ventilated, critical care patients, indirect calorimetry was used to measure resting metabolic rate for 7 days. Three estimation methods were compared with the cumulative measurement: the Penn State equations, the American College of Chest Physicians (ACCP) standard (25 kcal/kg body weight), and an extrapolated value based on the first measurement multiplied by 7 days.

Results: The cumulative difference between measured resting metabolic rate and the rate predicted by the Penn State equations was -468 ± 642 kcal (-3.7% ± 5.1% of the measured value). The difference for the ACCP was smaller, but variation was much wider (-387 ± 1597 kcal or -2.2% ± 11.9% of the measured value). The extrapolated value was -684 ± 1731 kcal (-4.1% ± 11.4% of measured expenditure).

Conclusion: On average, the Penn State equations predict resting metabolic rate over time within 5% of the measured value. This performance is similar to the practice of making 1 measurement and extrapolating it over 1 week. The ACCP method has an unacceptably wide limit of agreement.