Validation of 2 approaches to predicting resting metabolic rate in critically ill patients

Indirect calorimetry is the criterion method for determining resting metabolic rate for nutrition support in critically ill patients. However, calculation equations are more commonly used. In the current study we tested the validity of 2 such calculation systems.

Indirect calorimetry was performed with an open-circuit device in mechanically ventilated surgical, trauma, and medical patients at rest. Feedings were not stopped for the measurements. Two predictive equations by Ireton-Jones and 3 versions of a multivariate equation developed at our institution (referred to as Penn State equations) were then used to estimate resting metabolic rate. These estimates were compared on a percentage basis with the measured value of resting metabolic rate. Estimated resting metabolic rate within 10% of measured was considered accurate, whereas estimations >15% different from measured were considered large errors.

Forty-seven subjects were measured. A larger percentage of subjects were estimated accurately by the Penn State equations (72% in the best equation) than by the Ireton-Jones equations (60% in the best equation; not significant). The incidence of errors >15% of measured was significantly lower in the Penn State equation (11% of subjects) compared with the Ireton-Jones equation (32% of subjects) (p < .05).

The Penn State equation for resting metabolic rate in mechanically ventilated intensive care patients receiving nutrition support appears to be a valid clinical tool for determining energy goals in the absence of or as a supplement to indirect calorimetry. The Ireton-Jones equation performed less well, especially in that a higher number of large errors occurred.