Assessing hypermetabolism and hypometabolism in the postoperative critically ill patient

To determine whether a patient is hypermetabolic or hypometabolic, measured resting energy expenditure is compared with estimated (or predicted) energy expenditure. The latter is calculated using equations derived from measurements made in groups of healthy individuals. Body weight or body surface area are among the variables used in these equations. Yet, in critically ill patients, body weight often rises because of fluid resuscitation. This study examined the differences between using preoperative, postoperative, and ideal body weights on the determination of hypermetabolism and hypometabolism in mechanically ventilated, critically ill patients. When the elevated postoperative weights were used instead of the preoperative ones to predict energy expenditure, the degree of hypermetabolism was underestimated. Subtracting the weight of the cumulative net fluid balance from the postoperative weight was found to accurately reflect preoperative weight. The influence of various predictive equations on the magnitude of hypermetabolism was also explored. In conclusion, it is important, when determining hypermetabolism or hypometabolism, to consider the influence of the specific predictive equation used and the effect of resuscitation fluid.