Resting Energy Expenditure in Patients With Thermal Injuries

Abstract

Numerous equations have been used to estimate the energy needs of burned patients. Many of these equations were based on indirect calorimetry data collected in the early 1970s.^1,2 Since then, treatment and survival of burned patients have improved; thus, present energy requirements may differ from those estimated in the past. Several investigators have found that equations based on earlier studies overestimate energy requirements.^3-7 The investigators in this study reassessed the relationship between energy requirements and burn size by comparing measured resting energy expenditure (REE) by using indirect calorimetry in patients with burn injuries treated during 1972 to 1973 with measurements made in a similar group of burned patients treated during 1987 to 1989.

REE data were collected during 1987 to 1989 from 62 patients with burn size equal to 12% to 91% of total body surface area (TBSA) between 5 and 19 days postburn. These data were compared with data collected during 1972 to 1973 from 43 patients with burn size equal to 7% to 84% of TBSA between 7 and 18 days postburn. Resuscitation had been completed and all patients were hemodynamically stable. REE was estimated by indirect calorimetry. In the earlier study, expired gases were collected using a Douglas bag (no measurements were carried out in mechanically ventilated patients). In the more recently studied group of patients, a metabolic cart was used to measure expired air volumes and analyze gas concentrations, from which oxygen consumption and carbon dioxide production were determined (REE was calculated using the Weir equation⁸). Both spontaneously breathing (n = 38 via canopy system) and mechanically ventilated (n = 24) patients were studied. Age-and sex-specific basal metabolic rate (BMR) and body surface area (BSA) were estimated by using data from another study. ^9,10 A regression analysis relating measured REE (kcal/m² per hour) to percent TBSA revealed a linear relationship (r = .663) for the total population in the 1987-to-1989 study. The data generated from the canopy and ventilator groups were different and thus were evaluated using separate regression analyses. With the canopy-generated data, REE (kcal/m ² per hour) was linearly related to burn size (r = .724). A linear relationship was observed between the ratio of REE per BMR to %TBSA. These data were used to develop an equation for predicting energy requirements in nonmechanically ventilated burned patients: REE = BMR (0.89142 + 0.01335 × TBSA) × m² × 24 × activity factor of 1.25. The data from the ventilator-supported patients showed wide variance (r = 0.579), but the relationship remained linear with a larger intercept (indicative of a higher metabolic rate at any given burn size and probable error of any prediction of metabolic need).

Comparison between data generated during 1987 to 1989 and earlier data from 1972 to 1973 showed that the metabolic requirements were related to burn size in both patient populations, but there was a difference in the magnitude of caloric expenditure between the two groups. The data generated during the 1972-to-1973 period showed a curvilinear relationship of REE per BMR to %TBSA, whereas the more recent data show a linear relationship. The energy requirements of the current patient population were smaller than would have been predicted from the earlier data. The formulas developed from the earlier data overestimated energy requirements in 73% of the more recent group of patients.

The authors conclude that use of predictive equations based on older data overestimates energy requirements of patients receiving modern burn care. They recommend th use of individual measurements of metabolic rate and resetting caloric goals over time to avoid overfeeding.