The aim of the present study was the comparison of calculated and measured values of the energy expenditure of critically ill patients during the different phases of therapeutic mild hypothermia.
Five patients (mean age 40.6 years, range 23 to 68 years, 2 females) suffering from severe acute cerebral injuries who underwent mild hypothermia were prospectively included into the study. Indirect calorimetry measurements were made at intervals of 3-4 hours and subsequently, during the steady state, at least every 12 hours. The results were compared with the basal metabolic rate calculated by the Harris-Benedict equation.
A close linear correlation between body temperature and basal metabolism could be observed across a wide range of temperatures from 30.5 degrees C to 38.3 degrees C (r=0.82, p<0.001). One degree drop in temperature led to a 5.9% reduction in energy. During mild hypothermia, oxygen consumption was reduced by 71 mL/min (95% confidence interval 57 to 86 mL/min; p<0.001) as compared to base line. The basal metabolism rate was decreased by 30.3% (95% confidence interval 24.7 to 35.9%, p<0.001). The average value recorded was 16.7% below the values calculated in accordance with the Harris-Benedict equation (95% confidence interval 12.8 to 20.6%).
The immediate reduction in oxygen requirements achieved by hypothermia is linearly correlated with the reduction in temperature and the hypothermia induced reduction in oxygen requirement recorded by indirect calorimetry is considerably below that calculated in accordance with the Harris-Benedict equation. If indirect calorimetry should not be available and the Harris-Benedict equation is used, a corrective factor is therefore needed to avoid an inaccurate calorie administration.