DETERMINING ENERGY-EXPENDITURE IN PRETERM INFANTS - COMPARISON OF (H2O)-H-2-O-18 METHOD AND INDIRECT CALORIMETRY

Determining energy expenditure in preterm infants: comparison of (H2O)-H-2-O-18 method and indirect calorimetry. Am. J. Physiol. 263 (Regulatory Integrative Comp. Physiol. 32): R685-R692, 1992.-The doubly labeled water (H2O)-H-2-O-18 method used to estimate total energy expenditure (EE(Total)) is particularly sensitive to analytic error in preterm infants, because of their high percentage of body water and the high ratio of water flux to CO2 production. To evaluate further use of this method, the EE of 12 preterm infants was measured by indirect calorimetry and (H2O)-H-2-O-18 simultaneously and continuously for 5 days. Initial infant weight, age, and postconceptional age were (means +/-SD) 1,674 +/- 173 g, 4.4 +/- 2.6 wk, and 34.6 +/- 1.6 wk, respectively. The indirect calorimeter system included an air-temperature-controlled chamber and heart rate monitor. EE was measured by indirect calorimetry for 85.6 +/- 4.7% of study time and estimated from the linear regression of heart rate on EE for 14.4 +/- 4.7% of study time. The (H2O)-H-1-O-18 method entailed an initial dose of 100 mg (H2O)-H-2 and 250 mg O-18/kg and a final dose of 75 Mg O-18/kg; urine was collected twice daily. H-2 and O-18 enrichments were measured by gas-isotope-ratio mass spectrometry. EE was calculated from measured H-2 and O-18 dilution spaces (N(H), N(O)), turnover rates (k(H), k(O)), and measured respiratory quotient. The ratio of H-2 to O-18 dilution spaces was 1.01 +/-0.01 and the ratio of k(O) to k(H) was 1.16 +/- 0.04. Estimation of EE from (H2O)-H-2-O-18 and indirect calorimetry agreed within 1%, although individual variability in methods was large.