Measurement of Energy Expenditure in Preterm Infants. 2: Longitudinal Measurements † 1409

Abstract

Background: Indirect Calorimetry (IC) is increasingly used to measure Energy Expenditure (EE) in preterm and term infants. However little work has been performed on the longitudinal measurement of EE in these infants. Subjects: 11 healthy preterm infants <32wks gestation and < 1500g birthweight. Methods: EE was measured using IC(Datex Deltatrac II) during the course of three, 48hr nutrient balances, two prior to discharge and the third at 8wks corrected age. Cumulative measurements of resting EE and Respiratory Quotient (RQ) were recorded at minute intervals with interruptions for normal parental and nursing care. The infants were fed standard low birthweight formula milk throughout the study period. No weaning took place prior to the third nutrient balance.Results: A total of 27 measurements (mean length 20.7hrs; range 6.9 - 37.9hrs) were made on 11 infants (BWt 1.310±0.168kg, GA 208±10d, mean±SD). The three measurements were performed at post-natal ages (PNA) of 23±7, 32±8 & 127±9d, post-conceptual ages (PCA) of 232±10, 241±10 & 339±6 d and weights of 1.71±0.29, 2.10±0.37 & 4.83±0.72 kg respectively. EE was linearly related to energy intake (EI). On stepwise regression analysis 97% of the variability in EE was accounted for by EI with a further 2% by PNA. Although EI did not change between the three measurements(122±9 v 130±11 v 129±44 kcal/kg/d), EE increased significantly between the second and third measurements (48.5±5.4 v 51.8±3.3 v 59.0±11.3 kcal/kg/d; p<0.05). There was also a significant fall in RQ between the second and third measurements(0.94±0.05 v 0.95±0.03 v 0.89±0.03; p<0.002).Conclusion: Small but significant changes in EE and RQ are detectable using IC sequentially in preterm infants. EE is significantly affected by EI and PNA. The fall in RQ suggests that the primary energy source changes from carbohydrate to fat with increasing maturity.