Effect of birth weight and postnatal age upon resting energy expenditure in preterm infants.

Our objective was to establish the role of gestational age, birth weight, and postnatal age upon resting energy expenditure (REE) in incubated preterm infants. We hypothesized that at the time these infants are close to being weaned from their incubator, their REE is inversely related to gestational age or birth weight and directly related to postnatal age and weight gain. Infants born at a birth weight of 500 to 2000 g were eligible for the study when they reached a weight of 1500 to 2100 g. All infants were clinically and thermally stable while cared for in a skin servo controlled incubator. REE (kcal/kg body weight/d) was measured 2 hours after feeding while the infants were quietly asleep, using a Datex oxygen consumption analyzer (DELTATRAC II (TM); Datex-Ohmeda Instrumentarium, Helsinki, Finland), based on the principles of indirect calorimetry. There were 42 infants recruited in the study. In univariate analysis, no significant correlation was found between gestational age and REE, but REE was significantly and inversely correlated with birth weight ( R(2) = 0.243, P < 0.001). There was also a significant correlation between REE and postnatal age ( R(2) = 0.203, P = 0.003) and with weight gain ( R(2) = 0.176, P = 0.006). In backward stepwise regression analysis, the effect of birth weight or postnatal age or daily weight gain (g) upon REE remained significant even after taking into account sex, energy intake, and type of feeding. Birth weight, postnatal age, and daily weight gain significantly affect REE, even after taking into account energy intake, sex, and type of feeding. Weight may be a more important parameter in the control of thermoregulation of the preterm infant than gestational age.