1463 THE RESPIRATORY EXCHANGE RATIO IN GROWING LOW BIRTH WEIGHT INFANTS

Abstract

As part of an ongoing study of energy balance, 22 growing low birth weight infants were studied for three four hour periods during which minute by minute measurements of oxygen consumption and carbon dioxide production were made using a Kippdiaferometer calibrated with human expired gas. The three study periods were set one week apart so that each baby was studied over a two week interval. 16 babies were fed with formula (SIMILAC 70 kcals/100 gm) and 6 were fed with their own mother's expressed breast milk. Mean gestational age was 30 weeks (range 27 to 34 weeks) and mean birth weight 1.29 kg (range 0.98 kg to 1.7 kg). The age on entry to the study ranged from 5 to 29 days and average weight gain was 17 g/kg.day (range 12-22 g/kg.day). In 65 study periods mean oxygen consumption was 7.0 (S.D. 1.5) mls/kg.min, mean carbon dioxide production was 8.1 (S.D. 1.1) mls/kg.min and mean respiratory exchange ratio was 1.20 (S.D. 0.27). The respiratory exchange ratio increased with postnatal age, due both to a decrease in oxygen consumption and an increase in carbon dioxide production. A steady-state respiratory exchange ratio above unity indicates that processes other than oxidative catabolism are contributing to gaseous exchange, such as the conversion of glucose to fat. Such processes must be taken into account when calculating a thermal equivalent for oxygen consumption.