Insensible water loss (IWL) and metabolic rate (MR) were measured simultaneously in three groups of low birthweight (LBW) infants in a special incubator with air and wall temperature separately controlled as to keep the infants in their neutral thermal environment. Group I infants (n = 7) had a birthweight of 1.125 ± 0.192 kg and a gestational age of 20.0 ± 0.6 wk; group II infants (n = 8) had a birthweight of 1.730 ± 0.440 kg and a gestational age of 33.0 ±1.6 wk, and group III infants (n = 7) had a birthweight of 1.960 ± 0.230 kg and a gestational age of 38.3 ± 1.9 wk (small for dates).
In group I infants, IWL decreased significantly (P < 0.05) between the 1st and the 2nd wk of life from 54.1 ± 11.2 to 42.7 ± 12.7 ml·kg·d-1, whereas MR increased significantly (P < 0.01) from 48.8 ± 6.7 to 57.5 ± 8.0 kcal·kg·d-1. In group II infants, IWL in the 2nd wk of life was not significantly different from IWL in the 1st wk of life which was 26.3 ± 5.6 ml·kg-1·d-1, MR increased significantly (P < 0.001) from 45.7 ± 6.2 kcal·kg-1·d-1 in the 1st wk of life to 55.1 ± 6.3 kcal·kg-1·d-1·d-1 in the 2nd wk of life. In group III infants, IWL increased significantly (P < 0.05) between the 1st and 2nd wk of life from 23.7 ± 7.5 to 31.9 ± 9.4 ml·kg-1·d-1 with a concomittant increase in MR (P < 0.01) from 48.8 ± 7.1 to 61.0 ± 8.4 kcal·kg-1·d-1.
It is concluded that very LBW preterm infants in a neutral thermal environment have an increased evaporation through the skin resulting in an IWL that is much higher than expected from their MR, and that minimization of heat loss by radiation and convection is necessary to keep the infants in thermal equilibrium without a fall in body temperature and an increase in heat production.
Separate air and wall heating and extremely low air velocity may be important features of incubators to be designed for very LBW preterm infants.