SUMMARY It is generally accepted that low birthweight infants should be nursed at thermal neutral temperature-the environment in which oxygen consumption is at a minimum. Low birthweight infants do not, however, always show an increase in oxygen consumption at a temperature outside the neutral range, but react with a change in body temperature. We redefined therefore the neutral temperature for these infants as 'the ambient temperature at which the core temperature of the infant at rest is between 36&7 and 37 3°C and the core and mean skin temperatures are changing less than 0*2 and 0-3°C/hour respectively'. Using this definition, new guidelines of the neutral temperature have been made for healthy infants of 29-34 weeks' gestation. The neutral temperature during the first week of life is dependent on gestational age and postnatal age, whereas after the first week it depends on body weight and postnatal age. Using this definition and the guidelines, the appropriate environmental temperature for the individual patient can be chosen.Since the studies done by Budin in 1900,' it is known that maintaining the body temperature and therefore controlling the environmental temperature is important for the survival of low birthweight infants.Glass et al2 showed that the environmental temperature also influences growth. Although short exposures to cold stress has a facilitative effect on thermogenesis in newborns,3 it is generally accepted that these infants should be nursed in an environment that keeps heat loss at a minimum.4 The optimal ambient temperature at which the infant should be nursed is called the thermal neutral temperature and is defined as 'the range of environmental temperature within which the metabolic rate is at a minimum and within which temperature regulation is achieved by non-evaporative physical processes alone'.5Several guidelines for the neutral temperature have been published.69 These guidelines cannot, however, be used alone for setting the incubator temperature, as the temperature of the walls of the incubator,'0 humidity,' 12 and air velocity'3 are also important. Moreover, it is not known how the guidelines can be evaluated in individual patients, as it is impossible to measure metabolic rate continuously in each one. We defined the neutral 18 temperature in a more practical way and have drawn up new guidelines using this definition. Methods

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Total evaporative water loss, transepidermal as well as respiratory water loss was measured in 8 infants on day 1, 11 infants from day 2 to 8 and 8 infants after day 8. Measurements were performed at two levels of humidity, either vapor pressure of 16 or 25 mmHg (2 133 or 3 333 Pa). Evaporative water loss was 40% lower at the higher humidity. Neither metabolic rate nor body temperature showed a significant difference between the two levels of humidity. The effect of the change in humidity on the neutral thermal environment was calculated, the neutral temperature being 0.05 degrees C lower when the vapor pressure is increased by 1 mmHg (133.3 Pa). We conclude that a high humidity is of limited value in nursing infants born after 30-40 weeks.

The influence of sleep states on the metabolic rate and on the respiratory quotient (RQ) of low-birth-weight infants during continuous feeding was analyzed. Gestational age at birth varied between 29 and 35 weeks, postnatal age between 2 and 56 days and body weight between 0.81 and 2.11 kg. The mean oxygen consumption and carbon dioxide production were 10% higher, while the rise in SD was about 3-fold during REM sleep compared with NREM sleep. The RQ, however, was equal in both states. The sequence of the two sleep states did not have any influence on the analyzed parameters. Averaged over all the measurements, no statistically significant linear trend in oxygen consumption as a function of time could be found. However, small non-zero trends could be found, the direction of which appeared to depend on the sequence of the states.

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