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A radiant hood warmer, a device that heats the incubator roof independently of the incubator's main heat source, was used to study the thermal balance of 11 full term and 13 preterm (gestational age 25-34 weeks) infants exposed to an isolated elevation of incubator roof temperature at stable ambient air temperature and humidity. After initial measurements without active heating of the incubator roof, the hood warmer was set to 33 degrees C, 36 degrees C and finally (preterm infants only) to 39 degrees C. At least 18 min of measurements with the infant asleep were made at each hood warmer setting. In the term infants an increase in roof temperature from 30.5 degrees C to 35.6 degrees C resulted in an increase in skin temperature from 35.4 to 35.9 degrees C, and a decrease in radiative heat loss from 32.8 to 20.7 W/m2 exposed skin. In the preterm infants an increase in roof temperature from 31.0 to 38.4 degrees C led to an increase in skin temperature from 35.7 to 36.3 degrees C and a decrease in radiative heat loss from 34.1 to 13.0 W/m2 exposed skin. The increased inner roof surface temperature did not affect evaporative or convective heat loss, skin blood flow, respiratory water loss, oxygen consumption or transepidermal water loss in either group. Thus, at stable ambient air temperature and humidity, the increase in incubator roof temperature resulted in an increase in skin temperature and a decrease in radiative heat loss in both term and preterm infants.

The rate of evaporation from the skin (g/m2/h) was measured in 12 full-term and 16 preterm infants (gestational age 25-34 wk) both during incubator care and when nursed under a radiant heater. The method for evaporation rate measurement is noninvasive and based on determination of the water vapor pressure gradient close to the skin surface. Measurements were first made with the infant nursed in an incubator with a controlled environment with respect to humidity, temperature, and air velocity. The measurements in the term infants were performed at an ambient relative humidity (RH) of 50%, and in the preterm infants first at 50% and subsequently at 30-40%. Evaporation rate was then measured with the infant nursed under a radiant heater. In term infants, mean evaporation rate was 3.3 g/m2/h during incubator care (RH 50%) and 4.4 g/m2/h during care under the radiant heater. In preterm infants, the corresponding values were 15.5 g/m2/h in the incubator at RH 50%, 16.7 g/m2/h at RH 30-40%, and 17.9 g/m2/h under the radiant heater. It is concluded that the evaporative water loss from the skin depends on the ambient water vapor pressure, irrespective of whether the infant is nursed in an incubator or under a radiant heater. The higher rate of evaporation during care under a radiant heater is due to the lower ambient water vapor pressure and not to any direct effect of the nonionizing radiation on the skin.

Respiratory water loss, oxygen consumption, carbon dioxide production and skin blood flow were measured continuously in nine full-term infants on the first day after birth. After at least 18 min of measurements with the infant asleep in an incubator, with an air temperature of 33 degrees C and a relative humidity of 50%, the temperature of the incubator air was lowered to less than 27.5 degrees C. This resulted in a significant decrease in skin temperature and peripheral skin blood flow, while the infant's core temperature remained unchanged. At the same time, mean respiratory water loss increased from 3.7 to 6.1 mg/kg.min, which can be explained partially by the decrease in ambient humidity that accompanied the decrease in air temperature. In addition, mean oxygen consumption increased from 5.3 to 7.9 ml/kg.min and mean carbon dioxide production increased from 3.8 to 5.9 ml/kg.min. There was no concomitant increase in motor activity. Thus, when the newborn infants were exposed to cool air, they reacted with an increase in respiratory water loss, oxygen consumption and carbon dioxide production before their core temperature was affected and without increasing their motor activity.