Partitioning of heat losses and gains in premature newborn infants under radiant warmers.

The partition of heat loss into convective and evaporative components, and heat gain into metabolic rate of production and radiant heat needed to maintain thermal equilibrium was determined in ten premature neonates (weight 1.39 +/- .08 [SEM] kg, gestation 31 +/- 1 weeks) who were nursed naked and supine on open radiant warmer beds. Warmer beds were servocontrolled to maintain each infant's abdominal skin temperature at three different levels: 35.5, 36.5, and 37.5 degrees C. The quantity of radiant heat delivered by the warmer in vivo was measured directly and compared with the heat need calculated from the partition. Convective heat loss comprised the major component of net heat loss and increased significantly with servocontrol temperature from 2.86 +/- .24 to 3.27 +/- .23 kcal/kg/h (P less than .01), and to 3.72 +/- .26 kcal/kg/h (P less than .001). Evaporative heat loss increased with servocontrol temperature from .96 +/- .13 to 1.41 +/- .33 kcal/kg/h, and to 1.35 +/- .32 kcal/kg/h, but this increase was not significant. Metabolic rate decreased from 2.08 +/- .17 to 1.90 +/- .14 kcal/kg/h, and to 1.78 +/- .16 kcal/kg/h with increased servocontrol temperature, but this decrease was not significant. Radiant heat needed to maintain infants at higher temperatures increased from 1.73 to 2.80 kcal/kg/h, and to 3.32 kcal/kg/h. The radiant heat delivered by the warmer to infants was directly proportional to the heat need calculated from the partition (r = .68, P less than .001).