Randomized trials have shown that the neonatal mortality rate of low birth-weight babies can be reduced by keeping them warm. For low birth-weight babies nursed in incubators, warm conditions may be achieved either by heating the air to a desired temperature, or by servo-controlling the baby's body temperature at a desired set-point.

In low birth weight infants, to determine the effect on death and other important clinical outcomes of targeting body temperature rather than air temperature as the end-point of control of incubator heating.

Standard search strategy of the Cochrane Neonatal Review Group. Searches were made of the Cochrane Central Register of Controlled Trials (CENTRAL, The Cochrane Library, Issue 3, 2004) and MEDLINE, 1966 to April 2008.

Randomized or quasi-randomized trials which test the effects of having the heat output of the incubator servo-controlled from body temperature compared with setting a constant incubator air temperature.

Trial methodologic quality was systematically assessed. Outcome measures included death, timing of death, cause of death, and other clinical outcomes. Categorical outcomes were analyzed using relative risk and risk difference. Meta-analysis assumed a fixed effect model.

Two eligible trials were found. In total, they included 283 babies and 112 deaths. Compared to setting a constant incubator air temperature of 31.8C, servo-control of abdominal skin temperature at 36C reduces the neonatal death rate among low birth weight infants: relative risk 0.72 (95% CI 0.54, 0.97); risk difference -12.7% (95% CI -1.6, -23.9). This effect is even greater among VLBW infants.

During at least the first week after birth, low birth weight babies should be provided with a carefully regulated thermal environment that is near the thermoneutral point. For LBW babies in incubators, this can be achieved by adjusting incubator temperature to maintain an anterior abdominal skin temperature of at least 36C, using either servo-control or frequent manual adjustment of incubator air temperature.

Survival of low birth weight infants is increased by keeping them warm. The question of "how warm" has been addressed in randomized trials which compared a higher vs lower incubator air temperature. Silverman 1958 compared the effects on survival of a warmer condition (air temperature 31.7C, range 31.1-32.2) versus a cooler condition (air temperature 28.9C, range 28.3-29.4). Survival of low birth weight infants was enhanced in the warmer condition. Jolly 1962 found similar results.

The warmer conditions tested in these trials were still below the range of "thermoneutral" air temperatures for newborn infants, which Bruck 1961 determined to be 32-34C. Moreover, the body temperatures of the infants comprising Silverman's warmer group, especially of those under 1500 g birth weight, were lower than the body temperatures which Silverman 1966 found to be associated with minimal rates of oxygen consumption. Thus, it remained possible that even warmer or more finely regulated thermal conditions might further improve survival of low birth weight infants.

It became possible to test this hypothesis with the advent of a new incubator which incorporated radiant heat lamps whose output could be servo-controlled from the infant's skin temperature.

In low birth weight infants, to determine the effects on death and other important clinical outcomes of targeting body temperature rather than air temperature as the end-point of control of incubator heating.

Criteria for considering studies for this review

Search methods for identification of studies

Data collection and analysis

Description of studies

Risk of bias in included studies

Effects of interventions

The two trials which qualified for inclusion in this review were closely similar as regards the participants who were enrolled and in the thermal exposures which were tested. The meta-analysis of the results shows a very important reduction in death rate of low birth weight infants attributable to servo-control of abdominal skin temperature at 36C. The effect is particularly important in very low birth weight babies during the early neonatal period. The mechanism of the thermal effect in reducing death rates was not uncovered in these trials. The causes of death were not different between the two exposure groups.

No adverse biologic consequences of the close-to-thermoneutral exposure achieved by servo-control at 36C were noted in these trials. Buetow 1964 assessed temperature regulation of infants after their removal at 96 hours from the servo-controlled condition and found that they were actually better than the control infants at maintaining axillary temperatures of >35C during days 5-7. However, it should be noted that Glass 1968 and Glass 1969 showed that more prolonged (2-week) exposure of VLBW infants to a servo-controlled thermoneutral environment resulted in a reduction in their subsequent thermal stability in the face of a mild cold stress. The early and late effects of providing graded cold stimulation during the late neonatal period (i.e. after the time when the survival advantage of thermoneutrality has been shown) should be addressed in future trials. In particular, the possible effect of postnatal thermal stimulation of brown fat metabolism on later diet-induced thermogenesis should be investigated.

The particular incubators which were tested in these trials, which supplied supplementary heat using servo-controlled radiant heat lamps mounted on top of the incubator hood, are no longer being manufactured. Accidental dislodgement of the skin thermister used in servo-control can result in accidental overheating of the baby - a particularly serious problem when the servo-controlled heat source is a powerful radiant heater. Presently available incubators which incorporate servo-control use convective heating. Since there is no evidence that the route of heat transfer is itself critical in achieving thermoneutrality, the survival advantage shown with the particular incubators used in these trials can be expected to occur also with servo-controlled convective heating.

It should be noted that the set-point of 36C abdominal skin temperature which was chosen in these trials may be still slightly below the temperature required for strict thermoneutrality. Silverman 1966 estimated that minimal rates of oxygen consumption in VLBW infants occur when abdominal skin temperature is regulated at between 36-37C. When servo-control incubators are used in present-day care of VLBW infants in the first days of postnatal life, an abdominal skin temperature of 36.5C is recommended as the set-point to achieve thermoneutrality.

The possible benefits of "therapeutic" hypothermia in specific clinical circumstances were not investigated in the trials reviewed here. Cooling for term newborns with moderate or severe post-asphyxial hypoxic-ischemic encephalopathy has been investigated in recent trials. In this specific population, cooling has been shown to reduce both the death rate, and major neurodevelopmental disability among survivors (Jacobs 2007).

Implications for practice

Implications for research

Buetow 1964

Day 1964

Genzel-Boroviczeny 2007

Horns 2002

Perlstein 1976

Buetow 1964

Day 1964

Genzel-Boroviczeny 2007

Horns 2002

Perlstein 1976