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Servo-control for maintaining abdominal skin temperature at 36C in low birth weight infants

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Authors

John C Sinclair1

Background - Methods - Results - Characteristics of Included Studies - References - Data Tables & Graphs


1Departments of Pediatrics and Clinical Epidemiology and Biostatistics, McMaster University, Hamilton, Canada [top]

Citation example: Sinclair JC. Servo-control for maintaining abdominal skin temperature at 36C in low birth weight infants. Cochrane Database of Systematic Reviews 2002, Issue 1. Art. No.: CD001074. DOI: 10.1002/14651858.CD001074.

Contact person

John C Sinclair

Departments of Pediatrics and Clinical Epidemiology and Biostatistics
McMaster University
1200 Main Street West
Room 3N11F
Hamilton Ontario L8N 3Z5
Canada

E-mail: sinclair@mcmaster.ca

Dates

Assessed as Up-to-date: 14 April 2008
Date of Search: 06 April 2008
Next Stage Expected: 14 April 2010
Protocol First Published: Issue 2, 1998
Review First Published: Issue 2, 1998
Last Citation Issue: Issue 1, 2002

What's new

Date / Event Description
18 April 2008
Updated

This updates the review "Servo-control for maintaining abdominal skin temperature at 36C in low birth weight infants", published in the Cochrane Database of Systematic Reviews, Issue 4, 2004 (Sinclair 2004).

An updated search identified one new possibly eligible trial (Genzel-Boroviczeny 2007). This trial was not eligible for inclusion and has been added to excluded trials.

Conclusions unchanged.

11 April 2008
Amended

Converted to new review format.

History

Date / Event Description
17 November 2004
Updated

No new eligible trials

26 November 2001
New citation: conclusions changed

Substantive amendment

Abstract

Background

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.

Objectives

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.

Search methods

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.

Selection criteria

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.

Data collection and analysis

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.

Results

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.

Authors' conclusions

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.

Plain language summary

Servo-control for maintaining abdominal skin temperature at 36C in low birth weight infants

Low birth weight babies have a higher chance or survival if they are kept warm. The right conditions can be made by placing the baby in an incubator. The air can be heated to a desired temperature, or radiant heat lamps inside the incubator can adjust to the baby’s body temperature (servo-control). The review of trials found that keeping a baby’s skin temperature at 36C degrees by servo-control reduces the newborn death rate in low birth weight babies rather than setting a constant incubator temperature of 31.8C. More research is needed.

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Background

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.

Objectives

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.

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Methods

Criteria for considering studies for this review

Types of studies

Randomized or quasi-randomized controlled trials which test the effects of incubator heating servo-controlled from body temperature compared with control of incubator air temperature.

Types of participants

Low birth weight infants, enrolled in the early neonatal period.

Types of interventions

Servo-control of incubator heating, using the baby's body temperature as the end-point of thermal control, compared with air temperature control.

Types of outcome measures

Death
Death by postnatal age period
Death by birth weight group
Other important clinical outcomes

Search methods for identification of studies

This review used the search strategy of the Cochrane Neonatal Collaborative Review Group. Searches were made of the Oxford Database of Perinatal Trials, the Cochrane Neonatal Group's Neonatal Trials Register, the Cochrane Central Register of Controlled Trials (CENTRAL, The Cochrane Library, Issue 3, 2004) and MEDLINE. MEDLINE was searched for the period 1966 to 1997, and the search was updated in November 2001, June 2004 and April 2008, using the terms: exp temperature or body temperature or skin temperature or body temperature regulation, or oxygen consumption or exp energy intake, or incubator, infant or neonatal nursing or radiant (tw) and warm: (tw) and clinical trial and infant, premature or infant, low birth weight. Forward searches of Science Citation Index for citations of the two included trials, Buetow 1964 and Day 1964, were done for the period 1965 through 1998, and this was updated in June 2004, and again in April 2008.

Data collection and analysis

The standard method was used for conducting a systematic review, as described in the Cochrane Collaboration Handbook. Each included trial was assessed for blinding of randomization, blinding of the intervention, completeness of followup, and blinding of the outcome assessment. Categorical outcomes were analyzed using relative risk and risk difference, with their 95% confidence intervals. A fixed effect model was assumed for meta-analysis.

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Results

Description of studies

Two trials were found which met inclusion criteria: Buetow 1964 and Day 1964. The clinical details for each trial are given in the table, Characteristics of Included Studies. One additional possibly eligible trial was detected in the update of April 2008, but was excluded (Genzel-Boroviczeny 2007). This was a randomized cross-over trial in preterm infants which compared an incubator air temperature required to achieve an anterior abdominal wall skin temperature of 36.5C with that required to obtain a difference between abdominal wall and extremities of less than 2C. The reasons for exclusion of this trial are given in the Table of Excluded Studies.

Risk of bias in included studies

Day 1964 was a randomized trial, whereas Buetow 1964 was quasi-randomized in that allocation to the experimental group depended on the availability of the servo-controlled incubator at the time of the baby's admission. In addition, when twins were admitted to Buetow's trial, the first-born was systematically allocated to the servo-controlled incubator if it was available, and the second-born therefore entered the control group. For the trial as a whole, this resulted in imbalance between the two exposure groups for second-born twins, and they were excluded from the analysis in the report of Buetow and in this review. (For further details of trial quality and quality assessments, see table, Characteristics of Included Studies.)

Effects of interventions

Servo-control of abdominal skin temperature vs air control (Comparison 1)

Death (Outcome 1.1):

Among all babies enrolled, both Buetow 1964 and Day 1964 found that death rate tended to be lower in the group servo-controlled at 36C, but in neither trial did the difference reach statistical significance. However, the meta-analysis shows that servo-control at 36C substantially and statistically significantly lowers death rate, relative risk 0.72 (95% CI 0.54, 0.97); risk difference -12.7% (95% CI -1.6, -23.9).

Death (very low birth weight) (Outcome 1.2):

An analysis was done which was restricted to the lowest birth weight infants (Buetow, 1000-1500 g; Day 800-1599 g). Day found the death rate to be significantly reduced in these infants: relative risk 0.37 (95% CI 0.17, 0.83), whereas Buetow found a trend in the same direction which was not statistically significant. The meta-analysis shows that servo-control at 36C substantially and statistically significantly lowers death rate: relative risk 0.66 (95% CI 0.48, 0.90); risk difference -17.8% (95% CI -5.2, -30.3).

Time of death:

Buetow 1964 used life table statistics to analyze the effect of treatment group on time of death. It was noted that the greatest reductions in mortality occurred between the 2nd and 14th day of life.

Cause of death:

In both Buetow 1964 and Day 1964, the distribution of principal findings at necropsy was not different between the two exposure groups.

Other outcomes:

Buetow 1964 assessed infants' thermal stability after the servo-controlled incubator had been discontinued at 96 hours. Compared to the control infants, significantly more of the experimental group were able to maintain axillary temperatures of 35C or greater during the 5th, 6th and 7th days of life.

Discussion

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).

Authors' conclusions

Implications for practice

Servo-control of abdominal skin temperature at 36C reduces neonatal death rate in LBW infants, compared with air temperature control at 31.8C. During at least the first week after birth, LBW babies should be provided with a carefully regulated thermal environment. An abdominal skin temperature of 36C may be slightly below the temperature required for strict thermoneutrality in LBW babies. If thermoneutrality is the goal, incubator heating for such infants should be adjusted to maintain abdominal skin temperature at approximately 36.5C, using either servo-control or frequent manual adjustment.

Implications for research

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 low birth weight infants.

Potential conflict of interest

  • None noted.

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Characteristics of studies

Characteristics of Included Studies

Buetow 1964

Methods

Quasi-random allocation (dependent on availability of servo-controlled incubator). Blinding of allocation - no. Blinding of intervention - no. Complete follow-up - no. Blinding of outcome assessment - not relevant for death (all causes), can't tell for other outcomes.

Participants

181 very low birth weight newborn infants, birth weight range 1000-1500 g.

Interventions

Experimental group: servo-control of abdominal skin temperature at 36C using radiant heat, from admission for first 96 hours; then standard incubator providing ambient T 31.1-32.2C. Control group: standard incubator providing ambient T 31.1-32.2C.

Outcomes

Deaths from admission to day 28. Deaths by postnatal age interval. Cause of death. Temperature regulation after removal from servo-controlled incubator.

Notes

Second-born twins excluded from analysis: 5 of 94 experimental and 18 of 87 control infants were excluded on this basis.

Risk of bias table
Item Judgement Description
Allocation concealment? No

C - Inadequate

Day 1964

Methods

Randomized trial. Blinding of allocation - yes (for first part of study); no (for second of pair in last part of study). Blinding of intervention - no. Completeness of follow-up - can't tell. Blinding of outcome assessment - not relevant for death (all causes); can't tell for other outcomes.

Participants

125 low birth weight newborn infants weighing 600-1999 g. Later in the study admission was limited to those weighing 750-1749 g.

Interventions

Experimental group: servo-control of abdominal skin temperature at 36C using radiant heating, incubator air temperature 31.8C (range 31.1-32.2C); trial conditions maintained from admission until 1750 g or, if < 1750 g BW, until BW regained. Control group: incubator air temperature 31.8C (range 31.1-32.2C).

Outcomes

Death until hospital discharge. Duration of life among deaths. Cause of death. Axillary temperature range.

Notes

In first part of trial, randomization was done using table of random numbers, sealed envelopes; in second part of trial, coin toss was used, with next infant assigned to other group. Infants dying within 12 hours of birth were removed from the trial; number and numbers by group are not given.

Risk of bias table
Item Judgement Description
Allocation concealment? Yes

A - Adequate

Characteristics of excluded studies

Genzel-Boroviczeny 2007

Reason for exclusion

Outcomes limited to physiologic variables (vital signs, microvascular perfusion indices), measured very shortly (10 minutes) after reaching target thermal condition. No clinical outcomes assessed.

Horns 2002

Reason for exclusion

No clinical outcomes assessed.

Perlstein 1976

Reason for exclusion

Controls identified retrospectively.

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References to studies

Included studies

Buetow 1964

Buetow KC, Klein SW. Effect of maintenance of "normal" skin temperature on survival of infants of low birth weight. Pediatrics 1964;34:163-70.

Day 1964

Day RL, Caliguiri L, Kamenski C, Ehrlich F. Body temperature and survival of premature infants. Pediatrics 1964;34:171-81.

Excluded studies

Genzel-Boroviczeny 2007

Genzel-Boroviczény O, Seidl T, Rieger-Fackeldey E, Abicht J, Christ F. Impaired microvascular perfusion improves with increased incubator temperature in preterm infants. Pediatric Research 2007;61:239-42.

Horns 2002

Horns KM. Comparison of two microenvironments and nurse caregiving on thermal stability of ELBW infants. Advances in Neonatal Care 2002;2:149-60.

Perlstein 1976

Perlstein PH, Edwards NK, Atherton HD, Sutherland JM. Computer-assisted newborn intensive care. Pediatrics 1976;57:494-502.

Studies awaiting classification

  • None noted.

Ongoing studies

  • None noted.

Other references

Additional references

Bruck 1961

Bruck K. Temperature regulation in the newborn infant. Biology of the Neonate 1961;3:65-119.

Glass 1968

Glass L, Silverman WA, Sinclair JC. Effect of the thermal environment on cold resistance and growth of small infants after the first week of life. Pediatrics 1968;41:1034-46.

Glass 1969

Glass L, Silverman WA, Sinclair JC. Relationship of thermal environment and caloric intake to growth and resting metabolism in the late neonatal period. Biology of the Neonate 1969;14:324-40.

Jacobs 2007

Jacobs S, Hunt R, Tarnow-Mordi W, Inder T, Davis P. Cooling for newborns with hypoxic ischaemic encephalopathy. Cochrane Database of Systematic Reviews 2007, Issue 4. Art. No.: CD003311. DOI: 10.1002/14651858.CD003311.pub2.

Jolly 1962

Jolly H, Molyneux P, Newell DJ. A controlled study of the effect of temperature on premature babies. Journal of Pediatrics 1962;60:889-94.

Silverman 1958

Silverman WA, Fertig JW, Berger AP. The influence of the thermal environment on the survival of newly born premature infants. Pediatrics 1958;22:876-86.

Silverman 1966

Silverman WA, Sinclair JC, Agate FJ. The oxygen cost of minor changes in heat balance of small newborn infants. Acta Paediatrica Scandinavica 1966;55:294-300.

Other published versions of this review

Sinclair 1992

Sinclair JC. Management of the thermal environment. In: Sinclair JC, Bracken MB, editor(s). Effective Care of the Newborn Infant. Oxford: Oxford University Press, 1992:40-58.

Sinclair 2000

Sinclair JC. Servo-control for maintaining abdominal skin temperature at 36C in low birth weight infants. Cochrane Database of Systematic Reviews 2000, Issue 1.

Sinclair 2002

Sinclair JC. Servo-control for maintaining abdominal skin temperature at 36C in low birth weight infants. Cochrane Database of Systematic Reviews 2002, Issue 1. Art. No.: CD001074. DOI: 10.1002/14651858.CD001074.

Sinclair 2004

Sinclair JC. Servo-control for maintaining abdominal skin temperature at 36C in low birth weight infants. Cochrane Database of Systematic Reviews 2004, Issue 4. Art. No.: CD001074. DOI: 10.1002/14651858.CD001074.

Classification pending references

  • None Noted.

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Data and analyses

1 Servo-control of abdominal skin temperature vs air control

Outcome or Subgroup Studies Participants Statistical Method Effect Estimate
1.1 Death 2 283 Risk Ratio (M-H, Fixed, 95% CI) 0.72 [0.54, 0.97]
1.2 Death (very low birth weight) 2 228 Risk Ratio (M-H, Fixed, 95% CI) 0.66 [0.48, 0.90]

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Sources of support

Internal sources

  • McMaster University, Canada

External sources

  • National Institute of Child Health and Human Development, USA

This review is published as a Cochrane review in The Cochrane Library, Issue 3, 2008 (see http://www.thecochranelibrary.com External Web Site Policy for information). Cochrane reviews are regularly updated as new evidence emerges and in response to feedback. The Cochrane Library should be consulted for the most recent version of the review.