Temperature: NICU Handbook

Edward F. Bell, MD
Peer Review Status: Internally Peer Reviewed

The normal axillary temperature of a newborn infant, if correctly measured for 5 minutes, is from 36.5 to 37.4°C (mean + 1.5 S.D.). This range should be applied to both the term and premature infant. If measured for a shorter period (less than 5 minutes), the normal range is somewhat lower but has not been clearly defined. In order to avoid the incorrect diagnosis of hypothermia, an axillary temperature below 36.5°C should only be recorded and reported to the physician if read after the thermometer has been held in place for a full 5 minutes.

If the axillary temperature is above 37.4°C (or rectal temperature above 37.5°C), it is important to decide whether the infant has a fever or environmental hyperthermia. With fever, as seen sometimes with infection, the hypothalamic set point is elevated and the infant uses his thermoregulatory control systems to maintain a high body temperature. In some situations this high temperature may help to fight the infection. A febrile infant often has peripheral vasoconstriction with cool skin and extremities.

An infant with a fever may be difficult to cool; in fact, it may be undesirable to lower axillary temperature to normal if the skin is already cool. First, it is important to record both skin (probe) and axillary temperature of a febrile infant as well as the air temperature for the infant in an incubator. If the skin temperature is 36.0 to 36.5°C and the axillary (or rectal) temperature is 38.0°C or less, the thermal environment should not be changed. If necessary, the skin temperature set point can be lowered to 35.5 or even 35.0°C in order to keep the axillary temperature below 38.0°C. If a skin temperature of less than 35.0°C is required to keep the axillary temperature below 38.5°C, the fellow or staff neonatologist should be consulted.

The infant with environmental hyperthermia should be cooled to a normal axillary temperature by decreasing environmental heating or reducing thermal insulation (blankets or clothing). A bath or cooling mattress should not be necessary to correct hyperthermia caused by environmental overheating.

An infant with hypothermia can be rewarmed by a radiant heat or an incubator with higher air temperature. The simplest way to avoid overheating during rewarming with a radiant warmer is to use the skin temperature servocontrol with a set point of 36.5°C. The rate of rewarming is probably not critical.

Reference

Mayfield SR, Bhatia J, Nakamura KT, Rios GR, Bell EF. Temperature measurement in term and preterm neonates. J Pediatr 1984;104:271-275.

Edward F. Bell, MD
Peer Review Status: Internally Peer Reviewed

Background

Careful attention to providing the best possible thermal environment increases the chance of survival and the quality of outcome, particularly in the small premature infant.

Servocontrol is an electronic feedback system which functions as a thermostat to maintain a constant temperature at the site of a thermistor probe (usually on the skin over the abdomen) by regulating the heat output of an incubator or radiant warmer. Maintaining a constant abdominal skin temperature between 36.0 and 36.5ÁC is the simplest way to provide a "thermoneutral" environment, minimizing the number of calories needed to maintain normal body temperature and reducing the risks of cold stress or overheating.

Although either skin or air temperature control can be used safely for most infants, skin temperature servocontrol is probably better for very young, small (below 1500 g) infants because the desired control temperature is more easily determined. Servocontrol is the only acceptable method of heat regulation for the infant cared for under a radiant warmer.

Guidelines

The following guidelines apply to both the incubator and radiant warmer:

1. Insert probe plug securely into hole in heater unit.
2. Choose the desired abdominal skin temperature, usually 36.5ÁC. Some older infants will require a lower set point, e.g., 36.0ÁC to avoid overheating.
3. Check the setting of the control panel. Adjust if necessary.
4. Attach the probe to the exposed abdominal skin at mid-epigastrium, halfway between the xiphoid and the umbilicus. If the infant is prone, attach the probe to the skin over either flank (not between the scapulae). The probe should not be placed in the axilla.
5. Under the radiant warmer, protect the probe with a foil-backed shield.
6. Read the skin temperature from the temperature gauge on the heater unit. If it registers below the set point (36.5ÁC), the heater should be on. Check the heater indicator light or dial. If the heater is not on, check all connections.
7. If the skin temperature does not rise as quickly as you think it should, make sure the heater is on and WAIT. Increasing the set point will not cause faster warming.
8. When the abdominal skin temperature reaches the chosen set point, check the axillary or rectal temperature to be sure it is within the normal range (36.5 to 37.4ÁC).
9. Adjust the set point slightly if the axillary (or rectal) temperature is abnormal. Do not change the set point if the axillary (or rectal) temperature is normal.
10. Check frequently to be sure the probe is in solid contact with the skin. Poor contact will cause overheating. Entrapment of the probe under the arm or between the infant and mattress will cause underheating
11. Record incubator air temperatures along with infant skin and axillary (or rectal) temperatures. A clearly decreasing (or increasing) trend in incubator temperature may indicate the development of sepsis or a neurological problem.

Edward F. Bell, MD
Peer Review Status: Internally Peer Reviewed

Radiant warmer to incubator:

Most newly-admitted infants are cared for on radiant warmer beds in order to provide accessibility for resuscitation or procedures without jeopardizing thermal stability. As long as an infant remains critically ill and is likely to require resuscitation or frequent procedures, he should be kept on a warmer bed. Most very small infants (<1000 grams) can be kept warm more easily on a radiant warmer than in an incubator, since incubator air temperature drops rapidly when the door is opened. If a very small infant has difficulty maintaining normal body temperature on a radiant warmer, plastic food wrap can be stretched across the bed (from side to side). This reduces the movement of cool air over the baby's body surface.

When the condition has stabilized so that frequent procedures are not likely to be needed, an infant can be moved to a preheated incubator, on skin temperature servocontrol. Axillary temperature should be checked 30 minutes after moving the baby to the incubator, and every hour thereafter for four hours. The very small infant is at greatest risk of heat loss through an open incubator door. Whenever possible, procedures performed on a baby in an incubator should be performed through the ports (diaper change, vital signs, phlebotomy, etc.). Any infant who weighs less than 1000 grams should be cared for on a radiant warmer or in a servocontrolled double-walled incubator. If an infant consistently requires an air temperature above 37° C, it may be necessary to operate the incubator in the air temperature control mode to avoid periodic increases in air temperature to above 38° C, which may cause the heater to stop completely. Some infants who require a radiant warmer for temperature support may not need routine vital signs as often as other infants under radiant warmers. The frequency of vital signs may be reduced at the discretion of the infant's nurse and physician.

Incubator to bassinet:

If an infant has been maintained in an incubator operated by skin temperature servocontrol, the incubator should be changed to air temperature servocontrol in the following manner, before attempting the move the infant to a bassinet.

• Change to air temperature servocontrol setting the control temperature to equal the average incubator air temperature during the previous 24 hours (from the nursing notes).
• Check the baby's axillary temperature in 30 minutes and each hour for four hours.
• If the axillary temperature remains normal (36.5 to 37.4°C), disconnect and remove the skin probe if desired.

When an infant reaches 1700 to 1800 grams, has no respiratory distress and only occasional apnea, and has been stable in an incubator operated in the air temperature control mode with air temperature 32°C or less, an attempt can be made to move him to a bassinet as follows:

• Dress the infant in shirt and diaper and wrap him in a single blanket. Turn the air temperature control to 28°C.
• Check the baby's temperature in 30 minutes and each hour for four hours.
• If the baby's axillary or rectal temperature drops to below 36.5°C, reheat him in the skin temperature servocontrol mode until his skin temperature is 36.0°C and return to manual or air servocontrol mode as described above (try again in two or three days).
• If the baby's temperature is stable for eight hours, bundle him in extra blankets and move him to a bassinet.
• Check body temperature in 30 minutes and each hour for four hours; if the axillary or rectal temperature drops to below 36.5°C, return infant to incubator, reheat on skin temperature servocontrol as described above, then revert to air temperature control; try again in two or three days.

Reference

Bell EF. Infant incubators and radiant warmers. Early Hum Dev 1983;8:351-375.