NEONATAL THERMOREGULATION

Cold stress and hyperthermia may have serious consequences for all newborns. In small for gestational age and preterm infants (<2500 g) these consequences may be devastating and may increase both morbidity and mortality rates. The more premature an infant, the more susceptible to even the slightest alteration in environmental temperature. Yet all infants need to maintain specific thermal control in order to survive. Several factors lead to increased heat losses in the newborn infant. The neonate has a large skin surface area-to-body weight ratio, which increases heat and fluid loss. The fluid loss from the skin results in massive heat loss. The thin skin with blood vessels that are near the surface provides poor insulation, leading to further heat loss. Careful attention must be paid to the thermal environment from the moment of birth to the time they are capable of temperature regulation.

Prior to delivery infants are in an environment that maintains a stable body temperature. At the time of delivery, body temperature changes rapidly. The infant’s temperature may drop several degrees as he passes from a warm, protected uterine environment to that of a cool delivery room. Nursing or medical care for the infant may inhibit or delay his being warmed. Maternal analgesia that is transferred across the placenta prior to delivery may slow the infant’s metabolism enough to inhibit his ability to generate heat during the first few days of life.

Simple procedures such as vital signs, assessment, and diaper changes may place infants at risk for losing body heat. Frequent or prolonged heat-losing episodes in infants who have limited heat producing and conserving resources may lead to cold stress. Cold stress in turn may lead to physiologic changes that can compromise the infant. Protecting babies against heat loss improves survival and decreases metabolic demands (calorie expenditures).

Maintenance of an optimal thermal environment is considered one of the most important aspects of effective neonatal care and is basic to daily practice. The American Academy of Pediatrics (AAP) recognizes the importance of thermoregulation in neonatal resuscitation and includes it as part of the process of resuscitation.

Temperature measurement instruments include single-use paper thermometers, glass and mercury thermometers, and a variety of electronic thermometers. Each method is satisfactory for accurate temperature measurement when used correctly. Rectal temperature historically has been considered the most accurate measurement of core body temperature. The core temperature does not decline until the infant has lost the ability to produce heat. Axillary or skin surface temperatures may be lower than rectal temperatures by as much as 1ºC, but there is generally a difference of less than 0.4°C. Skin temperature readings taken over the site of large brown fat stores may yield a falsely high reading, because these skin areas tend to remain warmer. High evaporative losses may produce falsely high readings in abdominal skin temperature measurements. Inguinal site temperature measurement may be more closely aligned to rectal temperature and provide a less traumatic site for core temperature measurement. Infrared tympanic thermometers are used because the ear canal is a highly vascular region whose blood perfusion is the same as that which perfuses the hypothalamic region, the area responsible for temperature control. Temperature readings are obtained by placing a small probe into the ear canal, which should approximate the core temperature. When the tympanic thermometer is used correctly, there is only about a 0.3ºC to 0.5ºC difference compared with an axillary temperature, which is lower.

A change in measured temperature may not occur until the infant has lost the ability to generate heat. The infant may display subtle signs of distress:

  Tachycardia
   
  Tachypnea
   
  Short-term response: changes in behavior and response
   
  Long-term responses: poor growth patterns and behavioral changes

In cold stress, Tachypnea results from an increased need for oxygen as the result of an increase in metabolism. In heat stress, the infant becomes tachypneic to increase expiratory heat losses.

Assessing temperature fluctuations over a period of time, using the same method of assessment is more important in evaluating thermoregulation than the actual temperature value at one point in time. Assessing the infant for other factors, such as growth, oxygen needs, and feeding tolerance also contributes to the determination of appropriate thermal control. This assessment is done by reviewing growth charts, FiO2 needs, feedings, and emesis. Changes in growth patterns have frequently been overlooked as indicators of temperature instability. Energy demands for temperature control take precedence over the demands for growth. The infant who is experiencing slow weight gain or erratic growth patterns may be experiencing poor thermal control.

Humans are homotherms; capable of maintaining body temperature at a relatively constant level despite changes in the external environment. The ability of infants to regulate temperature in response to thermal stress is limited. Infants are unable to sweat in order to give off excessive heat when they become overheated.

The infant is capable of heat production through three mechanisms 1) voluntary muscle activity, 2) involuntary muscle activity, and 3) metabolism. Voluntary and involuntary muscle activity is limited and requires a chemical reaction utilizing large stores of energy. Term infants are capable of assuming a flexed position when cool and an extended position when overheated. The ability is limited in the premature infant, though it may be present to some extent.

Non-shivering thermo genesis appears to be the most consistent method of heat production in the neonate regardless of gestational age or birth weight. The major source of heat energy in the newborn is fatty acids. Thermo genesis is directly dependent on tissue oxygenation to utilize heat energy. Oxidized fatty acids generally are believed to derive from brown fat stores in the neonate.

Brown fat has high vascularization and is virtually nonexistent in preterm infants. Term infants have approximately 16 percent of body tissue mass as adipose tissue, but the preterm infant may have as little as 3.5 percent adipose tissue per body weight. Brown fat is located around the mediastinal structures, kidneys, scapulas, axilla and nape of the neck. Primitive brown cells first appear at 26-30 weeks gestation and ordinarily disappear by three to five weeks after birth.

Upon exposure to cold, thermal receptors in the skin (many of which are located in the face) signal the neonate’s central hypothalamus resulting in sympathetic nervous system arousal and the release of norepinephrine. The release of norepinephrine then stimulates the hydrolysis or breakdown of the brown fat. The rapid metabolism of brown fat produces heat, which warms the blood perfusing surrounding tissue. This heat is then transferred via the circulation to the rest of the body. This process consumes a lot of oxygen and glucose.

Asphyxia and hypoxia further compromise the infant’s ability to generate heat. Utilizing energy to produce heat requires an increase in oxygen consumption. In the hypoxic state, two molecules of adenosine triphosphate (ATP) are generated from a molecule of glucose instead of 38 molecules of ATP generated in the normally oxygenated infant. In order to produce heat energy in the hypoxic state, greater glucose stores must be utilized. Without sufficient oxygenation, asphyxiated or hypoxic infants have a decreased ability to generate heat. When the infant with already limited resources for heat production encounters environmental changes that threaten his ability to maintain an adequate temperature, a serious condition exists.

The metabolic rate gradually increases during the first week of life. Heat production also improves during the first few days of life with the institution of feedings. It is not clear why heat is produced. It may be due to increased metabolism during digestion, or it may be that heat can be generated when sufficient energy is provided via ingestion. Ingestion of human milk has been found to increase metabolism in low birth weight infants, leading to production of heat. Thermoregulatory needs gradually change as the infant grows, matures and feeds.

There are four mechanisms by which heat is lost 1) conduction, 2) convection, 3) radiation, and 4) evaporation. Conduction is the transfer of heat from one object to another by direct contact. The infant may lose heat from internal organs to the skin’s surface and from there to cool surfaces with which he is in contact. Immediately after birth, an infant placed on a surface that has not been pre-warmed will transfer heat to that surface. In order to prevent conductive heat loss, a scale, or resuscitation bed should be always have a pre-warmed blanket between its surface and the infant. Heat loss can continue to occur after the infant has been placed in a pre-warmed incubator if he is placed on cool x-ray plates, for example, or is changed to linens that have not been pre-warmed.

Convection occurs as heat is transferred from an object to the surrounding air. If the object is warmer than the surrounding air, heat will be given up to the atmosphere. If environmental temperatures exceed the temperature of the object, the object will gain heat. Convective heat losses occur as blood travels through the body and comes to the skin’s surface. As air currents pass over the thin skin surface, heat is given up to the environment. Convective heat is also lost via the respiratory tract as air and heat are expired. Heat loss can occur even when the infant is placed under a radiant warmer. A cool room or a high degree of air velocity may affect the efficiency of the warming bed. Another major source of heat loss through convection involves the use of oxygen. A cool gas flow over the infant’s face and head will cause heat losses that will not be readily apparent in the core temperature. Because the infant’s face and head are especially sensitive to cool air, only warmed, humidified oxygen should be used.

Radiation is the transfer of heat between two objects that are not in direct contact with each other. Heat is transferred from the warmer to the cooler object. Even an infant in an isolette has many opportunities for heat loss. Cool incubator walls are a large source of radiant loss. Incubators placed near air conditioner vents or cool windows or in drafts add to the potential heat loss. Double-walled incubators, infant heat shields, and radiant heaters over the incubator have demonstrated some success at decreasing radiant heat loss.

Evaporation losses occur as moisture from body surfaces is lost to the environment. At the time of delivery, the infant should be dried immediately to prevent rapid heat loss. Wrapping the infant in plastic can decrease the amount of evaporative loss. The smaller the infant and the lower the gestational age, the larger the evaporative heat losses. Evaporative losses can also occur through the respiratory tract. To decrease this heat loss, warmed, humidified oxygen should be used when supplemental oxygen is needed. The upper airway is sensitive to cold and may lower core body temperature. The infant who is tachypneic is at a greater risk of heat loss.

Heat gaining mechanisms include conduction, convection and radiation. Conduction as a source of heat gain for the infant is minimal. Warmed surfaces may prevent heat loss, but they are not efficient in providing heat. This important source of heat stabilization should not be ignored because prevention of heat loss is as important as heat gain for the infant. It should be used with caution to prevent thermal burns at relatively low temperatures.

Convection is one of the most common sources of heat gain. Convection incubators are frequently used as the easiest and safest method of maintaining a neutral thermal environment. Humidity is often provided as an adjunct to convection warming because it decreases the infant’s evaporative losses and allows him to maintain his temperature at a lower ambient temperature. Difficulties may arise if monitoring (using either a skin or air servomechanism) is not continuous. Although convection is a fairly efficient method of providing heat for the infant, he is unable to regulate heat gain and may become overheated in a short time.

Radiant warmers are a common way of providing heat to the infant. These are powerful, provide heat quickly and can be controlled via skin or rectal probe. A drawback to their use is the increase in evaporative losses experienced by the infant but can be buffered by using a clear plastic blanket. Although it restricts the caregiver in providing care, the plastic film allows complete visualization of the infant at all times. Under a radiant warmer, access to the infant can be maintained without reducing ambient temperature.

The use of an incubator heated by either convection or radiation has been shown to be as effective as a radiant warmer in achieving a neutral thermal environment. The percentage of humidity in the environment plays a role in determining ambient temperature in the incubator. Evaporative losses are minimized when humidity is kept between 50 and 80 percent. Low ambient humidity requires higher ambient temperature levels to maintain infant skin temperature between 36 and 37ºC. When humidity is low, radiation heat loss may be low in comparison to evaporation losses because the high ambient temperatures needed to maintain skin temperature warm the incubator walls. Use of an incubator in combination with high humidity can be effective in maintaining stable temperatures at lower ambient temperatures.

The effects of cold stress can be detected in all aspects of body functioning. An infant responds to cold stimulus with increased oxygen consumption, glucose utilization, and acid production. The prevention of cold stress is essential in protecting the infant from multisystem stress. The cardiovascular and respiratory systems manifest the most obvious symptoms:

	Peripheral vasoconstriction occurs to conserve heat.
	As central blood volume increases to compensate, pulse and blood pressure increase.
	Once central cooling has occurred, diuresis may result with a decline in pulse and blood pressure leading to decreased cardiac output.
	Arrhythmias may occur secondary to acidosis as fatty acids break down to generate heat.

The CNS can be affected by cold stress secondary to cardiovascular changes. With peripheral vasoconstriction the following occur:

	Cerebral blood flow diminishes
	Metabolic activity is compromised
	Electroencephalographic activity may decline
	Peripheral nerve conduction may also be delayed
	Pupils may become fixed and dilated

Metabolic response to cold stress encompasses fluid, electrolyte, and glucose aberrations:

	In the early stages, diuresis occurs
	If cold stress continues, glomerular filtration declines along with the reabsorption of sodium, water, and glucose
	Hypoglycemia occurs
	Metabolic rate rises
	Unstable glucose levels can lead to further acidosis and neurologic changes
	As the release of nonesterified fatty acid increases, the liver slows metabolism of glucose, inhibiting thermo genesis
	As liver function declines, drugs are metabolized and excreted more slowly
	Acidosis develops as tissue perfusion declines, lactic, pyruvic, and organic acids build
	Enzymatic action within the kidneys is blocked, preventing acid-base regulation through a diminished excretion of hydrogen ions
	Fluid balance is further complicated by poor gastrointestinal absorption and decreased peristalsis
	Acidosis continues with an increase in dissociation of the indirect bilirubin from albumin-binding sites
	An increase in nonesterified fatty acids is caused by their high affinity for the albumin-binding sites
	In the presence of high levels of nonesterified fatty acids, kernicterus can occur with relatively low bilirubin levels
	There is a risk for bleeding and thrombocytopenia because clotting factors may be altered
	An increase in hematocrit and viscosity of the blood, secondary to fluid shifts away from vascular space may be noted
	Lethargy may occur
	Refusal to eat may be noted
	Respirations become slow and shallow
	Cry is weak
	As the condition continues, edema or sclerema may occur

Hypothermia presents when the infant’s temperature drops below 36.3ºC in term infants and 36.5ºC in preterm infants. This occurs when the infant’s own attempts of trying to generate their own heat have failed. Severe cold stress can present with respiratory distress, hypotension, and hypoxia.

Maintenance of temperature stability should be focused on preventative measures. At delivery the baby should be dried thoroughly with pre-warmed blankets and those wet blankets removed. A cap should be placed on the baby’s head and the baby should be placed in skin to skin contact with mother and cover both with warm blankets. If a radiant warmer is used it should be pre-heated. Scales should be covered with warm cloth or diaper. For healthy term newborns, warm hands and stethoscopes prior to contact with baby. Use pre-warm beds, linens, and examining tables when possible. Position beds away from outside walls, windows, and drafts. Delay the initial bath until the body temperature has stabilized (minimum 3 normal temperatures on hour apart). Then a tub bath rather than sponge bath and dry quickly should be considered to reduce heat loss. Low birth weight infants (micro premies) should not be bathed for several weeks but may need to be wiped immediately after birth if maternally transferred infection is suspected to reduce risk to healthcare workers.

A gradual increase in the infant’s temperature is recommended to keep oxygen consumption to a minimum during rewarming. This is accomplished by the following:

  Incubator temperature should be adjusted 1ºC to 1.5ºC higher than the infant’s temperature.
   
  Hourly, the incubator temperature may be adjusted upwardly by 1ºC until the infant’s temperature has been stabilized.
   
  Caps, plastic warp, and heat shields should be removed to prevent them from interfering with heat gain.
   
  Feedings should always be warmed before being given to a cold stressed infant.
   
  Intravenous fluids may also be warmed by using blood-warming devices or by placing an extra length of tubing inside the incubator to allow the warmed environment to warm the fluids.
   
  Slow rewarming is recommended (it is not an exact science)

If the infant has been severely cold stressed, the temperature may continue to decline during the early stages of rewarming. Remember that the skin probe will read 36.5 degrees C before the core temperature is normal.

An infant who has a body temperature greater than 37– 37.5ºC may be considered abnormally warm. Heat stress, excluding the febrile state, should never occur in the neonate. When it does it is generally caused by improper use or monitoring of equipment to warm infants.

When core temperatures are elevated in febrile conditions, the skin temperature of the distal extremities remains cool in comparison to the skin temperature of the trunk. Hyperthermia can also be a sign of hyper metabolism when an infant is septic. The usual first step to approach treating hyperthermic infants is to remove external heating sources and by removing anything blocking heat loss (i.e. clothing).

Overheating can lead to a variety of responses many of which are similar to hypothermia with the exception of skin color. The skin is usually flushed or ruddy (plethoric) as apposed to pale and mottled. Other signs include:

	Hypo activity
	Restlessness
	Irritability
	Extended posture
	Flaccidity
	Tachycardia
	Tachypnea

Neutral thermal environment is the range of thermal environment in which the body temperature is normal, oxygen and caloric consumption is minimal and the least amount of metabolic energy is expended. This varies based on the infant’s age and weight. It is our goal to maintain an environment in which an infant does not expend a lot of energy to maintain a temperature within a normal range. Nurses can do this by providing external heat sources such as radiant warmers, isolettes, K-pads etc.

Servo-control mode is available with radiant warmers and isolettes. The infant’s skin temperature regulates the heater output. The nurse sets a control temperature where they wish the baby’s skin temperature to be. A skin probe is placed on the infant’s skin using an insulated probe cover. Some facilities use foam probe covers others use gel probe covers. Both of the covers have a reflective side to avoid falsely picking up the warmer temperature rather than the skin temperature. The warmer increases and decreases the heater output to maintain the set control temperature.

The critical factor is placement of the probe. In some facilities the practice is to place the skin temperature probe under the infant’s arm. When this is done there is a constant readout of the infant’s axillary temperature. This temperature is very close to the axillary temperature obtained during vital signs. There is a problem with this practice though when you consider the infants physiologic response to cold stresses and how the warmer is designed to function.

When an infant begins to become cooler the first area that drops in temperature is the skin. One of the first things an infant does in response to cold is to vasoconstrict the periphery (skin) to conserve what heat is on board. When an infant begins to respond to the cold the first response then is for slight decreases in skin temperature. When the probe is place on the skin of the exposed area, the warmer output responds to the subtle changes in skin temperature.

When the cold stress continues the axillary temperature drops. When the probe is placed under the infants arm the warmer still responds to a drop in temperature and maintains the infant at a normal temperature but the warmer output responds much later because by the time the axillary temperature has dropped the infant has had to use some of his energy to try and compensate before the warmer has kicked in to help. The appropriate placement is on the skin of the upper right quadrant of the abdomen or back (exposed skin, not lying on probe) and should be repositioned every time that the infant is repositioned. The probe should be attached firmly to the skin visible for inspection at all times.

Manual-control of radiant warmers should only be used in the immediate post delivery period and during resuscitation, to avoid over heating. Manual-control most frequently is used in isolettes. The control temperature is set for where you want to keep the environmental temperature (ambient temperature). The temperature probe can still provide information on the infant’s skin temperature but has no effect on the isolettes heater output.

The manual mode is used during the process of weaning infant’s to open cribs or for longer use when an infant no longer requires the specificity of the servo mode but is too small to wean to an open crib. In manual mode you can dress and bundle infants as long as their temperature is within normal parameters.

Other manual methods of assisting with temperature control include:

  Hats are useful for thermo-sensitive infants. A hat can cover 12-20 % of the infant’s body surface area.
   
  Plastic wrap over the intubated patient, under a radiant warmer, is used to minimize evaporative and convective heat loss. It is used for small premature infants.
   
  Heating pads (K-pads) can be used to decrease conductive heat loss.
   
  Warming diapers and linen prior to use by placing under warmer or in isolette can also decrease conductive loss. Avoid placing infants directly on cold surfaces.
   
  Remove wet linens frequently.

Blackburn, S., et al. (2001). Neonatal Thermal Care, Part III: The Effect of Infant Position and Temperature Probe Placement; Neonatal Network Vol. 20 No. 3, April, pp. 25-30.

Bissinger, R. (2004). Neonatal Resuscitation – Thermoregulation; emedicine.com.

British Columbia Reproductive Care Program Policy Manual (2003) Newborn Guidelines: Neonatal Thermoregulation.

Weber, Roberta; (2004) Neonatal Thermoregulation; Lecture Content