Temperature Regulation - Clinical Tree

Key Points

Providing an appropriate and stable thermal environment is important for newborns regardless of size or gestational age.
Neutral thermal environment (NTE) refers to the ambient temperature necessary to maintain normal metabolism.
Newborn (admission) hypothermia continues to be a global challenge, particularly in resource-limited settings.
Radiant warmers, warm blankets, thermal mattresses, head covering, plastic wrap (without drying), delayed bathing, and skin-to-skin care have been recommended to reduce neonatal hypothermia.
Interhospital transport, particularly for critically ill and/or low birth weight (LBW) neonates, increases the risk of thermal instability.

All newborns are at risk of experiencing physiologic instability due to aberration of their core temperature. Providing an appropriate and stable thermal environment is important for newborns regardless of size or gestational age (GA). Most of the temperature needs of the full-term infant will be associated with birth, or in rare circumstances with the development of an illness or environmental instability. It is imperative that providers involved with delivery or subsequent care understand the various causes and consequences of heat loss, the mechanisms of heat production by the infant, and the multiple management options for providing the correct thermal environment. A variety of clinical conditions present different management challenges depending upon the degree of immaturity, birth weight, and concurrent illness such as respiratory distress, sepsis, or asphyxia. Temperature regulation is an essential and important component of neonatal intensive care.

Nearly a century ago, long before the development of intensive care nurseries, research demonstrated an association between temperature control and increased survival in premature infants. Incubators first appeared in the 19th century in Europe, but it was not until the 1930s that they were incorporated into the care of premature infants at Michael Reese Hospital in Chicago. However, the concept that premature infants were unharmed by hypothermia (acting as if they were similar to a poikilothermic animal) prevailed until controlled trials demonstrated the associated morbidity and mortality of cooling in various birth weight groups. Often referred to as “cold stress,” a number of studies demonstrated the metabolic cost of hypothermia, resulting in a doubling or tripling of oxygen consumption, particularly in early-gestation infants. These studies were the first attempts at defining the optimal environmental temperature for maintenance of a normal body temperature and physiology, known as the neutral thermal environment (NTE). This quest has resulted in ever-increasing refinements of protective management techniques including hybrid incubators, radiant heaters, heated gel mattresses, plastic wrapping, heat shields, a laminar flow device, clothing, caps, room temperature control, and protocols for skin-to-skin care (SSC). Appropriate incorporation of the many management options will depend upon the needs of the individual infant related to his or her birth weight, maturity, degree of illness, and postnatal age.

Mechanisms of Heat Loss

The most vulnerable time for heat loss occurs during the first minutes after birth. It is of interest to consider the thermal environment of the fetus and how a stable body temperature is maintained despite variations in fetal metabolic activity associated with sleep cycles, muscular activity, respiratory movements (fetal breathing), and changes in maternal temperature. Maternal-fetal heat exchange occurs primarily via the umbilical vessels. Although the fetus maintains a differential of 0.5°C above maternal temperature, the extra heat is transferred from the umbilical artery to the umbilical vein as it courses back to the placenta such that the temperature of the blood returning to the placenta is nearly equal to the temperature of the blood leaving through the umbilical vein. Thus, the fetal environment is primarily dependent upon the maternal temperature. Significant elevations in maternal temperature have the potential to cause fetal harm, hence the standard advice given to pregnant women to avoid hot tubs. Maternal fever as a result of inflammation or infection is likely of higher risk to the fetus than moderate maternal exercise or ambient heat stress. The temperature gradient between the fetus and the mother may increase beyond 0.5°C during labor, uterine contractions, and it widens slightly with advancing GA.

There are four major mechanisms of heat loss that will vary over progressive days of nursery care (depending upon the maturity of the infant, growth, illness, and environmental factors). These are evaporation, radiation, convection, and conduction, each with variable contributions to heat loss ( Table 17.1 ; Fig. 17.1 ).

Table 17.1

Four Major Mechanisms of Heat Loss in Newborns: Estimated Percentages of Each, Based on the Environmental Temperature

	ENVIRONMENTAL TEMPERATURE
	30°C (%)	33°C (%)	36°C (%)
Radiation	43	40	24
Convection	37	33	5
Evaporation	16	24	56
Conduction	4	3	1

Fig. 17.1, Four modes of heat loss in the neonate.

Evaporation

Evaporative losses at birth may result in a fall of 2°C to 3°C within the first 30 to 60 minutes after birth if the newborn is extremely premature or if no wrapping, drying, or clothing is applied in a larger newborn. Delays in warming may occur if resuscitation or other medical care postpones drying. The first minutes after birth are already stressful because of the physiologic adaptions required for onset of breathing, absorption of fetal lung fluid, and circulatory changes.

The major cause of heat loss from evaporation following delivery is exposure of the infant’s large surface area of skin relative to his or her body mass while wet from amniotic fluid. The more immature the newborn, the larger the relative surface area ( Table 17.2 ). In full-term newborns, evaporation of water from the skin decreases until a body temperature of 36.6°C to 37.1°C is reached. Concomitant increase in skin blood flow does not appear to influence evaporative heat loss.

Table 17.2

Relative Skin Surface Area in Adults and in LBW Infants

	Relative Skin Surface Area (cm ² /kg)
Adult	250
1500-g infant	870
1000-g infant	1000
500-g infant	1400

Insensible water loss through the skin continues to contribute to evaporative heat loss, but this decreases over the first few days after birth. The rate of evaporative water loss depends upon the ambient humidity and increases at humidity levels below 50%. Evaporation is somewhat greater under a radiant warmer compared to an incubator. Studies have demonstrated large losses from transepidermal water evaporation over the first few days after birth in extremely LBW newborns ( Fig. 17.2 ). This suggests that the skin permeability of very premature neonates, especially during the first hours after birth, is very different from that of neonates born beyond a GA of 32 to 34 weeks.

Fig. 17.2, Transepidermal water loss in relation to gestational age at birth and at different postnatal ages in appropriately grown infants.

Radiation

Radiation is a major source of heat loss, thus it has been the focus of the majority of research studies which has led to the development of many devices to minimize that loss and to protect the infant from excessive energy expenditure. Radiant heat loss from the skin can be responsible for 40% or more of the daily heat loss (when air movement is low). Many variables can influence the degree of heat loss, including body surface area, environmental temperature, the type of external heat source, clothing, blankets, caps, heat shields, and swaddling. The full-term infant should not present ongoing concerns with radiant heat loss if certain measures of care are provided such as room temperature of 24°C to 26°C, appropriate SSC, clothing, and blankets. Premature infants and those who are growth restricted or ill with cardiorespiratory distress, sepsis, asphyxia, or other disorders will be at continued risk of radiant heat loss. This mechanism of heat loss is influenced by the mean temperature of the skin and the mean temperature of the surrounding walls, as well as the temperature gradient to a nearby object of lesser warmth even if the object is outside but near the incubator or the warmer.

Convection

Heat loss due to convection is determined by the airflow around the infant, the mean temperature of the ambient air, the mean temperature of the skin, and the exposed surface area of the infant. Different incubators have different airflow, humidity, and wall temperature capacities that can influence the degree of convective heat loss.

Conduction

Conductive heat loss contributes minimally to energy expenditure and primarily depends upon the thermal conductivity of the mattress, which is low in incubators and under radiant warmers. Skin adjacent to a colder object such as a radiograph plate or other cold instrument will result in some conductive heat loss.

Mechanisms of Thermoregulation

Our understanding of thermoregulation in the human infant comes from translational studies in animal models. From these studies we know that temperature sensors are located throughout the surface of the neonatal body. These receptors transmit signals to a central temperature controller within the hypothalamus which in turn regulates temperature homeostasis via multiple mechanisms. Thermoneutrality in response to hypothermia in the newborn is primarily accomplished via nonshivering thermogenesis as discussed below.

The optimal temperature for an infant is defined by a range of measurable skin temperatures that indicate a central or core temperature. Measurement sites include the axilla, rectum, and skin (the small size of the ear canal precludes tympanic measurement). The recommended range for normal axillary and rectal temperatures is 36.5°C to 37.5°C for full-term infants, 35.6°C to 37.3°C for preterm infants, and 36.7°C to 37.3°C for LBW infants. Axillary temperature measurement is intermittent with minimal errors related to placement of the thermometer, while rectal temperatures have a risk of trauma and inaccuracies due to depth or duration of insertion of the thermometer as well as passage of stool. Continuous temperature monitoring with an infrared skin probe placed over the midline of the upper abdomen or on the back below the scapulae permits accurate assessment of core temperature under various external heating arrangements.

The NTE refers to the ambient temperature necessary to maintain normal metabolism. For the average naked adult it is 25°C to 27°C, and for the full-term naked newborn it is 32°C to 34°C. For premature infants it will differ with GA and weight. Infants of 25 weeks’ GA or less will require an ambient temperature close to that in utero (i.e., 35°C to 37°C) until they are placed in an appropriate protective environment. LBW infants are particularly susceptible to temperature instability because of the absence of subcutaneous fat for insulation, decreased levels of brown fat for heat production, increased evaporative losses from a large body surface area relative to body weight, poor vasoconstrictor control, decreased spontaneous muscle activity, and transepidermal insensible water loss. Additionally, small for GA infants have a higher metabolic rate for weight. An increase in activity and ingestion of food in growing preterm infants will also increase the metabolic rate through altered energy expenditure (see Table 17.2 ).

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