INTRODUCTION

A. The importance of thermoregulation.

1. There are adverse consequences associated with both hypothermia and hyperthermia. Hypothermia is the major temperature abnormality in neonates. An increase in mortality and morbidity associated with hypothermia is well documented and is recognized as a problem worldwide.

a. In Europe during the mid to late 1800s, the use of an early crude form of incubator began and it was observed that more small babies survived.

c. In the 1950s, Silverman demonstrated that hypothermia increased morbidity and mortality (Silverman et al., 1958). More recent studies confirm this finding (Costeloe et al., 2000; Laptook et al., 2007).

d. The World Health Organization (WHO) recognizes thermoregulation of the newborn as a major threat to the health of newborns throughout the world (WHO, 1997).

e. The term “golden hour” has been used recently in reference to strategies implemented in the delivery room to improve the outcome of neonates. A major focus during the “golden hour” is prevention of heat loss (Bissinger and Annibale, 2010).

B. Mechanisms of heat transfer.

1. Heat transfer infers the transfer of heat from warm to cool. There are four mechanisms of heat transfer that affect neonates: conduction, convection, radiation, and evaporation. Table 6-1 lists mechanisms of heat transfer and activities that are associated with heat loss and heat gain as well as strategies to prevent both loss and gain.

a. Conduction is the transfer of heat via direct contact.

(1) Heat is lost when the neonate comes in contact with a cold surface such as a mattress, x-ray plate, or scale.

(1) Incubators transfer heat to infants through the circulation of warm air (convective heat gain).

(1) Radiant warmers provide this type of heat.

(3) If the walls of an incubator are cold, an infant may lose heat and be cold-stressed despite warm air circulating through the incubator (Blackburn, 2013).

d. Evaporation is heat transfer due to water vaporizing from the skin or respiratory tract into the drier surrounding air and, in the case of premature infants with thin permeable skin, through transepidermal water loss (TEWL).

e. In the delivery room, evaporation of amniotic fluid accounts for 25% of neonatal heat loss (Dahm and James, 1972).

f. High total body water, immature skin, and relatively large body surface area result in increased evaporative losses in premature infants. This is even more significant in very low birth weight (VLBW) and especially in extremely low birth weight (ELBW) infants.

1. Hypothermia.

a. Nurses caring for neonates should be able to identify infants who are at high risk for thermal instability. Infants who have decreased brown fat and decreased glycogen stores, those with high TEWL, and those who lack muscle tone would be considered at increased risk for hypothermia.

b. Infants at risk for hypothermia include premature and late preterm infants and infants who have intrauterine growth restriction, are critically ill, are sedated, or have neurologic problems. Infants with open skin defects that place them at risk for increased water loss, such as neural tube or abdominal wall defects, are also at increased risk (American Academy of Pediatrics [AAP] and American College of Obstetricians and Gynecologists [ACOG], 2007; Blackburn, 2013) (Box 6-1).

c. Symptoms of hypothermia include apnea, bradycardia, decreased cardiac output, poor oral feeding, feeding intolerance, hypoglycemia, hypotension, hypoxia, irritability, lethargy, weak cry, metabolic acidosis, peripheral vasoconstriction, pulmonary vasoconstriction, and respiratory distress (Box 6-2).

e. In severe cases of hypothermia, coagulation defects and death may result (Blackburn, 2013). Although shivering is rare, it may be seen in term infants after prolonged cold stress (Bissinger and Annibale, 2010).

2. Hyperthermia.

a. Infants are also at risk for hyperthermia because of the inability to dissipate heat. Close monitoring and attention to the thermal environment are indicated when caring for these infants.

b. Infants at risk for hyperthermia include those with hypermetabolism, including sepsis, cardiac problems, and drug withdrawal, those with dehydration, and infants with central nervous system (CNS) injury or malformation (see Box 6-1).

c. Symptoms of hyperthermia include apnea, CNS depression, hypernatremia from dehydration, irritability, lethargy, weak cry, poor oral feeding, seizures, tachycardia, and vasodilation (Brown and Landers, 2011) (see Box 6-2).

d. Severe hyperthermia may be associated with seizures and death.

3. Monitoring temperature (Smith et al., 2013).

a. Early detection and management of hypothermia and hyperthermia is essential to the well-being of neonates. Frequent temperature measurements and observation are key to early detection until the infant’s temperature is stable.

(1) Normal axillary ranges are 36.5° to 37.5°C (97.7° to 99.5°F) for the term neonate and 36.3° to 36.9°C (97.3° to 98.6°F) for the preterm neonate (Brown and Landers, 2011).

(2) Electronic or digital thermometers should be used according to the manufacturer’s specifications.

d. Rectal temperatures should not be used due to the risk of intestinal perforation and because the core temperature may not decrease until the neonate has totally decompensated.

(1) Rectal thermistors may be used to provide continuous monitoring during therapeutic cooling for hypoxic–ischemic encephalopathy (HIE).

(1) Avoid areas of brown adipose tissue (BAT) such as the axilla, as the temperature may be higher in these areas due to increased metabolic activity. BAT is discussed later in this chapter.

(a) Desired skin temperatures range from 36° to 37.5°C (96.8° to 99.5°F) (Bissinger and Annibale, 2010; Brown and Landers, 2011; Gomella, 2009; Malin and Baumgart, 1987).

(2) Abdominal temperatures greater than 36.5° C are associated with lower oxygen consumption in radiant warmers (Malin and Baumgart, 1987).

(3) A study on temperature and heart rate suggests a target of 36.8° to 36.9° C might be more optimal during transition in ELBW infants (Knobel et al., 2010).

(4) If both central (abdominal) and peripheral temperatures are monitored, the expected difference is 0.5° to 1° C (Turnbull and Petty, 2013). A difference of 2° C or greater has been associated with cold stress resulting in peripheral vasoconstriction in infants beyond the first day of life (Horns, 2002; Knobel et al., 2009; Lyon et al., 1997; Mok et al., 1991).

g. Ear thermometers are fast and easy to use but may be too large for premature infants. If the probe is not placed correctly, the surface temperature may be measured rather than an approximation of core temperature.

h. Infrared thermometers, also called temporal artery thermometers, measure the surface temperature without touching the infant by capturing heat radiated from the skin surface. More study is needed before these thermometers are used routinely in neonatal units.

PHYSIOLOGY OF THERMOREGULATION

A. In utero, the maternal temperature is the ambient environmental temperature for the fetus. The neonate’s temperature at birth will be reflective of the intrauterine temperature. If the mother has a fever, it is common for the neonate’s temperature to be elevated. It is important to distinguish between environmental, iatrogenic, and infectious causes of elevations in neonatal temperatures.

1. The intrauterine temperature may be 0.8° C (1.3° F) greater than the maternal oral temperature (Banerjee et al., 2004).

2. Maternal temperature increases as labor progresses and may increase by an average of 1° C during labor due to physical activity and stimulation of the thermoregulatory center in the hypothalamus (Blackburn, 2013).

3. Epidural anesthesia may also increase the maternal temperature. Temperatures as high as 37.5° to 38°C (99.5° to 100.4° F) have been reported.

4. Neonatal symptoms associated with increased maternal temperature in noninfected women include lower Apgar scores, hypotonia, hypoxia, and an increased need for resuscitation and oxygen at birth (Kattwinkel, 2011).

B. Once the infant is delivered, a number of physiologic conditions affect thermal stability, including the physical characteristics of the neonate, metabolic processes, and physical activity.

1. Physical characteristics of the neonate affecting neonatal thermoregulation including thermal receptors in the skin, surface area–to–body mass ratio, skin permeability, body posture, and the type and amount of subcutaneous fat.

a. Thermal receptors in the skin affect the infant’s response to cold stimuli (Blackburn, 2013).

(1) The most prominent and sensitive skin receptors are in the trigeminal area of the face.

(3) ELBW infants are unable to respond to hypothermia with vasoconstriction during the first 12 hours after birth (Knobel et al., 2009).

b. Neonates have a relatively large surface area–to–body mass ratio, which has an impact on thermoregulation.

(1) In term neonates, the surface area–to–body mass ratio is three times greater than that of an adult (Blackburn, 2013).

(2) In the preterm infant, the surface area–to–body mass ratio is five to six times greater than that of an adult. The surface area–to–body mass ratio in preterm infants increases as the gestation decreases.

c. Premature infants have very thin skin, allowing greater loss of water and heat than seen in term neonates. Thin permeable skin allows water to evaporate from the skin surface.

(1) Heat is lost by evaporation. This is known as TEWL (Sedin and Agren, 2006).

(2) At 25 weeks of gestation, premature infants can lose 15 times more water through their skin than term neonates. This water loss is inversely proportional to gestational age until about 33 weeks of gestation, when TEWL is similar to that of a term infant (Agren et al., 2006; Sedin and Agren, 2006).

d. Posture can affect body temperature by decreasing or increasing surface area for heat loss.

(1) Term neonates may lie in a tucked position to conserve heat or extend their arms and legs to dissipate heat.

(1) Infants have less insulating subcutaneous fat than adults. Term neonates have only 16% of body fat compared to 30% to 35% in adults; preterm infants have even less.

(a) Heat production through the metabolism of BAT is regulated by uncoupling protein-1, norepinephrine, and triiodothyronine (Bissinger and Annibale, 2010; Blackburn, 2013).

(b) BAT is metabolized during nonshivering thermogenesis (NST) to produce heat.

(c) A heavy concentration of blood vessels gives BAT its characteristic brown color and serves to conduct heat into the circulation.

(d) Metabolism of BAT involves the breakdown of triglycerides into glycerol and fatty acids and is dependent on the availability of oxygen, glucose, and ATP. If these elements are not available, as is the case during hypoxia, acidosis, and hypoglycemia, heat production will suffer (Cypess and Kahn, 2010).

(e) BAT is found around the kidneys and adrenal glands, at the nape of the neck, between the scapulas, in the mediastinum, and around the trachea, heart, lungs, liver, and abdominal aorta.

(f) The amount of available BAT is dependent on gestational age (Blackburn, 2013).

i. It begins to appear at 25 to 26 weeks, so only a minimal amount is available in VLBW infants.

ii. Infants less than 32 weeks of gestation do not have enough BAT to produce significant amounts of heat by NST (Knobel and Holditch-Davis, 2007).

iii. BAT increases significantly during the third trimester.

iv. BAT continues to increase in the early weeks after birth and is thought to play a role in thermoregulation until a child is approximately 2 years of age.

2. Metabolic processes generate energy by metabolism of fat, including BAT (above), glucose, and protein (Blackburn, 2013).

a. NST is the major method of heat production in the neonate.

f. Term neonates can increase their metabolic rate by 200% to 300% through a combination of metabolic and physical responses (Blackburn, 2013).

g. VLBW infants may only be able to increase their metabolic rate by 25%, while ELBW infants may not be able to respond at all.

h. Critically ill neonates may not have the reserves of oxygen, glucose, or energy to mount an increase in metabolic rate and adapt to an unstable thermal environment.

i. Hypothermia may lead to respiratory distress, hypoglycemia, and the production of lactic acid (see Box 6-2).

3. Physical activity can generate heat, but only late preterm and term infants have enough muscle tone to produce heat by this method and this may only be effective for a short time.

a. Increased physical activity, such as feeding, crying, restlessness, or hyperactivity, increases the metabolic rate.

c. Shivering is an involuntary method of heat production that is rarely seen in the neonatal population. The neonate only shivers after prolonged cold stress that leads to decreased spinal cord temperature (Blackburn, 2013).

MANAGEMENT OF THE THERMAL ENVIRONMENT

A. This aspect of neonatal care falls under the purview of the nurse. All infants have thermal management challenges in the delivery room and during transition. Premature infants have ongoing needs that must be addressed until they are able to maintain their own core temperature without metabolic consequences such as poor weight gain. It is crucial for the nurse to understand the importance of this role and to be versed in the use of equipment available to manage the thermal environment of newborn infants whether they are in the delivery room, in the nursery, or during transport.

1. Delivery room management (Bissinger & Annibale, 2010).

a. The primary challenge in thermal management of newborns is the prevention of hypothermia because the neonate is expected to transition from a warm, moist environment to a much colder, drier environment. Late preterm and preterm infants are at an increased risk of hypothermia, especially those who are less than 28 weeks of gestation at birth and/or weigh 1500 g or less at birth. A number of strategies have been described to maintain temperature at birth, including warming the room, drying the infant, providing skin-to-skin contact with the mother, utilization of heat loss barriers, use of external heat sources, and use of polyethylene plastic wrap or bag for neonates less than 29 weeks of gestation (Kattwinkel, 2011; McCall et al., 2010).

(1) Room temperature.

(a) A newborn’s temperature may fall rapidly. The infant’s temperature may drop 0.2° C to 1° C per minute due to evaporative losses from amniotic fluid, convective losses to cooler room air, and radiation to cooler structures in the room such as walls (Blackburn, 2013).

(b) The gradient between the ambient room temperature in the delivery room and the infant’s skin temperature promotes heat loss. The delivery room should be maintained above 25° to 26° C (77° to 78.8° F), or at least 26° to 27° C (78° to 80° F) for infants less than 29 weeks of gestation, to minimize convective heat loss (AAP & ACOG, 2012; Hammarlund et al., 1980; Kattwinkel, 2011; Knobel et al., 2005).

(2) Dry the infant immediately and remove wet linen (unless using a polyethylene bag) to minimize evaporative heat loss (Bissinger & Annibale, 2010).

(a) Use prewarmed absorbent towels to dry the infant gently, taking care not to damage the skin.

(a) If condition of the infant allows, drying and placing the infant against the mother’s bare skin, then covering with a warm blanket, allows heat from the mother to maintain the infant’s temperature (McCall et al., 2010).

(b) Skin-to skin holding can be used in the neonatal intensive care unit as well, even in VLBW infants. The conduction of heat from mother (or father) to the infant is high enough to compensate for increased heat loss through evaporation and convection while the infant is out of the humidified incubator (Karlsson et al., 2012).

(5) Barriers to heat loss (Bissinger & Annibale, 2010; McCall et al., 2010).

(a) Polyethylene bags or wraps should be used in infants less than 29 weeks of gestation to prevent evaporative heat loss and allow visualization, stabilization, and heat gain through radiation from the radiant warmer. The infant is placed in the wrap immediately and wrapped from the neck to the feet. Only the head is dried. Maintain the wrap until the infant’s temperature is stable in a humidified environment for 1 hour (AAP and ACOG, 2012; Knobel et al., 2005; Trevisanuto et al., 2010; Vohra et al., 2004).

(b) Use a polyethylene or insulated cap (not stockinette) after drying the baby’s head to prevent evaporative heat loss (Trevisanuto et al., 2010).

(c) Open skin defects (gastroschisis, myelomeningocele) present a major challenge to thermoregulation as the open defect presents a permeable surface for rapid fluid losses and increased evaporative heat loss. Cover the defect with sterile saline-soaked gauze and a sterile waterproof barrier to reduce evaporative losses. (See Chapter 29.)

(6) External heat sources.

(a) A radiant warmer or prewarmed incubator should be used in the delivery room to maintain the infant’s temperature during routine care and stabilization. Radiant warmers should always be used with a servo-control to prevent overheating (AAP and ACOG, 2012).

(b) Chemical mattresses, also called transwarmers, may be used to improve thermal control in VLBW infants. These may be used in the delivery room or during transport and should be removed once the infant is warm and in a stable thermal environment. Because heat is provided by conduction, a blanket should be placed under the neonate to prevent skin burns (AAP and ACOG, 2007, 2012; Brown and Landers, 2011; McCall et al., 2010).

(c) Avoid overheating if perinatal asphyxia is suspected. In some cases providing hypothermia strategies to preserve brain function may be appropriate. (See Chapter 34.)

(d) Use a prewarmed transport incubator to transfer neonates to the nursery. A transport incubator uses batteries and continues to provide heat during transfer. The incubator usually has equipment built in or attached for monitoring, suctioning, and providing respiratory support and intravenous infusions.

2. Nursery (AAP and ACOG, 2012).

a. The temperature in the nursery should be maintained at 22° to 27° C (72° to 78° F) (Bissinger and Annibale, 2010).

b. Humidity in the nursery should be maintained at 30% to 60% (Blackfan and Yaglou, 1933).

3. Equipment.

a. Nurses should be familiar with equipment used in their nursery. Equipment should be used according to the manufacturer’s directions and the unit’s policies and procedures.

(1) Incubators warm infants using convective heat.

(a) Incubator covers can be used to decrease radiant heat loss from incubator walls (Fig. 6-1).