I. BACKGROUND. Neonatal hypothermia after delivery is a worldwide issue, occurs in all climates, and if prolonged can cause harm and affect survival. Thermoregulation in adults is achieved by both metabolic and muscular activity (e.g., shivering). During pregnancy, maternal mechanisms maintain intrauterine temperature. After birth, newborns must adapt to their relatively cold environment by the metabolic production of heat because they are not able to generate an adequate shivering response. Brown fat is a source for thermogenesis in term newborns. It is highly vascularized and innervated by sympathetic neurons. When these infants face cold stress, norepinephrine levels increase and act in the brown fat tissue to stimulate lipolysis. Most of the free fatty acids (FFAs) are re-esterified or oxidized; both reactions produce heat. Factors that can increase risk for hypothermia include prematurity, intrauterine growth restriction, asphyxia, and certain congenital anomalies (e.g., abdominal wall defects, central nervous system [CNS] anomalies).

A. Premature infants experience increased mechanisms of heat loss combined with decreased heat production capabilities. These special problems in temperature maintenance put them at a disadvantage. Compared with term infants, premature infants have

1. A higher ratio of skin surface area to weight

2. Highly permeable skin which leads to increased transepidermal water loss

3. Decreased subcutaneous fat with less insulative capacity

4. Less-developed stores of brown fat and decreased glycogen stores

5. Poor vasomotor control

6. Challenges with adequate caloric intake to provide nutrients for thermogenesis and growth

7. Limited oxygen delivery if pulmonary conditions coexist

B. Cold stress. In the setting of resuscitation, newborn infants can be subject to acute hypothermia and respond with a cycle of peripheral vasoconstriction, causing anaerobic metabolism, metabolic acidosis, and pulmonary vasoconstriction. Hypoxemia further compromises the infant’s response to cold. Premature infants are at the highest risk for hypothermia and its sequelae (i.e., hypoglycemia, metabolic acidosis, increased oxygen consumption). After the immediate newborn period, the more common and chronic problem facing premature infants than actual hypothermia is caloric loss from unrecognized chronic cold stress, resulting in excess oxygen consumption and inability to gain weight. The use of low-reading thermometers (from 29.4°C/85.0°F) is recommended because temperature readings <34.4°C (94.0°F) can go undetected with routine thermometers.

C. Neonatal cold injury is a rare, extreme form of hypothermia that may be seen in low birth weight (LBW) infants and term infants with CNS disorders. Core temperature can fall below 32.2°C (90°F). It occurs more often in home deliveries, emergency deliveries, and settings where there is inadequate support regarding the thermal environment and practices needed to minimize heat loss. These infants may have a bright red color because of the failure of oxyhemoglobin to dissociate at low temperature. They may have central pallor or cyanosis. The skin may show edema and sclerema. Signs may include hypotension; bradycardia; slow, shallow, irregular respiration; poor sucking reflex; abdominal distention or vomiting; decreased activity; decreased response to stimulus; and decreased reflexes. Metabolic acidosis, hypoglycemia, hyperkalemia, azotemia, and oliguria can be present. Sometimes, there is generalized bleeding, including pulmonary hemorrhage. It is controversial whether warming should be rapid or slow. Setting the abdominal skin temperature to 1°C higher than the core temperature or setting it to 36.5°C on a radiant warmer will produce slow rewarming. In addition to rewarming, hypoglycemia should be corrected. The infant may benefit from a normal saline bolus (10 to 20 mL/kg), supplemental oxygen, and correction of metabolic acidosis. These infants should not be fed and should be carefully evaluated and treated for possible infection, bleeding, or injury.

D. Hyperthermia, defined as an elevated core body temperature, may be caused by a relatively hot environment, infection, dehydration, CNS dysfunction, or medications. Although the issue of infection is of clinical concern, awareness of environmental contributors such as phototherapy, incubators or warming table settings, or proximity to sunlight should be considered. If environmental temperature is the cause of hyperthermia, the trunk and extremities are the same temperature and the infant appears vasodilated. In contrast, infants with sepsis are often vasoconstricted and the extremities are cooler than the trunk.

E. Induced hypothermia. In recent years, there is experimental and clinical evidence that induction of controlled hypothermia can reduce neuronal loss and subsequent brain injury after a hypoxic-ischemic insult. It is a time-sensitive therapy and needs to be instituted within the first 6 hours after
birth to be most effective. Passive cooling in the delivery room and during stabilization, followed by transfer to a center that performs the treatment, should be considered when there is a history of an acute perinatal event (nonreassuring fetal heart tracings, cord prolapse, placental abruption), pH ≤7.0/base deficit ≥16 on cord gas or gas obtained within 1 hour of life, 10-minute Apgar score ≤5, or assisted ventilation initiated at birth and continued for at least 10 minutes. Target temperature range is 32.5°C to 34.5°C. Core temperature (typically measured rectally at referring hospital and on transport) should be monitored every 15 minutes (see Chapter 55).

A. Radiation. Heat dissipates from the infant to a colder object in the environment.

B. Convection. Heat is lost from the skin to moving air. The amount lost depends on air speed and temperature.

C. Evaporation. Heat is lost through conversion of water to gas. The amount of loss depends primarily on air velocity and relative humidity. Wet infants in the delivery room are especially susceptible to evaporative heat loss.

D. Conduction. Heat is lost due to transfer of heat from the infant to the surface on which he or she lies.

IV. NEUTRAL THERMAL ENVIRONMENTS minimize heat loss. Thermoneutral conditions exist when heat production (measured by oxygen consumption) is minimal and core temperature is within the normal range (Table 15.1).

1. Standard thermal care guidelines include maintaining the delivery room temperature at 72°F (American Academy of Pediatrics [AAP])/25°C (World Health Organization [WHO]), immediately drying the infant (especially the head), removing wet blankets, and wrapping the newborn in prewarmed blankets. It is also important to prewarm contact surfaces and minimize drafts. A cap is useful in preventing significant heat loss through the scalp.

2. Examination in the delivery room should be performed with the infant under a radiant warmer. A skin probe with servocontrol to keep skin temperature at 36.5°C (97.7°F) should be used for prolonged examinations.