Preterm infants are predisposed to heat loss because they have a high ratio of surface area to body weight (5 times more than the adult), little insulating subcutaneous fat, and reduced glycogen and brown fat stores. In addition, their hypotonic (“frog”) posture limits their ability to curl up to reduce the skin area exposed to the colder environment.
Mechanisms of heat loss in the newborn include the following:
Radiation. Heat loss from the infant (warm object) to a colder nearby (not in contact) object.
Conduction. Direct heat loss from the infant to the surface with which he or she is in direct contact.
Convection. Heat loss from the infant to the surrounding air.
Evaporation. Heat loss by water evaporation from the skin of the infant. Immediately after delivery, evaporative heat loss may contribute to more than 50% of all heat loss. Thereafter its magnitude is inversely proportional to the degree of immaturity. The underdeveloped stratum corneum results in higher skin permeability for the extremely low birthweight infant (<1000 g BW). Transepidermal water loss as high as 6–8 mL/kg/h may be seen in the most immature infants during the first weeks of life.
Consequences of excessive heat loss. Those related to the compensatory augmentation in heat production through the increase in metabolic rate include the following:
Insufficient oxygen supply and hypoxia from increased oxygen consumption.
Hypoglycemia secondary to depletion of glycogen stores.
Metabolic acidosis caused by hypoxia and peripheral vasoconstriction.
Decreased growth.
Apnea.
Pulmonary hypertension as a result of acidosis and hypoxia.
Consequences of hypothermia. As the capacity to compensate for the excessive heat loss is overwhelmed, hypothermia will ensue.
Clotting disorders such as disseminated intravascular coagulation and pulmonary hemorrhage can accompany severe hypothermia.
Shock with resulting decreases in systemic arterial pressure, plasma volume, and cardiac output.
Intraventricular hemorrhage.
Severe sinus bradycardia.
Increased neonatal mortality.
Treatment of hypothermia. Rapid versus slow rewarming continues to be controversial, although the trend is toward more rapid rewarming. Rewarming may induce apnea, hypotension, and rapid electrolyte shifts (Ca++, K+); therefore, the hypothermic infant should be continuously and closely monitored regardless of the rewarming method. One recommendation is to rewarm at a rate of 1°C/h unless the infant weighs <1200 g, the gestational age is <28 weeks, or the temperature is <32.0°C (89.6°F) and the infant can be rewarmed more slowly (with a rate not to exceed 0.5°C/h). Another recommendation is that, during rewarming, the skin temperature not be >1°C warmer than the coexisting rectal temperature.
Equipment
Closed incubators. Usually used for infants who weigh <1800 g. Closed incubators are convectively heated (heated airflow); therefore, they do not prevent radiant heat loss unless they are provided with double-layered walls. Similarly, evaporation loss is compensated for only when additional humidity is added to the incubator. One disadvantage of incubators is that they make it difficult to closely observe a sick infant or to perform any type of procedure. Body temperature changes associated with sepsis may be masked by the automatic temperature control system of closed incubators. Such changes will hence be expressed in the variations in the incubator's environmental temperature. An infant can be weaned from the incubator when his or her body temperature can be maintained at an environmental temperature of <30.0°C (usually when the body weight reaches 1600–1800 g). Enclosed incubators maintain a neutral thermal environment by using one of the following devices:
Servocontrolled skin probe attached to the abdomen of the infant. If the temperature falls, additional heat is delivered. As the target skin temperature (36.0–36.5°C) is reached, the heating unit turns off automatically. A potential disadvantage is that overheating may occur if the skin sensor is detached from the skin or the reverse if the infant is lying on the probe-attached side.
Air temperature control device. The temperature of the air in the incubator is increased or decreased depending on the measured temperature of the infant. Use of this mode requires constant attention from a nurse and is usually used in older infants.
Air temperature probe. This probe hangs in the incubator near the infant and maintains a constant air temperature. There is less temperature fluctuation with this kind of probe.
Radiant warmer. Typically used for very unstable infants or during the performance of medical procedures. Heating is provided by radiation and therefore does not prevent convective and evaporative heat loss. The temperature can be maintained in the “servo mode” (ie, by means of a skin probe) or the “nonservo mode” (also called the “manual mode”), which maintains a constant radiant energy output regardless of the infant's temperature. Serious overheating can result from mechanical failure of the controls, from dislodgment of the sensor probe, or from manual operation without careful monitoring. Deaths are associated with hyperthermia-induced radiant warmers. On manual mode, such as in the delivery room, they should be used only for a limited period. Insensible water loss may be extremely large in the very low birthweight (VLBW) infant (up to 8 mL/kg/h). Covering of the skin with semipermeable dressing or the use of a water-based ointment (eg, Aquaphor) may help reduce insensible transepidermal water loss.
Temperature regulation in the healthy term infant (weight >2500 g). Studies have shown that a healthy term infant can be wrapped in warm blankets and placed directly into the mother's arms without any significant heat loss.
Place the infant under a preheated radiant warmer immediately after delivery.
Dry the infant completely to prevent evaporative heat loss.
Cover the infant's head with a cap.
Place the infant, wrapped in blankets, in a crib.
Temperature regulation in the sick term infant. Follow the same procedure as that for the healthy term infant, except place the infant under a radiant warmer with temperature servoregulation.
Temperature regulation in the premature infant (weight 1000–2500 g)
For an infant who weighs 1800–2500 g with no medical problems, use of a crib, cap, and blankets is usually sufficient.
For an infant who weighs 1000–1800 g
A well infant should be placed in a closed incubator with servo-control.
A sick infant should be placed under a radiant warmer with servo-control.
Temperature regulation in the extremely low birthweight (ELBW) infant (weight <1000 g). See Chapter 12.
In the delivery room. Considerable evaporative heat loss occurs immediately after birth. Consequently, speedy drying of the infant has been emphasized as a very important aspect of the management of the ELBW infant. A more efficient and different approach has been advocated whereby the infant is placed in a plastic bag from feet to shoulders, without drying, immediately at birth.
In the nursery. Either the radiant warmer or the incubator can be used, depending on the institutional preference. More recently hybrid devices such as the Versalet Incuwarmer (Hill-Rom Air-Shields, Batesville, IN) and the Giraffe Omnibed (Datex-Ohmeda; GE Medical Systems, Finland) have become available. They offer the combined features of radiant warmer and incubator with controllable humidity in a single device, allowing for seamless conversion between modes as deemed clinically necessary.
Radiant warmer
(a) Use servo-control with the temperature for abdominal skin set at 36.0–36.5°C.
(b) Cover the infant's head with a cap.
(c) To reduce convective heat loss, place plastic wrap (eg, Saran Wrap) loosely over the infant. Prevent this wrap from directly contacting the infant's skin. Avoid placing the warmer in a drafty area.
(d) Maintain an inspired air temperature of the hood or ventilator of ≥34.0–35.0°C.
(e) Place under the infant a heating pad (K-pad) that has an adjustable temperature within 35.0–38.0°C. To maintain thermal protection, it can be set between 35.0 and 36.0°C. If the infant is hypothermic, the temperature can be increased to 37.0–38.0°C (controversial).
(f) If the temperature cannot be stabilized, move the infant to a closed incubator (in some institutions).
Closed incubator. Excessive humidity and dampness of the clothing and incubator can lead to excessive heat loss or accumulation of fluid and possible infections.
(a) Use servo-control, with the temperature for abdominal skin set at 36.0–36.5°C.
(b) Use a double-walled incubator if possible.
(c) Cover the infant's head with a cap.
(d) Keep the humidity level at ≥40–50% (as high as 89% if needed).
(e) Keep the temperature of the ventilator at ≥34.0–35.0°C.
(f) Place under the infant a heated mattress (K-pad) that has an adjustable temperature within 35.0–38.0°C. For thermal protection, the temperature can be set between 35.0 and 36.0°C. For warming a hypothermic infant, it can be set as high as 37.0–38.0°C.
(g) If the temperature is difficult to maintain, try increasing the humidity level or use a radiant warmer (in some institutions).