Thermal Environment of the Intensive Care Nursery

Thermal management of the newborn infant is a cornerstone of neonatal care: The field accommodates important advances in care provision spanning from pioneering studies on chronic cold stress and neonatal incubation to kangaroo mother care, prevention of low admission temperatures, and therapeutic hypothermia. Globally, cold stress remains a contributor to neonatal mortality and morbidity, and the potential impact of optimal thermal care provision on infant health is huge. A basic understanding of the physics and physiology of heat exchange enables the undertaking of adequate measures to reduce heat loss and monitor body temperature, measures that often are simple and easy to apply. Without thermal care, a term 5-kg newborn may quickly and easily become hypothermic, increasing its need for endogenous heat production and posing a risk for cardiorespiratory instability and failure to feed properly, states that may be detrimental to an otherwise healthy or only slightly depressed infant. On the other hand, with adequate preparation, equipment, and training, the body temperature of a 500-g extremely preterm infant can be kept within the normal range, aiding a smooth transition to extrauterine life. In general, adequate thermal management depends at least as much on sound knowledge as on the use of high-tech devices such as incubators and radiant warmers.

Heat Exchange Physics

The Four Modes of Heat Exchange

Direct heat exchange between the infant and its environment occurs through the skin and through the respiratory tract. With use of different techniques, this heat exchange may be calculated and estimated with reasonable accuracy. The four modes of heat exchange—convection, radiation, evaporation, and conduction ( Box 35.1 )—all influence thermal balance, and the relative contribution of each mode changes with maturity, postnatal age, disease state, and care environment. Analysis of the contribution from the different modes of heat exchange makes it straightforward to reduce heat loss in any given situation, minimizing the need for delivery of heat through another route. Together with proper temperature monitoring, such an approach simplifies thermal management and renders individual estimates of heat loss unnecessary.

Box 35.1

The Modes of Heat Exchange

Convection

Air movement causes warm air close to the skin to move away, resulting in heat loss.
The relative magnitude of heat loss through convection is similar in term and preterm infants.
Convective heat loss is much increased when air velocity is high (forced convection).
Convective heating is the mode by which modern intensive care incubators operate.
Convective heat exchange is reduced by any measure that will minimize air movement close to the skin (e.g., limiting incubator porthole opening to one side at a time, providing clothing and/or blankets).

Radiation

Radiant heat transfer occurs from the infant to surrounding cooler surfaces and to the infant from radiant heaters and warm-light phototherapy devices.
The magnitude of heat exchange depends on the temperatures of the surfaces facing the infant and on the body surface area exposed.
Heat radiation is reduced by clothes, blankets, double incubator walls, heat shields, flexible plastic wrap, and bags.

Evaporation

Evaporation of fluid from the skin surface implies loss of heat (approximately 2.4 kJ/g water).
Evaporation is the dominant route for heat loss from the wet neonate in the delivery room but is substantially reduced by immediately wiping the infant dry at birth.
Very immature infants have poor skin barrier function, leading to large ongoing losses of water and heat for several days to weeks after birth.
Evaporation is reduced in a high vapor pressure environment.

Conduction

Inadvertent conductive heat loss is negligible in most care situations, because mattresses used in incubators and cots are made of insulating material.
Care must be taken when using equipment with an active warming function. If switched off, a gel mattress such as those used in some radiant warmer beds and/or an unheated water-filled mattress will be a very effective conductive cooler for any size neonate.
Conductive cooling is the method by which all systems for hypothermia treatment operate.
Conductive heat delivery (e.g., heated mattress or skin-to-skin care) is a simple and effective way to rewarm infants and stabilize body temperature.

Heat Exchange at Term Birth

The wet newborn infant will be exposed to moving air and surfaces that have a much lower temperature than that in utero. Unless the infant is wiped dry and covered by a blanket, heat loss through evaporation and radiation to surrounding surfaces of the delivery room will be high and lead to rapid cooling. Heat loss through convection is also significant unless airflow velocity is reduced (e.g., by covering, clothing). As soon as the term or moderately preterm infant is dry, radiant heat loss will dominate.

Evaporative Heat Loss Is Related to Infant Maturity and Ambient Humidity

As loss of fluid also implies loss of heat, evaporative fluid loss is an important component of heat exchange, particularly in extremely preterm infants. A functionally competent skin barrier develops gradually during fetal life. Consequently, the magnitude of water loss through the skin is related to maturity at birth, and the tiniest infants may have evaporative heat losses that are many times higher than those of a term infant under similar environmental conditions ( Fig. 35.1 ). In the preterm infant relatively rapid postnatal skin maturation occurs, leading to decreased evaporative loss over the first postnatal days and weeks ( Fig. 35.2 ). This maturation explains the gradual reduction in the environmental temperature needed to obtain thermal balance, even though evaporative heat loss still exceeds radiant heat loss for about a week in the most immature infants (see Fig. 35.2 ).

Fig. 35.1, Heat exchange through evaporation in relation to gestational age. Measurements from the first day of life during incubator care at an ambient humidity of 50%.

Fig. 35.2, Heat exchange in relation to postnatal age in different gestational age groups.

Evaporative heat losses are inversely related to ambient humidity ( Fig. 35.3 ), and measures to increase the vapor pressure close to the skin thus simplify fluid and thermal management. Note that the impact of postnatal age and ambient humidity on evaporative loss is less clinically important in newborn infants with a gestational age of more than 28-30 weeks and in the extremely preterm infant beyond the first postnatal week.

Fig. 35.3, Heat exchange through evaporation in relation to gestational age at different ambient relative humidity.

Respiratory Loss of Heat

Respiratory water and heat exchange take place by the combined processes of evaporation and convection. These processes occur as the temperature and vapor pressure of the inspired air rapidly equilibrates in the airway. Consequently, the losses are related to air temperature and humidity and directly proportional to the rate of breathing. Provision of a warm and humid environment (e.g., humidified incubator) and/or assisted ventilation with use of adequately warmed and humidified (saturated at ≥37.0°C) gas will reduce respiratory loss of water and heat to a minimum and aid temperature stability. Only in situations in which cold (pressurized) gas is delivered or when ambient temperature and humidity are low, such as during transport in a cold climate, will the heat loss through the airway be of clinical significance.

Thermal Physiology

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