THERMOREGULATION

Paula Forsythe

Overview

In utero, the fetus is dependent on the mother as the source of heat. At birth, the neonate transitions from a warm environment to one that is much colder. After delivery, the neonate must assume self-regulation of body temperature. This is one of the most significant challenges faced by every newborn. Failure to regulate temperature has serious implications for many body systems and may cause even healthy newborns to experience respiratory, metabolic, and cardiovascular complications, all of which can be prevented by diligent nursing care.

Background

Thermoregulation is the ability to regulate heat production and loss to maintain normal body temperature between 36.5°C to 37.5°C (American Heart Association and American Academy of Pediatrics, 2015; Brand & Boyd, 2015; Knobel-Dail, 2014). It is a key physiologic requirement for survival. In utero, fetal body heat is the result of the infant’s rapid metabolic rate and heat transferred from the mother via the placenta and uterus. The result is a body temperature 0.3°C to 0.5°C higher than the mother (Asakura, 2004; Knobel-Dail, 2014). Heat loss and cold stress, which often occurs during delivery and in the minutes that follow, stimulate the newborn’s skin and thermal receptors, especially the trigeminal area of the face, to signal the hypothalamus to conserve or produce heat (Brand & Boyd, 2015; Karlson, 2013). The hypothalamus activates the sympathetic nervous system and norepinephrine is released triggering several adaptations vital to maintaining body temperature (Brand & Boyd, 2015; Chaplain Maternal Newborn Regional Program [CMNRP], 2013; Karlson, 2013). Norepinephrine increases metabolism, respiratory rate, oxygen consumption, and the utilization of glucose. Norepinephrine also vasoconstricts peripheral blood vessels to minimize environmental heat loss and retain core body heat, as well as vasoconstricts pulmonary vessels, increases pulmonary vascular pressure, and shunts deoxygenated blood away from the lungs through the ductus arteriosus to the aorta. It also stimulates brown fat metabolism (Brand & Boyd, 2015; Karlson, 2013). This constellation of physiologic reactions places the newborn, especially those born premature and/or ill at risk for hypoxemia, hypoxia, hypoglycemia, anaerobic metabolism, and acidosis, which can lead to cell damage and even death (Brand & Boyd, 2015; CMNRP, 2013; Karlson, 2013).

Neonates rely on nonshivering thermogenesis to metabolize brown adipose fat (BAT) and release energy. BAT contains mitochondria, fat vacuoles, sympathetic nerve endings, and an abundant blood supply (Asakura, 2004; Brand & Boyd, 2015). BAT develops primarily during the third trimester of pregnancy and is deposited around the kidneys, adrenal glands, mediastinum, scapulae, and 225axilla. BAT serves as the primary source for heat generation during the perinatal and postnatal period. Metabolism of brown fat requires oxygen and glucose and is stimulated by nerve endings that activate lipase, resulting in lipolysis, fatty acid oxidation, and the generation of heat, which warms circulating blood, transferring heat throughout the body (Brand & Boyd, 2015; CMNRP, 2013; Karlson, 2013).

Although central thermoregulatory mechanisms are present at birth, they are developmentally deficient and not well differentiated (Asakura, 2004; Knobel-Dail, 2014) placing neonates, particularly the smallest and most critically ill, at great risk for heat loss or hypothermia. Heat loss occurs by evaporation, conduction, convection, and radiation. Factors that impact the rate of body heat transfer or loss in the neonate include a large surface area to body mass ratio; decreased amounts of subcutaneous fat and BAT; high body water content; immature, nonkeratinized skin that allows transepidermal water and heat loss; vasoconstrictive, motor and metabolic function; and differences in temperature between the neonate and environment. Without appropriate environmental modifications and nursing care interventions, the newborn loses body heat at a rate of up to 1°C per minute (Brand & Boyd, 2015; Karlson, 2013).

Hyperthermia, a condition in which the body temperature higher than 37.5°C (Karlson, 2013), is less common in the neonatal period but can result in deleterious effects. Neonates are at high risk for hyperthermia because of their inability to dissipate heat (Brand & Boyd, 2015

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