The normal core temperature in adults is 37°C. The core temperature is that of the brain and abdominal and chest organs (Johnson & Taylor 2006) and should remain constant despite changes in the external environment. Peripheral temperature fluctuates depending on the temperature of the external environment and there may be approximately 1°C difference between core and peripheral temperature (Blows 2001). The body seeks to maintain a core temperature of 37°C as this is optimum for the activity of the enzymes that regulate biochemical changes and physiological functions (Coad 2006).

Maintaining this optimum temperature is therefore a balance between heat production and heat dissipation. Heat is produced during cellular metabolism, particularly in the liver and muscle cells (Blows 2001) and is dissipated via the blood to the peripheries of the body. Temperature receptors in the skin detect changes in temperature and send nerve impulses to the cerebral cortex and hypothalamus in the brain. The hypothalamus also detects the temperature of the blood directly, as do sensors in some internal organs, the spinal cord and major blood vessels (Guyton & Hall 2007). The hypothalamus activates changes necessary to maintain the core temperature at 37°C.

When core temperature exceeds 38°C, this is termed pyrexia (Mallik et al. 1998). If the temperature rises above 37°C the sympathetic nervous system is activated and sweating or diaphoresis is induced, enabling the body to cool by a process of evaporation. Dilation of the peripheral skin vessels results in blood being brought closer to the skin surface and heat is lost through radiation. Voluntary acts, like shedding clothes or drinking, are also activated.

Heat loss can be summarized as follows:

Pregnant women often have warm hands and feet, due to a seven-fold increase in blood flow to the extremities (Coad 2006). The basal metabolic rate is raised, resulting in increased heat generation of up to 35% (Johnson & Taylor 2006) and an increase in maternal temperature of 0.5°C. Following childbirth temperature can be elevated to 38°C for 24 hours and this may be attributed to dehydration (McKinney et al 2000). Maternal temperature may also rise on the second or third days postnatally, when milk production begins and persists for a few days (Johnson & Taylor 2006).

A temperature of up to 37.2°C is acceptable for a baby in a neonatal unit or postnatal ward. A temperature below 36.5°C is considered hypothermic (Baston & Durward 2001). Coad (2006) describes how heat transfer is affected by two gradients: the internal gradient involves heat transfer from the core to the surface of the baby and the external gradient involves heat transfer from the body surface to the environment. Heat is lost rapidly at birth as the external environment is cooler than the uterus. The baby is also wet with amniotic fluid, evaporation of which leads to rapid cooling. Neonates have a high surface area to body weight ratio. The surface area from which they can lose heat is much larger than the body mass that can generate heat. They have less subcutaneous fat than adults, which means that their blood vessels are nearer the skin surface facilitating rapid transfer of heat from the core to the skin surface. The head is particularly prone to heat loss, comprising 25% of the neonate’s surface area (Stables 2005).

The neonate can generate heat in response to a drop in environmental temperature. Brown adipose tissue (BAT), a highly vascular tissue packed with mitochondria, is stimulated to produce heat following the release of adrenaline and noradrenaline from the sympathetic nervous system. This form of thermogenesis is known as ‘non-shivering thermogenesis’ (NST) dependent on sufficient supplies of oxygen and glucose. The neonate’s natural attitude of flexion also helps prevent heat loss (Burroughs & Leifer 2001). Allowing the neonate to become cool is potentially lethal. A cold baby will utilize stores of glucose and increase its oxygen consumption in an attempt to increase its basal metabolic rate (BMR) through the metabolism of BAT. Eventually this process may result in low blood sugar (hypoglycaemia) and lack of oxygen to the tissues (hypoxia) leading to a lowering of blood pH (acidosis). The fatty acids released by the metabolism of brown fat can also interfere with the transportation of bilirubin to the liver resulting in hyperbilirubinaemia (McKinney et al 2000). The neonate is less well equipped to lose heat purposefully, peripheral vasodilatation being the main source of heat loss (Coad 2006). Although the neonate has sweat glands they are immature and limited in their capability to increase heat loss by evaporation.

With the development of electronic thermometers the use of the ear as a site has become more common. It is a generally acceptable site for all people, however there are mixed results reported from respective research on the accuracy of this method for both adults and children (Purssell 2007).

This is the most appropriate site for taking the neonate’s temperature. The thermometer should be positioned under the axilla, perpendicular to the chest wall and the arm held in place for the required length of time, depending on the manufacturer’s guidelines for the type of thermometer in use. Any procedure involving the baby should avoid undue removal of clothing. The axilla is also the site of choice in clients whose condition renders them unable to tolerate an oral thermometer (see above) or who may inadvertently bite it.

This site for temperature estimation is not recommended for routine practice due to the potential risk of trauma to the rectal mucosa in babies and embarrassment in adults.