CHAPTER 22 Maintaining body temperature

Introduction

The aim of this chapter is to provide an understanding of the factors and processes involved in thermoregulation, i.e. the maintenance of body temperature at a near-constant level. An understanding of the physical, physiological and behavioural mechanisms will allow for a more rational approach to treatment for patients whose thermoregulatory system is disturbed. There are a number of reasons why body temperature might rise or fall from the ‘normal’ range, but before any decisions are made about treatment, the first step is to ensure that measurement is accurate.

The remarkably narrow range of normal temperature, together with the fact that infections cause body temperature to rise, allows us to use temperature as a measure of disease. However, the practice of temperature measurement and monitoring is often based on ritual rather than on rational decision-making. The responsibility of the nurse is to:

• understand the mechanisms for ‘normal’ control of body temperature at approximately 37°C

• ensure that temperature measurements are accurate and that the body site used for temperature measurement is appropriate

• monitor and evaluate the impact of any treatments used to regulate body temperature.

It is for the above reasons that patients should have their temperature measured and assessed by a registered nurse or nursing student under supervision.

Normal body temperature

‘Body temperature’ can be a misleading term because the body is not at a uniform temperature. For example, skin temperature varies considerably. The skin has been described as a mosaic of temperatures and even ‘core’ (deep body) temperature can vary slightly. For this reason it is good practice to use the same body site for each temperature measurement and to report the temperature of the site used, e.g. ‘Axilla temperature of Mr A on admission to hospital was 36.9°C’ or ‘Oral temperature of Mrs B was 37.1°C’.

The temperature of the tissues of the body

Surface temperature

Under most circumstances, the skin surface or ‘body shell’ is the coolest part of the body. The organs, blood and deeper tissues are described as the body ‘core’ (Tortora & Derrickson 2006). Because the temperature of the skin is influenced by the temperature and humidity of the air (Figure 22.1), measurement of skin temperature is not a reliable method to detect temperature changes of the organs and deep body tissue (Box 22.1). This may be most important when caring for patients with critical illness where accurate temperature measurement is likely to be of importance in the diagnosis of infection.

Figure 22.1 Core temperature and temperature of the skin surface at various sites in a hot, a thermoneutral and a cold environment.

(Based on an original figure by Aschoff & Wever, cited in Stainer et al 1984, with additional data from Childs C.)

Deep body (core) temperature

The organs of the head and thorax (the main components of core tissues), e.g. heart, liver and brain, are ‘insulated’ from environmental conditions by bone, fat and skin. The liver, kidneys, brain and myocardium have a high metabolic rate and consequently a higher temperature than tissues with a lower rate of metabolic activity such as smooth muscle and skin (Houdas & Ring 1982).

In a recent publication by Moran et al (2007) the authors undertook a prospective observational cohort study to investigate whether tympanic measurements were a reliable measurement technique in the critically ill patient. Different conventional methods for body temperature measurement were compared. Access the paper by Moran and colleagues on the website to find out whether the authors considered tympanic membrane temperature to be a valuable measurement method in critically ill patients.

See website Box 22.1

See website Research abstract 22.1 for information about a study by LeFrant et al (2003) that demonstrated the variation in core temperatures in the different organs, muscles, etc.

See website Research abstract 22.1

Regulation of body temperature

Mammalian thermoregulation is controlled by the brain, specifically the pre-optic region within the anterior portion of the hypothalamus (POAH) (see website Figure 22.1 for anatomy of the hypothalamus). Incoming (afferent) signals to the hypothalamus from thermoreceptors in skin and organs are transmitted to and integrated within the POAH, which then responds by transmitting outgoing (efferent) signals to the body so that heat can be either generated/conserved or lost.

See website Figure 22.1

For example, if the temperature of the blood bathing the cells of the hypothalamus starts to rise above ‘normal’ (37°C), as in a fever, the outgoing, or efferent, signals stimulate ‘effector’ mechanisms so that heat is lost from the body. Conversely, if the incoming blood is at a lower temperature than the reference (set-point) temperature (i.e. <37°C), mechanisms to conserve or produce heat will be activated. In this way, deep body temperature is prevented from rising or falling from the biological set-point of 37°C. The pathways that help to maintain body temperature at a constant level are shown in Figure 22.2.

Figure 22.2 Control of body temperature. A. Responses to a cool environment or lowered core temperature. B. Responses to a warm environment or raised core temperature. BAT, brown adipose tissue; C, convection; K, conduction; NST, non-shivering thermogenesis; R, radiation.

Mechanisms of heat conservation and heat production

In early studies, patients suffering from serious burns (see Ch. 30) were found to have a very high metabolic rate and also a very high body temperature (Childs 1994). Wilmore (1977) reported that metabolic heat production in severe burns was almost double that of a healthy person of the same age. Later, it was found that improvements in surgical management, wound care and analgesia ameliorated the enormous increases in metabolic rate and demand for energy (Childs 1994) although patients remained febrile. Part of the high metabolic ‘cost’ of burn injury could be reduced by early surgery (to promote healing of the burn wound) and pain management but fever can persist and is a cause of raised metabolic rate, especially if the patient develops a burn wound infection.

Any activity or event that increases the rate of chemical reactions in cells and the rate of oxygen uptake by the cell increases metabolic rate and thus the amount of metabolically produced heat (Frayn 1997). Body heat is produced in two ways: chemically by non-shivering thermogenesis, and physically by the rhythmical contractions of skeletal muscle, i.e. shivering thermogenesis.

Non-shivering thermogenesis

Although peripheral vasoconstriction is very effective in conserving body heat, additional sources of heat may need to be generated to restore body temperature to ‘normal’. This can be achieved by non-shivering thermogenesis (NST). As its name implies, NST does not involve muscular contraction to stimulate endogenous heat production, but relies upon release of the thermogenic hormone, noradrenaline, under the control of the sympathetic nervous system. Although muscle tissue is the most important source of chemically produced heat, other important heat-producing organs are the brain and liver.

Shivering thermogenesis

People who are cold shiver. The heat conservation area of the brain stimulates mechanisms that increase muscle tone, i.e. shivering, such that heat production increases to five times the basal rate. Shivering occurs in most skeletal muscles but the greatest intensity of shivering thermogenesis is in the jaw and neck and is least in the legs. The repetitive contraction of muscle is not, however, a very economical process, as only 40% of the heat generated during shivering is retained by the body and shivering does not continue indefinitely (see Hypothermia, p. 627).

Mechanisms of heat loss

High air temperature and strenuous exercise will raise the temperature of the blood. If the heat gained during the exercise is not matched by an equivalent rate of heat loss, heat-losing mechanisms are initiated and heat-conserving mechanisms are inhibited (Figure 22.2). The series of events that protects the brain and core tissues from reaching dangerously high temperatures is as follows:

• A high air temperature warms the skin. The rise in skin temperature is detected by skin thermoreceptors which send afferent signals to the thermoregulatory centre in the POAH.

• The first reaction of the person is to reduce body insulation by removing some clothing.

• At the same time, blood vessels in acral regions (hands, feet, ear lobes and nose) dilate so that more blood is brought to the surface of the skin. Areas of the body such as the face and ears will then appear ‘flushed’, pink and warm.

• Providing the air temperature is lower than the skin temperature, heat will be lost by radiation and convection (Table 22.1). If, however, the air temperature is higher than skin temperature, the person will gain heat from the environment. This is why evaporative heat loss becomes so important when the temperature gradient between skin and air is narrow, i.e. when skin temperature is close to air temperature.

• Stimulation of sweat glands in hot conditions is controlled by the sympathetic nervous system. Production of a fluid consisting mostly of water, but also containing salt, urea, lactic acid and potassium ions, onto the skin causes cooling when the thermal energy needed to transfer the fluid to a gas is removed from the body, i.e. from the skin surface to the air. This transfer of energy from a fluid to a gaseous state is called vaporisational heat loss and occurs during heat loss by sweating.

Table 22.1 Principles of heat transfer

Route for Heat Loss	Principle	Relevance to Clinical Practice
Radiation (R)	Transfer of energy in the form of electromagnetic waves. The human body emits heat as infrared radiation. At the same time all dense objects (furniture, buildings, other people) are also radiating heat. The rate at which heat is emitted from the human body is dependent upon the temperature difference (gradient) between the skin and other objects and surfaces in the room. If the skin is hotter than the average temperature of objects in the room, heat will be lost. If the objects in the room are hotter, the body will gain heat	A person, naked, sitting quietly in a room at 25°C loses between 50 and 70% of heat by R, the major route for heat loss under such conditions. As air temperature increases, the temperature gradient between skin and air falls such that less heat is lost by this route. As air temperature rises towards skin temperature (35°C), the gradient will be so small that very little heat loss can take place by this route. Evaporative heat loss then becomes an important route for heat loss
Convection (C)	Air (or water) next to the body warms and moves slowly away because warm air is less dense and rises. As it moves away from the body, cooler air replaces it. This process can be speeded up if a strong draught (e.g. electric fan) is used to force the air away and cause a rapid replacement of warmed air with cool air	Nurses frequently increase the rate of heat loss by convection by placing electric fans close to their patients. This can be a very efficient way to lower skin temperature, but frequently the cool stimulus results in an inappropriate response, i.e. peripheral vasoconstriction. Heat retention within core tissues can cause core temperature to rise rather than fall. In febrile patients who have an elevated set-point, the use of electric fans is likely to cool the skin and return heat to the tissue
Conduction (K)	Heat loss by K involves transfer of thermal energy from atom to atom. The skin must be in contact with cooler or hotter objects for heat exchange to take place by K	Critically ill patients are often nursed on special beds designed to reduce the incidence of pressure ulcers. These beds are often maintained at a constant temperature to help prevent heat loss from the body. Sometimes the thermostat can fail and patients have been known to overheat or to cool because the temperature of the bed is too high or too low. Patients must be protected from body temperature disturbances of such an iatrogenic nature
Evaporation (E)	Evaporation of water from the skin and respiratory passages is the most important route for heat loss in hot conditions. The evaporation of water occurs when energy transforms water and sweat droplets to a gas. The heat (or thermal energy) needed to drive this process is taken from the body. Thus the more water there is on the skin, the more heat is taken from the body to turn it into a gas (vaporisation). The more heat removed from the body in the process of E, the more the body cools. The function of sweat (produced under the control of the sympathetic nervous system) as an agent for vaporisational heat loss can be enhanced by spraying the body with a fine mist of warm water	Patients with a high core temperature may not always sweat. During a rise in rectal temperature the body responds as though it were too cold and if patients are observed carefully it will be seen that their skin is dry. At this stage the hypothalamic set-point is above normal but the body continues to activate heat-conserving mechanisms to achieve the new set-point temperature. Only when the patient has reached the new central temperature set-point will heat loss by all routes (E included) be activated. Thus when nurses notice that patients with a high core temperature are sweating, it is more likely to indicate that core temperature has reached the new set-point. A fall in body temperature may then follow

Fluctuations in a healthy person’s temperature

Humans are able to make both physiological and behavioural adjustments to maintain deep body temperature close to 37°C. Like many mammals, humans have the ability to increase the amount of heat in the body as air temperature falls, or to increase heat loss when conditions become uncomfortably hot. It is for this reason that humans are able to live successfully in most climates on Earth.

Normal temperature range

Although central or hypothalamic temperature is ‘set’ at a relatively constant level, fluctuations do occur in healthy people, e.g. following exercise. No harm is done to the cells of the body by a change in temperature, providing core temperature does not rise above or fall below critical limits. Indeed, the ability of the thermoregulatory system to stimulate changes in heat production or heat loss indicates that the system is operating efficiently.

Website Figure 22.2 illustrates the range of ‘normal’ temperature in health.

See website Figure 22.2

In the early morning or during cold weather, body temperature may fall to 35–36°C. After moderate exercise, and also, for example, in crying babies, core temperature may rise to approximately 38°C; after hard exercise, body temperature can reach 40°C without causing harm. However, an increase of approximately 5°C above 37°C indicates a serious disruption of thermoregulation and a person’s life may be at risk if deep body temperature rises above 43°C or falls below 24°C.

Circadian rhythm

Body temperature fluctuates in a characteristic pattern over a 24-h period. It is thought that this pattern is a result of regular, but normal, deviations of the body’s own thermostat or hypothalamic set-point temperature. As with many other physiological functions, thermoregulation displays a circadian rhythm (diurnal variation). This rhythm persists during short periods of night work. Eventually, however, regular night work will reverse the normal pattern so that lower temperatures occur during the day and higher temperatures at night.

In general, the lowest temperatures recorded over a 24-h cycle will be approximately 0.5°C lower than the afternoon temperature. By the evening, body temperature may be as much as 1.0°C above the early morning temperature. In the newborn, a circadian rhythm of core temperature is not fully developed (Waterhouse et al 2000).

An awareness of the normal changes in deep body temperature in health is necessary if nurses are to interpret temperature measurements correctly. For example, a moderately elevated axilla temperature recorded during the afternoon should be monitored closely before any action to lower the temperature is taken because of the cyclical nature of body temperature, otherwise treatment for pyrexia could be instigated unnecessarily.

Other factors affecting a healthy person’s temperature

Babies and young children

As long ago as 1937, Bayley & Stolz showed that rectal temperature begins to rise during the first 7 months of life, remaining fairly constant until the age of 2 years, after which it begins to fall. Average rectal temperature at age 1 month was reported to be 37.1–37.2°C, at 8 months 37.6–37.7°C, and at 18 months 37.7°C. By the time the children in the Bayley & Stolz study approached their third birthday, rectal temperature had settled to 37.1°C.

Higher temperatures in early childhood are thought to be a result of increased cellular and metabolic activity. The young of most mammals have a large body surface area relative to their body weight. As a result, babies have a higher risk of hypothermia because they cannot shiver and cannot increase their body insulation, as can adults, by putting on more clothes. This means that a naked baby will lose relatively more heat than an adult for every square metre of their body surface. Babies are therefore reliant on parents and carers to protect them from extremes of environmental temperature.

To counteract increased heat loss, babies have an important source of body heat: brown adipose tissue (BAT), a specialised type of fat cell (Cannon & Nedergaard 2004). BAT is a unique type of adipose tissue, shown by Hull (1976) to be an important source for heat production in small hibernating mammals, and has been described as the ‘hibernating gland’. BAT is also stimulated in the human newborn infant after birth. Under the control of the sympathetic nervous system, the function of BAT is to transfer energy from food into heat (Cannon & Nedergaard 2004) and probably plays a role in the evolutionary success of mammals. BAT can be found around the kidneys, between the shoulder blades, around the great vessels and deep within the axillae (Frayn 1997). BAT is richly supplied with blood and oxygen and this dense blood supply is responsible for its brown colour. It becomes much less important in maintaining the temperature of the body as a baby gets older. In adults, BAT is scarce and probably not functional (Klaus 2004).