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Exercise and eating cause an elevation in temperature (Marieb and Hoehn 2010).

Extremes of age affect a person’s response to environmental change. While young people will shiver at a temperature of 36°, most people over the age of 80 will not shiver until the body temperature falls to 35.1° (Kenney and Munce 2003). Thermoregulation is inadequate in the newborn and especially in lowbirthweight babies. In older people, there is an increased sensitivity to cold and the body temperature is generally lower (Nakamura et al. 1997).

Related theory

Hypothermia

Hypothermia is defined as a core temperature of 35°C that causes the metabolic rate to decrease (Trim 2005). Hypothermia may be classified as mild (32–35°C), moderate (28–32°C) and severe (less than 28°C) (Cuddy 2004). This occurs when the body loses more heat and is subsequently unable to maintain homoeostasis (Neno 2005). If the temperature does fall below 35°C, the patient will start to shiver severely (Edwards 1997). However, hypothermia frequently escapes detection due to symptoms being nonspecific and an oral thermometer’s failure to record in the appropriate range (Marini and Wheeler 2010). It can occur in all ages, although the elderly are at particular risk, and is often multifactorial in origin. It can arise as a result of:

environmental exposure

medications that can alter the perception of cold, increase heat loss through vasodilation or inhibit heat generation, for example alcohol, paracetamol

metabolic conditions, for example hypoglycaemia and adrenal insufficiency

the exposure of the body and internal organs during surgery and the use of drugs which dampen the vasoconstrictor response (Marini and Wheeler 2010).

Surgical patients having procedures longer than 1 hour have increased disruption to normal homoeostatic mechanisms resulting in a drop in temperature. Complications can include cardiovascular ischaemia, delayed wound healing and increased risk for wound infections and increase in postoperative recovery time (Wagner 2006).

Hyperthermia

Sudden temperature elevations usually indicate infection, making it prudent to perform a directed physical examination and, if indicated, obtain appropriate cultures and institute antibiotics. However, although infection is the most common explanation, several lifethreatening noninfectious causes of fever are frequently overlooked (Marini and Wheeler 2010) (Table 12.4).

Table 12.4 Non-infectious causes of hyperthermia

Agonist drugs Malignancy

Alcohol withdrawal Malignant hyperthermia

Anticholinergic drugs Neuroleptic malignant syndrome

Allergic drug or transfusion reaction Pheochromocytoma

Autonomic insufficiency Salicylate intoxication

Crystalline arthritis (gout) Status epilepticus

Drug allergy Stroke or central nervous system damage

Heat stroke Vasculitis hyperthyroidism

Fever caused by pyrexia (elevated body temperature) is the result of the internal thermostat resetting to higher levels. This resetting of the thermostat is the result of the action of pyrogens, which are chemical substances now known to be cytokines. Cytokines are chemical mediators, which are involved in cellular immunity (Marieb and Hoehn 2010). They enhance the immune response and are released from white blood cells, injured tissues and macrophages. This causes the hypothalamus to release prostaglandins, which in turn reset the hypothalamic thermostat. The body then promotes heatproducing mechanisms such as vasoconstriction. As a result of vasoconstriction, heat loss from the body surface declines, the skin cools and shivering begins to generate heat. These ‘chills’ are a sign that body temperature is rising (Marieb and Hoehn 2010) and are often referred to as ‘rigors’. A rigor is marked by shivering and the patient complains of feeling cold. The temperature quickly rises as a result of the normal physiological response to cold. This results in the following physiological changes.

Thermoreceptors in the skin are stimulated, resulting in