In health, the core temperature of humans is maintained by the thermoregulatory centre in the hypothalamus at a constant 37°C. Heat is produced by cellular metabolism, and is lost through the skin by vasodilatation and sweating and in air expired from the lungs. Sweating occurs when the ambient temperature is greater than 32.5°C and during exercise. The evaporation of sweat is an important mechanism in keeping the skin cool and the body temperature down.
Acclimatization to a hotter climate takes 1-2 weeks. There is a gradual increase in sweating, and the sweat has a lower salt content. This process allows increased evaporation.
These are painful cramps in the muscles (usually of the legs) after exercise. They often occur in fit young people who are well acclimatized when they take vigorous exercise in hot weather. The symptoms are thought to be the result of a low extracellular sodium caused by replenishment of water but not salt during prolonged sweating. The cramps respond to salt and water replacement and can be prevented by increasing dietary salt intake.
This usually occurs in subjects who are not acclimatized and who undertake heavy exercise. It typically occurs in troops who are suddenly landed in a hot climate without prior acclimatization. Heat exhaustion is caused by water depletion, or salt and water depletion, due to sweating. Water loss can be as high as 5-6 litres per day, and up to 20 g of salt can be lost. Common symptoms are giddiness, generalized fatigue, weakness and syncope. Many patients are not seriously affected, but they may go on to develop hypotension, a rise in body temperature to 38-40°C, and signs of volume depletion. Dehydration and delirium can eventually occur. Sweating usually continues until the late stages. The serum sodium can be high in water depletion, but is normal or low if both water and salt are depleted.
The patient is removed from the heat and cooled using cold sponging and fans. Oral rehydration with both salt and water may be all that is required; 25 g of sodium chloride and 5 litres of water in the first 24 hours is given, with adequate .replacements thereafter. In severe heat exhaustion, intravenous fluid is required. Isotonic saline is usually given, depending on the level of sodium in the serum. Careful monitoring is required and any subsequent potassium loss must be corrected.
Heat stroke is an acute life-threatening situation when the body temperature is above 41°C. The patient suffers from headache, nausea, vomiting and weakness. The skin is hot. Sweating is often absent, but this is not invariable, even in severe heat stroke. Neurological involvement leads to confusion, delirium and eventually coma. Heat stroke occurs in hot, humid climates with little cooling wind, even without exercise. Patients are usually unacclimatized; in some, sweating is limited owing to prickly heat (i.e. inflammation of the sweat glands after prolonged exposure to high temperatures). Old age, diabetes and alcohol are all further precipitating factors. The diagnosis is clinical. The patient must be rapidly removed from the hot area, and then cooled with sponging and ice if available.
Unconscious patients need to be managed in intensive care and rapid cooling with ice packs started. Fluids may be required, but these must be given with care as hypovolaemia is not present in many patients. Prompt treatment is essential and can lead to a rapid and complete recovery; any delay may be fat~l.
• Cerebral oedema
• Renal and hepatic failure
Treatment of the complications is described in the appropriate chapters.