Education And Debate

ABC of Sports Medicine: Temperature and Performance - II: Heat

BMJ 1994; 309 doi: https://doi.org/10.1136/bmj.309.6954.587 (Published 03 September 1994) Cite this as: BMJ 1994;309:587
  1. E L Lloyd

    The general principles of heat balance were considered in the article on cold (20-27 August, p 531).

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    Heatstroke

    Heatstroke develops when the body is unable to get rid of the excess heat being produced. There will be a varying combination of high environmental temperature, high humidity, lack of wind, vigorous activity, heat retaining clothing, and dehydration. Early symptoms include excessive sweating, headache, nausea, dizziness, hyperventilation, and disturbance of consciousness. Consciousness may be lost or clouded and there may be hallucinations. There may be muscle twitching or convulsions and loss of control of the body sphincters. In severe cases there may be deep coma with pinpoint pupils. They may be in shock with tachycardia. Tachpnoea is often present and breathing may be difficult and vomit inhaled. The patient feels warm or hot and has a high core temperature (rectal usually >41°C). Sweating may or may not be present. Diagnosis depends on a high index of suspicion.

    Treatment of heatstroke

    1. Lay the patient flat and raise the legs

    2. Cool by removing clothing within modesty, spraying with warm or tepid water, and fanning with warm air Do NOT use ice baths, ice packs, cold sponging, or blowing cold air. This may kill the patient because the cold stimulus causes vasoconstriction, thus reducing heat loss, and triggers shivering, thus increasing heat production

    3. Rehydrate with sodium rich fluid like 0.9% saline. Several liters may be required to restore or maintain blood pressure. Intravenous bicarbonate will be needed to counteract the metabolic acidosis but this is best done in hospital by measuring acid/ base status. Hydrocortisone (repeated doses of 100 mg intravenously) may be necessary if the blood pressure is falling

    4. Transfer to the intensive care unit in a hospital

    Management

    Without correct treatment a heatstroke victim is in great danger of developing irreversible damage in the brain, kidneys, liver, and adrenal glands or of death. Disseminated intravascular coagulation may occur. Treatment should be started as early as possible. Unnecessary cooling is much safer than waiting for a definite diagnosis.

    Local effect of heat

    Sunburn

    Unprotected exposure to sun causes sunburn (tanning is impossible without risking a burn). It accelerates skin aging, may cause drug photosensitisation, and depresses immune responses in the skin - leading to recurrent herpes labialis - for example in skiers. It also increases the risk of skin cancer- basal and squamous cell carcinomas on exposed areas and melanomas anywhere on the body. The risks are increased by altitude, by reflective surfaces (such as fresh ice, snow, water (especially morning and evening), sand, metal, concrete, and by wind (“windburn” is exacerbated sunburn). Sunburn can occur unexpectedly during cloudy weather.

    Sunburn prevention

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    Treatment of sunburn

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    Other effects of heat Dehydration, loss of performance, collapse

    In high environmental temperatures 60% of cardiac output may pass through the skin for cooling and sweat production, and unacclimatised people will therefore perform less well in heat. Loss of fluid of 1% of body weight (600-800 ml) by sweating will lead to reduced performance. To prevent dehydration people should drink at least an extra cupful of water every hour (1 1\2-2 litres a day) and be weighed three times a days because rapid weight loss in the heat means water loss not fat loss. People losing weight should therefore drink more - possible even 10 litres - to maintain body weight. Urine should be plentiful and light coloured. Urine that is dark, strong smelling, or of reduced volume means the person has a large fluid deficit.

    Salt depletion from sweating causes tiredness, irritability, giddiness, fainting, cramps, and loss of performance. During the first 10 days in the heat additional salt should be taken as salt tablets or dilute (0.1%) solution. A normal diet should provide sufficient of the other electrolytes.

    Hyperventilation is usually an indication of possible heatstroke, with respiratory alkalosis developing to compensate for metabolic acidosis. Occasionally a person who has a reduced ability to sweat, increases heat loss by tachypnoea and by panting, producing uncompensated respiratory alkalosis with unconsciousness and tetanic spasm. This occurs after a competitor has appeared normal at the end of the event, and it will resolve completely with simple rest. In heatstroke there is unlikely to be tetanic spasm, but hyperventilation in an athlete should lead to a suspicion of heatstroke.

    Precautions to take when exercising in the heat

    Clothing should be white, light weight, and loose fitting. Natural fibres are safer than synthetic ones. During endurance exercise small quantities of fluid should be drunk at frequent intervals and water sprayed on the skin at every opportunity. The use of sweat inhibiting deodorants should be avoided, and people should not exercise in the heat immediately after a glucose or high carbohydrate feed because blood is diverted from the skin into the gastrointestinal tract. Endurance events should be cancelled if the wet bulb/globe thermometer index exceeds 28°C.

    After exercise

    * Drinks should be cool but not iced.

    * Showers should not be too cold because cold on the skin stimulates heat production and causes vasoconstriction thus reducing heat loss. People would stay hot and start sweating immediately after coming out of the shower and, at worst a cold shower might precipitate heatstroke. A safe and effective cooling method is to allow a cool shower to play over only the head, neck, hands, wrists, feet, and ankles.

    Acclimatisation to heat

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    Acclimatisation, which should be specific for the destination environment - that is, dry or humid heat - produces beneficial pysiological changes.

    Adaptation occurs over 10-14 days of heat exposure (the biggest changes in days 3-5), but is lost within a few weeks unless exposure to heat is repeated regularly at intervals of four days or less. Exercise training by itself is less effective than regular heavy exercise in the heat - for example, in a hot room or sauna or wearing an impermeable track suit.

    The photographs are reproduced with permission of Allsport

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