Acute heat illnesses


  • Suspect always heatstroke when physical activity is accompanied by hot skin, altered consciousness, hypotension, hyperventilation, nausea or diarrhoea. Sweating may be excessive but will stop especially when dehydration develops.
  • Measuring the rectal temperature is in the field setting the most reliable method for determining the internal body temperature to identify severe heat illnesses. In heatstroke, rectal temperature is usually over 39°C. Axillary or ear temperature measurements may lead to wrong conclusions.
  • In heatstroke the first aid is cooling, which must be started without delay. Also an infusion of isotonic physiological saline solution is started as soon as possible.

Predisposing factors

  • Poor physical condition, considerable overweight
  • Hypovolaemia
  • Physical activity in warm conditions
  • Wrong type of clothing
  • Poor air conditioning
  • Sleep deprivation and circadian rhythm disturbance
  • Hot vehicle, particularly for children
  • Heart failure, diabetes, hyperthyroidism, dementia
  • Medication (tricyclic antidepressants, phenothiazines, SSRIs, anticholinergics, antihistamines, diuretics, beta-blockers, some NSAIDs when used regularly)
  • Alcohol, psychostimulants
  • Convalescence (common cold or gastroenteritis)
  • Age (children and the elderly)
  • Poor adaptation to heat (travelling to hot climate or the first days of a heatwave at home)
  • Previously suffered heatstroke


  • The most severe type of illness caused by heat and may lead to death.
  • Fitness events and working in hot conditions involve a risk of heart illness. When protective equipment is used or when the work takes place in humid conditions the body temperature may rise quickly even if the ambient temperature is not very high.

Symptoms and findings

  • In an acute heatstroke associated with physical stress, the consciousness is lost without prior symptoms or after short prodromal symptoms (disorientation, inappropriate behaviour).
  • In a slowly developing heatstroke the prodromal symptoms such as poor appetite, weakness, nausea, diarrhoea and disorientation may appear in the course of a few days. Dehydration develops insidiously and decreases sweating.
  • Rectal temperature is usually 39°C or more (up to 45°C).
  • Hypotension, tachycardia (over 100/minute)
  • Skin is often dry in the slowly developing type and sweaty in the type associated with physical stress. Cessation of sweating with associated goose-pimpling and cold shivers is in both forms a sign of a severe disturbance of temperature control.
  • Signs of dehydration often appear.
  • Laboratory findings
    • Hypernatraemia due to dehydration
    • Hypokalaemia is common in the first phase.
    • Hypoglycaemia sometimes appears after physical stress (drinking a lot of energy drinks containing short-chain carbohydrates may predispose to hypoglycaemia).
    • Renal failure occurs in the late phase, and hyperkalaemia and occasionally hypocalcaemia are found. In the acute form, renal insufficiency will develop in up to 30–35% of the patients, in the more slowly developing form in less than 5% of the patients.
    • On the ECG, ST changes, T inversions and conduction abnormalities are seen (may sometimes simulate myocardial infarction).

Differential diagnosis

  • Septicaemia, epileptic seizure, intracranial bleeding, ordinary syncope (normal body temperature)


  • Basic life support
  • Side position
  • Cooling as quickly as possible (starting on the scene)
    • The best method is to sprinkle, pour or spray water on the entire skin, and at the same time fan the patient with 2–3 fans or clothes.
    • Immersing in cold water is the fastest way of cooling healthy body. The method is not recommended in cooling elderly patients, children, or patients with underlying conditions, due to the strong impact on circulation.
    • Ice packs are beneficial if positioned correctly. Good places include armpits, neck and groin area.
  • Oxygen
  • Sufficient diuresis
  • I.v. drip: isotonic NaCl, possible hypernatraemia should not to be worsened by infusing the wrong salts.
  • Transportation to hospital (intensive care unit) after treatment to lower the body temperature has started. Continue treatment during transportation.
  • Convulsions of the large muscles produce a lot of heat. Diazepam is the primary medication for managing the convulsions.

Laboratory tests

  • Glucose (quick test)
  • Baseline values for plasma potassium and sodium, if possible. Along with fluid therapy the electrolyte balance may change rapidly, and monitoring of the laboratory parameters should be regular during the initial phase of the treatment.
  • Basic lood count with platelet count (leucocytosis is often associated with dehydration)
  • CRP (to differentiate from infection; should be analysed as early as possible)
  • Acid-base balance
  • Plasma creatinine
  • Plasma creatine kinase (isoenzymes as well), AST and lactate dehydrogenase
  • Coagulation status (bleeding time, prothrombin time, activated partial thromboplastin time (aPTT)
  • Lumbar puncture if there is any suspicion of central nervous system infection or subarachnoidal bleeding


  • DIC (disseminated intravascular coagulation; the most common cause of death)


  • Adequate intake of fluids is ensured during physical strain.
  • Fluid resuscitation: 4 dl of water before strenuous physical activity, and 1–2 dl every 20 minutes during the activity (e.g. a marathon).
  • When working in a hot environment, the rhythm of work and breaks is adjusted according to the environmental conditions.
  • Wearing of appropriate clothing that reduces heat load and protection of the head against direct solar radiation should be taken care of.
  • Fluid balance must be maintained during the working day.
  • Although hot weather may reduce appetite, regular eating secures the preservation of body energy stores and maintains for its part the fluid and sodium balance.
  • Rehydration must be adjusted to the heat effect in the environment and to the level of physical strain. Unnecessary excessive overrehydration may also lead to a disturbance of the electrolyte balance in the body.


  • Caused by the heat stress directly on an uncovered head.
  • Symptoms: headache, irritation, nausea, dizziness and other symptoms originating in the central nervous system.
  • Treatment: taking shelter in a cool place, rest and drinking.

Heat oedema

  • Hypertension and overweight predispose to oedema of the lower extremities.
  • Treatment: rest, raising the legs and plenty to drink. Avoid diuretics if there is no pre-existing disease indicating their use.

Heat cramps

  • Heat cramps occur most commonly in the calf muscles, especially when only water is used for rehydration during prolonged exercise.
  • Treatment: oral energy drinks containing long-chain carbohydrates, 0.1% salted water (half a teaspoon of NaCl in 2 litres of water) or in severe cases i.v. infusion of isotonic saline.

Heat exhaustion

  • Often a precursor to heatstroke. The main underlying factor is incorrect hydration.
  • Mild heat exhaustion is often an underdiagnosed condition that may increase proneness to accidents and erroneous actions in the work and may impair general physical and mental functional ability.
  • Three subtypes can be distinguished according to the nature of dehydration.

Hypertonic dehydration

  • If fluid loss in not compensated, hypertonic dehydration will develop (hypernatraemia). It is usually caused by physical exercise and heat.
  • Symptoms: tiredness, weakness, hyperventilation, disorientation, thirst and high temperature. (Remember serious bacterial infections in the differential diagnosis).
  • Treatment: drinking water

Hypotonic dehydration (salt deficiency symptoms)

  • If the patient is rehydrated with water alone, salt deficiency symptoms may develop slowly.
  • Symptoms: headache, weakness, nausea and GI symptoms. A feeling of thirst and elevated temperature is not as common as in hypertonic dehydration. Serum sodium is low, and in severe cases AST is elevated.
  • Treatment: i.v. infusion of isotonic physiological saline

Isotonic dehydration

  • Deficiency of both water and salts. Plasma sodium is normal.
  • Treatment: isotonic saline-glucose infusion or an oral glucose-salt solution. The oral solution should be hypotonic with regard to NaCl.
  • Fluid replacement should be moderate in elderly persons who already have developed symptoms of a heat illness and it should be adequately monitored: electrolytes, central blood circulation.

Other type of hyperthermia

  • Hyperthermia can also be associated with the following conditions:
    • malignant neuroleptic syndrome (Neuroleptic malignant syndrome (NMS))
    • thyrotoxicosis (history!)
    • phaeochromocytoma
    • anaesthesia-related hyperthermia
    • overdose of cocaine, ecstasy or amphetamine.
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