Describe heat cramps, heat exhaustion and heat stroke.

The Spectrum of Heat Illness

Heat Stroke — Predisposing Factors and Findings

• Predisposing factors — a high ambient temperature with high humidity, which prevents evaporative cooling and is the crucial combination; the extremes of age; unaccustomed exertion, particularly in labourers, soldiers and athletes; obesity; alcohol; dehydration; heavy or occlusive clothing; cardiovascular disease; and drugs impairing sweating (anticholinergics, antihistamines, phenothiazines) or heat loss (beta-blockers, diuretics)
• Two forms — classical (non-exertional), in the elderly and the chronically ill during a heat wave, developing over days; and exertional, in young fit people exercising or labouring in the heat, developing within hours and often with sweating still present, which makes it easy to miss
• Autopsy findings are non-specific — rapid onset of rigor mortis and rapid putrefaction because the body is already hot; generalised congestion; petechial haemorrhages in the serous membranes and the brain; cerebral oedema; and evidence of disseminated intravascular coagulation, rhabdomyolysis and acute tubular necrosis in those who survived some hours. The diagnosis rests on the circumstances and the recorded temperature, not on the postmortem findings
• India records substantial mortality in heat waves, concentrated in outdoor labourers, the elderly and the homeless, and it is expected to rise; heat action plans and the recording of heat-related deaths have become a public health priority

Management and Applied Aspects

• Cool immediately and aggressively — remove clothing, move to the shade, and cool by evaporative methods (spraying with tepid water and fanning) or cold-water immersion, which is the fastest method and is preferred in exertional heat stroke. Ice packs to the neck, axillae and groins supplement it. The target is to bring the core temperature below about 39°C rapidly and then stop, to avoid overshoot
• Time is the determinant of outcome; mortality relates directly to the duration of hyperthermia, so cooling begins at the scene and does not wait for transport
• Antipyretics are useless — paracetamol and aspirin act on the hypothalamic set point, which is not what is wrong in heat stroke; and aspirin may worsen bleeding. This is a common and futile error
• Monitor for rhabdomyolysis, renal failure and DIC, and check the creatine kinase, potassium and coagulation
• Record the ambient temperature and the circumstances in any suspected heat death; the diagnosis is circumstantial, and the autopsy alone will not support it
• Distinguish heat stroke from malignant hyperthermia and the neuroleptic malignant syndrome, which present with hyperthermia and rigidity but follow anaesthesia and antipsychotic drugs respectively and have specific treatments
• Consider heat stroke in a summer death in India where the body is found hot with rapid rigor and decomposition; it is under-diagnosed, and the circumstances must be sought at the time because they cannot be reconstructed
• Humidity matters as much as temperature, because it prevents evaporative cooling; a moderately hot humid day is more dangerous than a hotter dry one
• Exertional heat stroke may still be sweating, which makes it easy to dismiss as exhaustion in a young fit person — the temperature and the mental state decide
• Cool at the scene, not on arrival; mortality tracks the duration of hyperthermia and transport time is time spent hot
• Stop cooling at about 39 degrees to avoid overshoot into hypothermia
• Check creatine kinase, potassium and coagulation; rhabdomyolysis, hyperkalaemia and disseminated intravascular coagulation follow and are what kill the survivors
• Ask about drugs that impair sweating — anticholinergics, antihistamines and phenothiazines — which convert a tolerable day into a lethal one
• Record the ambient temperature and humidity in any suspected heat death; the autopsy findings are non-specific and the diagnosis is circumstantial