Heat Related Illness

An increase in body temperature can cause symptomatic illness. This usually comes about as a result of conditions of:

• High ambient temperature
• High relative humidity
• Physical exertion

There is a spectrum of heat-related illness ranging from heat cramps, through heat exhaustion to heatstroke.
Temperature alone does not define the type of heat-related illness, but does so in combination with the associated symptoms and signs.

Incidence

Relatively low in UK, estimated at around 40 cases of heat-related mortality per million population annually.¹
A heat wave in France in 2003 was estimated to have caused over 11,000 deaths, particularly amongst the elderly in residential care.²
The incidence can rise when customs or beliefs cause groups of people to be exposed to the heat for long periods of time such as Hajj, the annual Muslim pilgrimage in Saudi Arabia.

Risk factors

• Environmental - hot & humid
• Age - infants and elderly
• Physical - obesity, dehydration, unacclimatised, unusual exertion, inappropriate clothing, sleep deprivation, sunburn, sweat gland dysfunction
• Medical conditions - alcoholism, anorexia, cardiac illness, cystic fibrosis, dehydration, delirium tremens, dermatological conditions with decreased sweating, diabetes insipidus, epilepsy, poorly-controlled diabetes mellitus, febrile illness, gastroenteritis, previous heat-related illness, hypokalaemia, Parkinson's disease, spinal injuries, thyrotoxicosis
• Drugs - alcohol, anticholinergics, alpha-adrenergics, antihistamines, tricyclic antidepressants, selective serotonin re-uptake inhibitors, diuretics, phenothiazines,³ beta blockers, Ca-channel blockers, LSD, PCP, cocaine, amphetamines, ecstasy, aspirin, lithium

Heat cramps

• Body temperature often elevated, usually <40ºC
• Intense thirst with muscle cramps and tachycardia
• Sweating and heat dissipation mechanisms preserved
• Normal alertness and higher functions with no neurological problems

Heat exhaustion

• CNS function is usually largely preserved, but sufferers may experience mild confusion, irritability and poor co-ordination.
• Heat dissipation is still functioning, and temp. is usually <41ºC.
• Patients may experience nausea, oliguria, weakness, headache, thirst, occasionally syncope, sinus tachycardia, orthostatic hypotension.
• They often complain of being hot and appear flushed and sweaty.

Heat stroke

• This is a combination of hyperthermia (>41°C classically, though can present with lower temperatures), often with loss of the capacity to dissipate heat (may not be sweating) and CNS impairment.
• Loss of ability to sweat is often a late and ominous sign.⁴
• Hyperventilation is almost invariable, with hypotension and shock occurring commonly.
• If the condition progresses to more severe form (core temperature >41.5°C) it can cause widespread damage in particular to brain, liver, kidney and muscle.
• The thermoregulatory centre may fail so that patient actually feels cold with dry, vasoconstricted skin leading to a vicious cycle.

• The history of exposure to adverse environment ± physical exertion usually clinches the diagnosis, but sepsis and alternative causes of fever (particularly malaria if relevant area/travel history) should be considered as a cause or precipitant.
• In patients taking phenothiazines or other anti-psychotics, consider neuroleptic malignant syndrome. Those on SSRIs or other serotonergic medication may be suffering from serotonin syndrome.
• If there is a history of recent inhalational anaesthetic consider malignant hyperpyrexia.
• Recreational drug toxicity, particularly involving cocaine, amphetamines and ecstasy is an increasingly common cause of hyperthermia.

• Rectal temperature
• U&E
• Glucose
• Urate - may predict acute renal failure
• Liver function tests
• Creatine kinase - may indicate rhabdomyolysis
• Full blood count
• Arterial blood gases
• Urinalysis - may show myoglobinuria
• ECG
• Chest x-ray - to check for aspiration/pulmonary oedema

Begin (as always) with resuscitation ABC.

• Airway protection may be necessary as coma, fits and vomiting are common. Intubate, but avoid using suxamethonium.
• Breathing should be checked and supported, as necessary.
• Circulatory support is given with IV fluids as 0.9% NaCl or 5% Dextrose.
  • Avoid K+ containing fluids.
  • Gradually reduce of Na⁺ if hypernatremic.
  • If inotropes are required, try to use those with less alpha activity e.g. dopamine.
• Rapid cooling - aiming for <40ºC a.s.a.p.⁵
  • Strip patient, spray with tepid water & use gentle fanning (cools at ~0.3°C/min).
  • Apply ice packs to neck, axillae & groins (cools at ~0.1°C/min).
  • Ice-bath immersion has been shown to be the most effective cooling method, though it is often technically impractical.^6,7
  • Gastric/rectal/peritoneal lavage & cooled cardiopulmonary bypass/haemodialysis can also be used in expert hands.^4,8
  • Modify or discontinue cooling methods once temp. <38.5ºC to avoid overshooting. Antipyretics are ineffective, as is dantrolene.^6,9
• Benzodiazepines and non-depolarising muscle relaxants should be used to control shivering and fits.
  • Neuroleptics may be used to treat excessive shivering associated with cooling.
• Catheterisation should be considered to monitor urine output.
• Complications should be treated as they arise - see below.

• Hyperkalaemia
• Hypocalcaemia
• Acidosis
• Rhabdomyolysis
• Disseminated intravascular coagulation
• Hepatic/renal failure
• Ventricular fibrillation (often fatal)
• Initial successful resuscitation and 'lucid interval' leading to deterioration due to irreversible thermal shock to the central nervous system¹⁰

With rapid cooling, sufficient rehydration and careful management of complications survival rates for heatstroke approach 90% in most modern centres.⁴Poor prognostic indicators include:

• Coagulopathy
• Lactic acidosis (in absence of severe physical exertion)
• Rectal temperature > 42.2ºC
• Coma lasting > 4 hrs
• Acute renal failure
• Hyperkalaemia
• Very high transaminase level
• Prolonged period of hyperthermia

Adverse effects can be avoided by:

• Adequate hydration
• Avoiding exercising in heat
• Acclimatisation