a) Define heat stroke and describe the two forms of heatstroke, highlighting the differences between these two conditions. (20% marks)
b) Describe the clinical features of heatstroke and the biochemical and haematological changes that may occur. (40% marks)
c) Discuss the cooling strategies in heat stroke. (40% marks)
a) Heat stroke is defined as a core body temperature usually in excess of 40ºC with associated central nervous system dysfunction in the setting of a large environmental heat load that cannot be dissipated. Classic (nonexertional heat stroke) affects elderly individuals with underlying chronic medical conditions that impair thermoregulation, prevent removal from a hot environment, or interfere with access to hydration or attempts at cooling. These conditions include cardiovascular disease, neurologic or psychiatric disorders, obesity, anhidrosis, physical disability, extremes of age, and the use of recreational drugs and certain prescription drugs. Exertional heat stroke generally occurs in young, otherwise healthy individuals who engage in heavy exercise during periods of high ambient temperature and humidity. (2 marks)
b) The first clinical signs are often neurological and may include restlessness, delirium, seizures and coma. Multiple organ involvement may occur including signs of distributive shock with a hyperdynamic profile with hypovolaemia as a consequence of dehydration and reduced organ perfusion and associated lactic acidosis. There may be hyperventilation with respiratory alkalosis and hypoxia from acute lung injury. The main biochemical abnormalities include hyperglycaemia, hypophosphataemia, raised hepatic and muscular enzymes and an elevation of acute phase proteins. The haematological findings include leucocytosis, thrombocytopenia and activation of coagulation and fibrinolysis. (4 marks)
c) Cooling Strategies in Heat Stroke:
Water and fan: Evaporative and convective cooling:
Body sprayed with lukewarm water and fans are used to blow air over the moist skin.
Suppression of heat:
Agitated and shivering patient can generate heat. That can be suppressed with the use of benzodiazepines (such as lorazepam, midazolam) and chlorpromazine paralysing agents may be required
Cold water immersion:
Immersion of patient in ice water: non-invasive, rapid but makes patient monitoring difficult
Application of ice packs:
Ice packs can be placed in axillae, neck and groin: excellent method for intubated patient, poorly tolerated by non- intubated patients
Can be applied on smooth, hairless surfaces like: palms, cheeks, soles: rapid cooling
Cold thoracic, gastric and peritoneal lavage: invasive but rapid
Cooling catheters: invasive, rapid
Cooling blankets: non-invasive, can set the temperature
Cold IV fluids
Cooling recommendations are primarily based on observation studies
There is no definitive study supporting any particular approach to cooling in classic heat stroke
Pharmacological agents like dantrolene are ineffective and not indicated in heat stroke
Alcohol sponge baths should be avoided due to risk of absorption of alcohol through skin
Definition of heat stroke
- Failure of thermoregulation due to impaired heat dissipation, characterised by severe hyperthermia, dry skin and a decreased level of consciousness
Exertional heat stroke
- Increased body thermogenesis due to exercise, and the failure of otherwise normal healthy thermoregulatory mechanisms
Non-exertional heat stroke
- Impaired thermoregulatory mechanisms and increased body temperature under condtionals of normal rates of thermogenesis
Clinical signs of heat stroke
- Raised body temperature
- Neurological dysfunction - restlessness, delirium, coma
- Hyperdynamic circulation; distributive shock (Shahid et al, 1999)
- Dry skin (usually)
Characteristic laboratory findings in heat stroke
- ABG: acidosis, probably mixed metabolic; as well as respiratory alkalosis and hypoxia
- BSL: elevated (catecholamines)
- FBC: haemolysis, thrombocytopenia, anaemia, raised white cell count
- EUC: renal failure, hyperkalemia
- CMP: hypophosphataemia and hypocalcemia (Knochel & Caskey, 1977)
- LFTs: raised transaminases and bilirubin. Specifically, AST and LDH will be raised.
- CK: elevated
- Urinary myoglobin
- Coagulopathy (DIC): raised PT and APTT
- Raised acute phase inflammatory markers (CRP, ferritin)
Cooling strategies for heat stroke
- Evaporation of cold water sponges
- Ice packs
- Immersion in ice water
- Contact cooling by blankets and jackets
- Iced gastric, colonic, bladder, or peritoneal lavage
- Infusion of cold intravenous fluids
- Invasive technique such as cooling of the dialysis circuit, or ECMO
There is not specific approach which is thought to be more effective than other approaches. For instance, in a letter to Intensive Care Medicine, Hadad et al (2005) pointed out that in the Israeli Defence Forces, with tap water and a fan one is able to achieve a core temperature rate drop of 1°C every 9 minutes. Costrini (1990), looking at different ways of cooling down overheated athletes, suggested ice water immersion to be the best method. A more detailed discussion of cooling methods is carried out in the chapter on inducing therapeutic hypothermia. The college, in their answer to Question 22 from the second paper of 2018, mention alcohol sponge baths as a discredited alternative. This practice has been discredited since the 1960s, when it killed children (Senz et al, 1959) and adults (Wise, 1969) by producing a surprising amount of alcohol absorption (they were using mainly isopropyl "rubbing" alcohol). On the other hand, if your objective is to achieve heroic levels of intoxication, percutaneous obsorption is a valid method (Puschel et al, 1981).
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