Hyperthermia and Heat Stroke

Quick Answer

What are the critical anaesthetic considerations for heat stroke?

Heat stroke is a life-threatening hyperthermic emergency with core temperature >40°C and neurological dysfunction. Rapid cooling is the priority. Key principles:

1. Rapid cooling - "Cool first, transport second" - goal 0.15-0.2°C/min; target <38.5°C within 30-60 minutes
2. Method - Cold water immersion (most effective); evaporative + convective cooling; ice packs to neck/groin/axilla
3. Differentiation - Exertional (young athletes, rhabdomyolysis) vs classic (elderly, chronic disease); both treated similarly
4. DIC risk - Common in severe cases; requires FFP, cryoprecipitate, platelets as indicated
5. No antipyretics - Paracetamol/aspirin ineffective (hypothalamic set point normal); dantrolene only if MH/NMS suspected
6. Fluid resuscitation - Hypovolaemia common but be cautious (risk of cerebral/pulmonary oedema)
7. Supportive care - Protect airway, treat seizures, monitor for multi-organ failure

Clinical Pearl: Time is brain in heat stroke. Every minute of temperature >40°C causes cellular damage. The most effective cooling method is cold water immersion (ice bath) for exertional heat stroke in the field - it can achieve cooling rates of 0.15-0.35°C/min. "Cool first, transport second" is the mantra for field management of exertional heat stroke.

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Clinical Overview

Definitions

Epidemiology

Global and Australian context:

Australian context:

• Summer peaks (December-February)
• Northern Australia year-round risk
• Athletes: Australian Rules Football, rugby (pre-season training)
• Workers: Mining, agriculture, construction
• Vulnerable populations: Elderly (especially during heat waves), Indigenous remote communities

Risk factors:

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Pathophysiology

Heat Balance

Heat gain vs heat loss:

Mechanisms of heat stroke:

Cellular Effects of Hyperthermia

Critical thresholds:

• 40°C: Risk of organ damage

• 42°C: Critical danger zone (cell death)

• 43°C: Usually fatal if sustained

Multi-Organ Effects

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Classification and Clinical Features

Exertional Heat Stroke (EHS)

Characteristics:

Clinical presentation:

• Collapse during/after exertion
• Hot, sweaty skin
• Altered consciousness (confusion, agitation, seizures, coma)
• Tachycardia, hypotension
• Muscle cramps, pain (rhabdomyolysis)

Classic (Non-Exertional) Heat Stroke

Characteristics:

Clinical presentation:

• Altered mental status (delirium, coma)
• Hot, dry skin
• Respiratory alkalosis (hyperventilation)
• Cardiovascular collapse
• Multi-organ failure

Comparison

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Initial Management

Prehospital Care

Field priorities:

Cooling methods in the field:

"Cool first, transport second" (EHS only):

• Immerse in ice bath until temperature <38.5°C
• Then transport
• Cooling in field better than delayed cooling in hospital
• Does NOT apply to classic heat stroke (elderly) - rapid transport needed

Contraindications to cold water immersion:

• Unconscious patient (risk of drowning)
• Altered mental status (cannot protect airway)
• In these cases: Evaporative cooling + ice packs + transport

Emergency Department Management

Immediate priorities:

Airway management:

• Intubate if:
  • GCS <9
  • Airway reflexes impaired
  • Respiratory failure
  • Seizures requiring control
• Avoid suxamethonium if rhabdomyolysis >24 hours (hyperkalaemia risk)

Cooling Methods

Target cooling rate: 0.15-0.2°C per minute

Recommended approach:

Exertional:

1. Cold water immersion (if alert) or
2. Ice packs + evaporative cooling (if obtunded)

Classic:

1. Evaporative cooling (mist + fan)
2. Ice packs to neck/axilla/groin
3. Cooling blanket adjunct

Technique details:

Evaporative cooling:

• Undress patient
• Spray lukewarm water over skin (not cold - causes vasoconstriction)
• High-flow fan across body
• Continuous application
• Most practical for hospital setting

Cold water immersion:

• Best for exertional heat stroke
• Ice bath or cold water bath
• Monitor closely (ECG, temperature)
• Remove when temp <38.5°C (to avoid overshoot)

Ice packs:

• Neck (carotid area)
• Axilla
• Groin
• High blood flow areas

When to stop cooling:

• Core temperature <38.5°C
• Stop active cooling (risk of hypothermia/overshoot)
• Monitor for rebound hyperthermia

Fluid Management

Principles:

• Hypovolaemia common (dehydration, vasodilation, sweating)
• But risk of pulmonary/cerebral oedema
• Careful fluid resuscitation

Approach:

Fluid choice:

• Isotonic crystalloid (0.9% NaCl or Hartmann's)
• Cold fluids may help cooling but volume limited
• Consider albumin if significant capillary leak
• Blood products if DIC/haemorrhage

Cautions:

• Avoid over-resuscitation
• Monitor for pulmonary oedema (CXR, SpO2)
• Monitor for cerebral oedema (pupils, CTA)

Pharmacological Therapy

Antipyretics:

• NOT effective in heat stroke (hypothalamic set point normal)
• Paracetamol: No benefit; hepatotoxic in this context
• Aspirin: No benefit; bleeding risk with DIC
• NSAIDs: No benefit; renal risk

Dantrolene:

• Controversial in heat stroke (not MH)
• May be used if MH or NMS cannot be excluded
• Dose: 2.5 mg/kg IV, repeat to 10 mg/kg total
• Mechanism: Inhibits calcium release from sarcoplasmic reticulum
• Evidence for heat stroke limited; case reports only

Benzodiazepines:

• For agitation, seizures, shivering during cooling
• Diazepam 5-10 mg IV or midazolam 2-5 mg IV
• Shivering control important (generates heat)

Other:

• Proton pump inhibitors (stress ulcer prophylaxis)
• DVT prophylaxis (LMWH when coagulation normalised)
• Antibiotics only if infection suspected
• Insulin for hyperglycaemia

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Complications and Organ-Specific Management

Rhabdomyolysis

Common in exertional heat stroke:

• CK often >10,000-50,000 U/L
• Myoglobinuria (tea/cola coloured urine)
• Risk of AKI

Management:

• Aggressive fluid resuscitation
• Target urine output >100-300 mL/hr
• Urine alkalinisation (NaHCO3)
• Mannitol (once volume replete)
• Avoid nephrotoxins

Disseminated Intravascular Coagulation (DIC)

Common in severe cases, especially classic heat stroke:

Lab targets:

• INR <1.5
• Fibrinogen >1.5 g/L
• Platelets >50,000

Acute Kidney Injury

Multifactorial:

• Hypoperfusion
• Rhabdomyolysis (myoglobin)
• Direct thermal injury
• DIC

Management:

• Fluid resuscitation
• Rhabdomyolysis protocol
• Avoid nephrotoxins
• Renal replacement therapy if indicated

Neurological Complications

Hepatic Failure

Common:

• Transaminases often >10,000 U/L
• Jaundice
• Coagulopathy

Management:

• Supportive
• N-acetylcysteine may have role (antioxidant)
• Liver transplant rarely needed (usually recovers)

Cardiovascular Management

Hypotension:

• Fluid resuscitation
• Noradrenaline if refractory
• Avoid vasopressin (splanchnic ischaemia risk)

Arrhythmias:

• Correct electrolytes (especially K+, Mg2+, Ca2+)
• Correct temperature
• Standard ACLS protocols

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Differential Diagnosis

Malignant Hyperthermia (MH)

Neuroleptic Malignant Syndrome (NMS)

Serotonin Syndrome

Thyroid Storm

Sepsis/SIRS

• Can cause hyperthermia + encephalopathy
• Look for source of infection
• Blood cultures
• Antibiotics if indicated

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Special Populations

Athletes

Prevention:

• Heat acclimatisation (10-14 days)
• Hydration protocols
• Scheduled rest breaks
• Wet bulb globe temperature monitoring
• Remove from play if symptomatic

Return to play:

• One week minimum after EHS
• Clearance from physician
• Gradual return
• Avoid heat for several weeks

Elderly

Classic heat stroke risk:

• Reduced thirst sensation
• Reduced sweating
• Chronic disease
• Medications (diuretics, anticholinergics, beta-blockers)
• Social isolation
• Poverty (no air conditioning)

Prevention:

• Check on elderly during heat waves
• Air conditioning or cooling centres
• Hydration reminders
• Medication review

Children

Increased risk:

• Higher surface area to volume ratio
• Higher metabolic rate
• Reduced sweating capacity
• Depend on adults for hydration

Prevention:

• Never leave in cars
• Frequent hydration breaks
• Avoid exercise in extreme heat
• Sun protection

Indigenous Communities (Remote Australia)

Risk factors:

• Extreme heat in central/northern Australia
• Poor housing (limited air conditioning)
• Outdoor occupations
• Chronic disease burden
• Limited healthcare access

Management challenges:

• Distance from hospital
• Retrieval services needed
• Prevention education
• Heat health alert systems

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ANZCA Final Examination Focus

High-Yield Topics

Written examination:

Viva scenarios:

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Assessment Content

SAQ 1: Heat Stroke Management (20 marks)

Question:

A 22-year-old football player collapses during a pre-season training session on a hot day (38°C, 80% humidity). He is confused, agitated, and combative. His rectal temperature is 41.2°C. He is sweating profusely. Heart rate is 140, BP 90/60. There is no access to an ice bath at the scene.

a) What is the diagnosis, and what features differentiate it from heat exhaustion? (4 marks)

b) Describe your immediate management in the field. (8 marks)

c) What complications would you monitor for in hospital? (8 marks)

Model Answer:

a) Diagnosis and Differentiation (4 marks):

Diagnosis (2 marks):

• Exertional heat stroke (EHS)
• Core temperature >40°C (41.2°C)
• CNS dysfunction (confusion, agitation, combativeness)
• Context: Exercise in hot, humid conditions

Heat exhaustion vs heat stroke (2 marks):

b) Field Management (8 marks):

Immediate cooling (4 marks):

• Strip off all clothing and equipment immediately
• Move to shade or cool area
• Begin active cooling immediately (do not wait for transport):
  • Evaporative cooling: Spray skin with lukewarm water
  • High-flow fan across body continuously
  • Place ice packs or cold packs to neck, axilla, groin (high blood flow areas)
• If cold water available (bottles, hoses): Wet skin completely + fan
• Target cooling rate 0.15-0.2°C/min

Airway and positioning (1 mark):

• If consciousness impaired, place in recovery position
• Protect airway (risk of aspiration if vomiting)
• High-flow oxygen if available

Fluids (1 mark):

• Oral fluids if conscious and able to swallow (not primary treatment)
• IV access and cold IV fluids if available (0.9% NaCl or Hartmann's)
• Do not delay cooling for IV access

Monitoring (1 mark):

• Continuous rectal temperature monitoring (essential)
• ECG if available
• Stop cooling when temperature <38.5°C (avoid overshoot)

Transport (1 mark):

• If using evaporative cooling effectively, can transport during cooling
• Continue all cooling measures during transport
• "Cool first, transport second" less critical than with ice bath
• ALS/ambulance activation

c) Hospital Complications (8 marks):

Cardiovascular (2 marks):

• Hypotension (vasodilation, hypovolaemia)
• Arrhythmias (sinus tachycardia common; VT/VF possible)
• Cardiomyopathy
• Management: Fluids, vasopressors, antiarrhythmics, monitoring

Renal (2 marks):

• Acute kidney injury (hypoperfusion, rhabdomyolysis)
• Myoglobinuric renal failure
• Management: Aggressive hydration (>100-300 mL/hr), urine alkalinisation, avoid nephrotoxins, dialysis if indicated

Neurological (1 mark):

• Seizures, cerebral oedema, encephalopathy, coma
• Management: Benzodiazepines, neuroprotection, ICP monitoring if severe

Muscle (1 mark):

• Rhabdomyolysis (common in EHS)
• Management: As above for renal protection

Haematological (1 mark):

• DIC (less common in EHS than classic, but possible in severe cases)
• Management: FFP, cryoprecipitate, platelets as needed

Metabolic (1 mark):

• Lactic acidosis, electrolyte abnormalities (hyper/hypokalaemia, hypocalcaemia), hepatic dysfunction
• Management: Supportive, correct abnormalities

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Viva Scenario: Intraoperative Hyperthermia

Scenario:

You are anaesthetising a 16-year-old boy for appendicectomy. He has been anaesthetised for 45 minutes. Suddenly his temperature rises from 36.8°C to 39.5°C. Heart rate increases to 150. EtCO2 is 65 mmHg. The patient is becoming rigid.

Examiner: "What is your differential diagnosis and immediate management?"

Candidate Response:

"This is an acute intraoperative hyperthermia with tachycardia, hypercapnia, and rigidity - a classic presentation for malignant hyperthermia. My differential would be:

1. Malignant hyperthermia (most likely) - Triggered by volatile anaesthetics or suxamethonium; presents with hyperthermia, tachycardia, hypercapnia, rigidity, acidosis
2. Sepsis - Pre-existing infection, pyrexia
3. Thyroid storm - If history of hyperthyroidism
4. Pheochromocytoma - Catecholamine surge
5. Iatrogenic warming - Over-heating from warming devices

However, the combination of rapid temperature rise + tachycardia + hypercapnia + rigidity is pathognomonic for MH.

My immediate management would be:

1. Call for help - MH crisis is life-threatening; need experienced assistance
2. Stop triggers - Immediately discontinue all volatile anaesthetics; turn off vaporisers; flush circuit with high-flow O2; change to clean machine if available or use charcoal filters
3. Hyperventilate - Increase minute ventilation 2-3× to wash out CO2
4. Dantrolene - 2.5 mg/kg IV immediately; can repeat up to 10 mg/kg total; this is life-saving
5. Cooling - Active cooling: ice packs, cooling blanket, cold IV fluids; stop all active warming
6. Monitoring - Arterial line; ABG (expect severe acidosis); CK (will rise); lactate; electrolytes (K+ may be high); coagulation (DIC risk)
7. Fluids - Aggressive crystalloid resuscitation (rhabdomyolysis will occur)
8. Treat complications - Bicarbonate if pH <7.1; insulin/glucose for hyperkalaemia; antiarrhythmics if needed
9. Postoperative - ICU admission; dantrolene 1 mg/kg q6h × 24 hours; monitor for late complications (DIC, ARF, compartment syndrome)

Documentation:

• Document all anaesthetics used
• Family counselling (genetic condition; first-degree relatives need testing)
• Report to MH registry

The key is immediate recognition and dantrolene administration - this is life-saving."

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References

1. Bouchama A, Knochel JP. Heat stroke. N Engl J Med. 2002;346(25):1978-1988. PMID: 12075060
2. Epstein Y, Roberts WO. The pathopysiology of heat stroke: an integrative view of the final common pathway. Scand J Med Sci Sports. 2011;21(6):742-748. PMID: 22092222
3. Casa DJ, McDermott BP, Lee EC, et al. Cold water immersion: the gold standard for exertional heatstroke treatment. Exerc Sport Sci Rev. 2007;35(3):141-149. PMID: 17632428
4. Roberts WO. Exertional heat stroke during a cool weather marathon: a case study. Med Sci Sports Exerc. 2006;38(7):1197-1202. PMID: 16826030
5. Hew-Butler T, Rosner MH, Fowkes-Godek S, et al. Statement of the Third International Exercise-Associated Hyponatremia Consensus Development Conference, Carlsbad, California, 2015. Br J Sports Med. 2015;49(22):1432-1446. PMID: 26205594
6. Armstrong LE, Casa DJ, Millard-Stafford M, et al. American College of Sports Medicine position stand: exertional heat illness during training and competition. Med Sci Sports Exerc. 2007;39(3):556-572. PMID: 17473783
7. Lopez RM, Casa DJ, Jensen KA, et al. Comparison of two cooling methods on thermoregulatory responses before, during, and after exertional heat stress. J Athl Train. 2010;45(4):386-394. PMID: 20617910
8. Proulx GI, Ducharme MB, Kenny GP. Effect of water temperature on cooling efficiency during hyperthermia in humans. J Appl Physiol (1985). 2003;94(4):1317-1323. PMID: 12496132
9. Costrini A. Emergency treatment of exertional heatstroke and comparison of whole body cooling techniques. Med Sci Sports Exerc. 1990;22(1):15-18. PMID: 2406331
10. Hadad E, Rav-Acha M, Heled Y, et al. Heat stroke: a review of cooling methods. Sports Med. 2004;34(8):501-511. PMID: 15248729

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File generated for ANZCA Final Examination preparation. Last updated: 2026-02-03