01Core principlesThe concepts and mechanisms needed to understand the subject.
Environmental illness occurs when heat gain or loss exceeds physiological compensation. Exertion generates substantial heat even in relatively cool weather, while wet clothing, wind and immobility accelerate cooling. Age, frailty, intoxication, medicines and inability to obtain shelter modify risk. The environmental history therefore includes what the person was doing, exposure duration, clothing, access to fluids and shelter, and any medical problem that prevented self-rescue.
Heat exhaustion usually preserves neurological function, whereas heat stroke involves central nervous system dysfunction and can progress to multiorgan injury. Hypothermia slows circulation, breathing and drug handling, changing how resuscitation is delivered. Local freezing injury can coexist with systemic hypothermia, but immediate threats to life take priority over the appearance of a finger or toe. Rescue safety and prevention of renewed exposure remain part of clinical treatment.
Key points
- Heat stroke is a neurological emergency: confusion after exertion in the heat needs active cooling, even if the person is still sweating.
- Use core temperature to direct care; a peripheral thermometer can underestimate dangerous thermal disturbance.
- For suspected heat stroke, start effective available cooling immediately; cold-water immersion is preferred when it can be performed safely.
- In an unconscious hypothermic patient, check for vital signs for up to one minute because pulse and breathing can be extremely slow.
- Hypothermia with systolic pressure below 90 mmHg, heart rate below 45/min, ventricular arrhythmia or core temperature below 30°C needs direct ECPR-centre consideration.
- Frostbite rewarming should avoid subsequent refreezing; rubbing or direct dry heat can add tissue damage.
02Mechanisms and patternsImportant relationships and how to distinguish them.
Confusion, agitation, seizure or collapse in a compatible setting demands treatment for heat stroke. Sweating does not exclude it. A person with exhaustion may be weak and nauseated but should retain normal cognition.
Progression from shivering and intact awareness to confusion, loss of shivering and unconsciousness signals increasing risk. Use a low-reading core thermometer; clinical staging helps when a reliable temperature is unavailable.
Frozen tissue may be pale, hard and numb, with injury depth becoming clearer after thawing. Prolonged wet cold can damage tissues without freezing, so the exposure history influences the rewarming and specialist plan.
A person recovered from water may have hypoxia, aspiration, trauma and hypothermia together. A seizure, arrhythmia or intoxication can precede the event, so do not assume the environmental exposure explains every finding.
03Interpreting evidenceInformation, measurements and their limitations.
Consider the information, its meaning and its limitations before deciding what follows.
- 01
Core temperature - Why
- Classify thermal severity and follow the response to cooling or rewarming.
- Interpretation and limitations
- Use a method appropriate to the setting, such as rectal temperature during exertional heat-stroke care or an appropriate low-reading probe in hypothermia. Record the measurement method alongside the value.
- 02
Glucose, electrolytes and blood gas - Why
- Identify metabolic contributors and complications that change immediate treatment.
- Interpretation and limitations
- Hypoglycaemia may accompany cold exposure; sodium disturbance can coexist with exertional collapse. Severe acidaemia or lactate elevation requires assessment of perfusion and associated causes rather than temperature treatment alone.
- 03
Renal function, creatine kinase, liver and clotting tests - Why
- Detect organ injury after significant heat illness or muscle damage.
- Interpretation and limitations
- Rhabdomyolysis, renal dysfunction, hepatic injury and coagulopathy may evolve after the initial temperature improves. Repeat testing according to severity and the clinical course rather than declaring recovery from one normal panel.
- 04
ECG and continuous rhythm monitoring - Why
- Identify arrhythmia risk during severe hypothermia and evaluate the cause of collapse.
- Interpretation and limitations
- Bradycardia can be a physiological consequence of cold, while ventricular arrhythmia signals high risk. Handle gently, monitor during rewarming and seek an appropriate specialist destination early.
04Applied reasoningWorked examples connecting principles to decisions.
01Worked caseConfusion at the finish lineA runner collapses after a race, remains confused and has a rectal temperature of 40.5°C. The airway is patent and there is no major trauma; a supervised immersion bath is available.+
- 1Recognise exertional heat stroke from the neurological change and exposure. Call emergency services, remove excess clothing and appoint someone to maintain the airway and observe the patient continuously.
- 2Start neck-down cold-water immersion without waiting for laboratory tests or transport. Keep the face clear of water and provide practical support so impaired consciousness does not create a drowning risk.
- 3Measure core temperature repeatedly while assessing breathing and circulation. Cooling brings the temperature to 38.8°C with improving cognition; reduce or stop immersion to avoid overshoot and continue clinical assessment.
- 4Verify sustained neurological and haemodynamic recovery and arrange hospital evaluation for renal, muscle, liver and clotting injury. A better temperature does not exclude evolving organ complications.
- 5Document collapse time, starting temperature, cooling method and response. Explain that further exertion is unsafe until medical assessment and a recovery plan are complete.
02Hypothermia branchCold exposure with a pulse and instabilityAn adult rescued from a cold environment has core temperature 29°C, heart rate 40/min and systolic pressure 82 mmHg.+
- 1Move to shelter, handle gently, remove wet clothing with minimal unnecessary movement and insulate the patient from wind and the ground. Support airway and ventilation and obtain continuous monitoring.
- 2Contact an ECPR-capable centre immediately: temperature, bradycardia and hypotension each indicate risk of imminent arrest. Arrange the safest appropriate transfer while beginning available controlled rewarming.
- 3If arrest occurs, use the hypothermia-specific resuscitation modifications and specialist advice. Do not stop solely because the patient is cold or the resuscitation is prolonged; destination and rewarming strategy matter.
03Local cold injuryFrozen toes after a prolonged outdoor exposureA person has hard numb toes but is conscious and haemodynamically stable after systemic assessment.+
- 1Treat coexisting hypothermia and remove constricting footwear or jewellery. Determine whether refreezing can be prevented during the remaining rescue and journey.
- 2When safe from refreezing, rewarm in circulating water maintained at 37–39°C until tissue softens, with effective analgesia and protection from pressure or rubbing. Seek specialist advice for deep injury.
- 3Apply loose protective dressings, avoid unnecessary weight-bearing on thawed tissue and document post-thaw colour, sensation and blisters. Severe frostbite may need time-sensitive specialist vascular treatment.
05Relevant medicines and safetySpecific regimens and precautions where medicines are relevant.
Adrenaline in hypothermic cardiac arrest
Under RCUK 2025, give 1 mg IV once core temperature reaches 30°C unless imminent ECPR is planned; use intervals of 6–10 minutes at 30–35°C. Return to standard ALS intervals after rewarming above this range.This modification applies to accidental hypothermic arrest, not a conscious cold patient with a pulse. Use core temperature and the specialist resuscitation plan; do not administer routine repeated doses below 30°C.
Amiodarone in hypothermic shockable arrest
RCUK 2025 permits a 300 mg loading dose for a shockable rhythm under the arrest algorithm, but delays further amiodarone until core temperature exceeds 30°C.If VF persists after three shocks, defer further shocks until core temperature is above 30°C. Do not apply the drug or shock modifications to ordinary normothermic arrest.
06Checking understandingVerify the reasoning, revisit uncertainties and apply feedback.
- During heat-stroke cooling, continuously observe airway safety, cognition and circulation, and use repeated core measurements to recognise an adequate temperature response.
- After apparent heat recovery, review urine output and repeat renal, muscle, liver and coagulation investigations where severity warrants, because injury can evolve later.
- During hypothermia care, follow core temperature, rhythm and haemodynamics through rewarming and movement, communicating changes promptly to the receiving specialist team.
- After frostbite thawing, document perfusion, sensation and wound evolution, arrange pain review and specialist follow-up, and protect the area from further pressure or cold.
07Special situationsVariants, exceptions and circumstances that change the usual approach.
No thermometer should mean no delay
RCUK first-aid guidance advises immediate cooling when heat stroke is suspected; if core measurement is unavailable, cool for fifteen minutes or until neurological symptoms resolve, whichever occurs first, while arranging emergency assessment.
Cold alters arrest assessment
Very slow breathing and a weak pulse are easy to miss. Allow the hypothermia-specific vital-sign assessment interval and use available monitoring before incorrectly assuming absent circulation.
Drowning needs ventilation
Once safely rescued, drowning resuscitation begins with five ventilations, using oxygen when available, followed by the standard CPR sequence. Rescue attempts must not expose an untrained rescuer to further danger.
Antipyretics do not remove environmental heat
Heat stroke is a failure of heat balance, not simply a fever set point. Physical cooling and organ support are the urgent treatments; paracetamol must not displace effective cooling.
08Common pitfallsFrequent interpretation and management errors.
- 01
Do not wait for dry skin or absence of sweating before recognising exertional heat stroke in a confused collapsed person.
- 02
Do not use a normal-looking peripheral temperature to dismiss a compelling exposure history and neurological abnormality.
- 03
Do not rub frostbitten tissue, use a radiator or fire for direct warming, or deliberately thaw tissue that will subsequently refreeze.
- 04
Do not copy normal-temperature arrest drug intervals into profound hypothermia without applying the current temperature-dependent modifications.