Environmental Medicine

Hyperthermia and heat stroke

In hyperthermia the hypothalamic set-point is normal and heat load simply outstrips heat loss, so antipyretics have nothing to act on and physical cooling is the only disease-modifying treatment.

In a nutshell

Hyperthermia is heat gain or failed heat loss at a normal set-point, so antipyretics do nothing and cooling is the treatment. Immerse neck-down in cold water at 1 to 26°C until the rectal temperature is below 39°C, and cool first, transfer second.

Classic presentation

A club runner collapses confused and seizing 2 km from the end of a half marathon on a mild day, still sweating heavily.

Key points

Heat stroke is recognised on central nervous system dysfunction in a hot or exertional context. No guideline sets a temperature threshold for the diagnosis.
With no core temperature available, cool for 15 minutes or until neurological symptoms resolve, whichever comes first. Overshoot into hypothermia is a real harm.
Exertional heat stroke can occur at any ambient temperature, and sweating is often preserved. Hot dry skin describes classical heatwave heat stroke, not every case.
Heat syncope and heat exhaustion need only a cool place, simple external cooling and fluids. Heat stroke needs the fastest cooling method available.
Rhabdomyolysis, hyperkalaemia and acute kidney injury declare hours after cooling. The rhabdomyolysis and hyperkalaemia chapters carry the thresholds and the treatment.
Malignant hyperthermia is the one hyperthermic syndrome with a BNF dose: dantrolene 2 to 3 mg/kg, then 1 mg/kg, maximum 10 mg/kg per course.
Serotonin syndrome and neuroleptic malignant syndrome have their own chapters. Drug history and time course discriminate better than examination: hours after serotonergic change, days after dopamine blockade.
Sepsis and central nervous system infection never leave the differential, however good the heat history is, and either can coexist with heat stroke.

First-line investigation

Rectal core temperature taken while cooling continues, plus a capillary glucose in the first minute; a normal reading after bystander cooling does not exclude heat stroke.

Management

Recognise, call, and start cooling in the same minute

Confusion, agitation, disorientation, seizure or unresponsiveness in a hot or exertional context is heat stroke until proved otherwise. Call 999.^1,2
Move into shade or a cool room, strip excess clothing, and start active cooling. Do not wait for a thermometer, a cannula or a bed.^1,4
Cool first, transfer second. Keep cooling in the ambulance and in the resuscitation room.¹

Cool to below 39°C, then stop

Gold standard: whole-body neck-down immersion in cold water at 1 to 26°C, continued until the rectal temperature falls below 39°C.¹
No bath available: ice sheets, commercial ice packs, a cold shower, a fan alone, or evaporative cooling with mist and a fan.¹
No core temperature obtainable: cool for 15 minutes, or until the neurological symptoms resolve, whichever comes first.¹
Stop at the target. The endpoint is a number, below 39°C, not an impression: cooling has no natural brake and running it on causes hypothermia.¹

Oxygen, glucose, fluid

Oxygen titrated to saturations 94% to 98%, or 88% to 92% if at risk of hypercapnic respiratory failure. Below 88% is life-threatening hypoxaemia.^1,10
Capillary glucose below 4.0 mmol/L: 15 to 20 g oral glucose if awake and swallowing safely, otherwise 15 to 20 g intravenously over 15 minutes.^1,6
Shock: 0.9% sodium chloride or Hartmann's 500 mL intravenously over less than 15 minutes, reassessed after every bolus. No heat-specific regimen exists.¹¹
Hypovolaemia by NICE: systolic pressure below 100 mmHg, heart rate above 90, capillary refill over 2 seconds, or a National Early Warning Score of 5 or more.¹²
No paracetamol. The set-point is normal, so there is nothing to lower, and its ceiling is 4 g daily by mouth in an uninjured liver.^1,3

Malignant hyperthermia, the one syndrome this chapter treats

It occurs only under general anaesthesia, in a susceptible person exposed to a potent inhalational anaesthetic or suxamethonium.^9,14
Commonest early signs: rising end-tidal carbon dioxide with a rising heart rate. Suxamethonium causes masseter spasm in 70% of susceptible patients.^8,14
Stop the trigger and cool. Dantrolene 2 to 3 mg/kg by rapid intravenous injection, then 1 mg/kg repeated if necessary, maximum 10 mg/kg per course.¹³
Dantrolene is given only under the supervision of someone experienced in its use for malignant hyperthermia.¹³
The managing anaesthetist refers to the UK Malignant Hyperthermia Investigation Unit, St James's University Hospital, Leeds, the only UK contracture-testing centre. Relatives need testing.^15,9

The complications arrive after the temperature falls

Rhabdomyolysis, hyperkalaemia and acute kidney injury declare hours later. ECG, hourly urine output, and repeated creatine kinase, potassium, creatinine, liver function, clotting and lactate.^4,12
Heat exhaustion: cool place, unnecessary clothing off, water or an oral rehydration or isotonic drink, and skin cooled by spraying or sponging and fanning.²
Heat exhaustion should resolve within 30 minutes. Still unwell at 30 minutes is a 999 call, not a review.²

Prevention during a heat-health alert

UKHSA heat-health alerts run from 1 June to 30 September and are colour-coded by likely impact. Out-of-season alerts can still be issued.¹⁶
Check on people aged 65 and over, children under 5, pregnant women, people living alone, and people on multiple medicines or manual outdoor workers.^5,16
Close windows, curtains and blinds by day and open them at night; avoid the sun between 11am and 3pm; review medicines that impair sweating.⁵

Exam traps

Paracetamol does not treat heat stroke. There is no raised set-point for it to lower, and it adds injury to a liver heat has already damaged.
A temperature below 40°C does not exclude heat stroke if bystanders have already cooled the patient.
Stop active cooling below 39°C. Cooling past the target causes hypothermia, and the endpoint is the number, not how the patient looks.
Cool first, transfer second. Time at temperature drives the injury, so cooling on scene beats arriving sooner.
Malignant hyperthermia happens only under general anaesthesia. A hyperthermic patient on the ward who has had no anaesthetic does not have it.
Dantrolene has a BNF dose for malignant hyperthermia only. It has none for neuroleptic malignant syndrome, and relabelling the anaesthetic dose would invent a UK recommendation.
Muscle rigidity, leucocytosis and a raised creatine kinase point to neuroleptic malignant syndrome; all three may be absent in serotonin syndrome.

Illustrations

Active external cooling for heat illnessClinical treatment photograph showing a heat casualty undergoing active external cooling; rapid whole-body cooling, preferably cold-water immersion for severe or exertional heat stroke, is central to management.Neill (Sgt), No. 1 Army Film and Photographic Unit, Wikimedia Commons · Public domain

Key sources

This page is exam revision material, not medical advice, and must not be used for patient care. Always check drug doses against the BNF and current guidance. Full disclaimer.