AKT · Acute care/Metabolic & endocrine

Hyperthermia & hypothermia

Failure of thermoregulation — heat gain exceeding loss, or the reverse

Overview

Core temperature outside the range the body can compensate for. Heat stroke is core temperature above roughly 40 °C WITH central nervous system dysfunction, and is distinguished from heat exhaustion by that CNS involvement. Hypothermia is core temperature below 35 °C, graded mild (32–35), moderate (28–32) and severe (<28).

Recognise

Heat stroke: core >40 °C with confusion, seizures or coma; hot skin that may be dry (classic) or sweaty (exertional)
Heat exhaustion: temperature <40 °C, normal mental state — thirst, cramps, nausea, faintness
Hypothermia, mild: shivering, tachycardia, cold diuresis, clumsiness
Hypothermia, moderate–severe: shivering STOPS, bradycardia, hypotension, falling GCS, dilated pupils, rigidity — can mimic death
ECG in hypothermia: bradycardia, prolonged intervals and the Osborn (J) wave; risk of VF rises sharply below 28 °C

Red flags

Heat stroke with seizures, coma or oliguria → rhabdomyolysis, DIC and multi-organ failure; cool immediately, do not wait for the temperature to be confirmed
Hypothermia <28 °C — the myocardium is irritable and rough handling alone can precipitate VF

Differentials & how to tell them apart

SepsisSource of infection with a raised CRP; heat stroke follows exertion or environmental exposure

Neuroleptic malignant syndrome / serotonin syndromeDrug exposure with rigidity or clonus — the drug history is the discriminator

Thyroid stormGoitre, atrial fibrillation, suppressed TSH

Myxoedema comaHypothermia WITH bradycardia, hyporeflexia with slow-relaxing reflexes and hyponatraemia

HypoglycaemiaCapillary glucose — a mandatory early test in both directions, and easily missed

Investigations

LOW-READING or oesophageal/rectal core thermometer (a standard tympanic thermometer under-reads and will mislead you). ECG, capillary glucose, U&E, CK, LFT, clotting/DIC screen, ABG/lactate, FBC. Look for the cause: infection screen, TFT (myxoedema coma), toxicology, and a trauma survey in anyone found collapsed outdoors.

Management

Heat stroke — rapid active cooling to about 39 °C, aiming to start within minutes. Hypothermia — gentle rewarming with minimal handling.

1Heat stroke: ABCDE, remove from the heat, undress, cold-water immersion or evaporative cooling with ice packs to groin/axillae/neck; cooled IV fluids. STOP active cooling around 39 °C to avoid overshoot.
2Hypothermia: remove wet clothing, insulate, warm environment. Mild — passive external rewarming. Moderate–severe — active warming (forced-air blanket, warmed IV fluids and humidified oxygen).Gate: Core <28 °C, cardiac instability or arrest → critical care for extracorporeal rewarming (ECMO/bypass)
3In hypothermic cardiac arrest, resuscitation is prolonged and drugs are withheld below 30 °C (and given at doubled intervals between 30–35 °C), because they accumulate without effect. Defibrillation attempts are limited to three until the core is above 30 °C.Gate: No return of circulation but core still <32 °C → keep going; "not dead until warm and dead"

IV crystalloid (cooled for hyperthermia, warmed for hypothermia) — Volume replacement plus a route for temperature control

Benzodiazepines — For shivering during cooling, and for seizures

Key points

Two rules that decide outcomes. In heat stroke, minutes matter — cool first and investigate afterwards, and stop at 39 °C. In severe hypothermia, the rule is the opposite: move slowly, handle gently, and do not stop resuscitating on temperature alone, because full neurological recovery after prolonged hypothermic arrest is well documented.

Monitor & prognosis

Continuous core temperature and cardiac monitoring; CK, renal function, glucose and clotting.

Heat stroke carries a high mortality once multi-organ failure sets in, and neurological damage can be permanent. Hypothermia, if rewarmed before arrest, generally recovers fully.

Source: Resuscitation Council UK — special circumstances · NICE CKS