Environmental Medicine

Hypothermia

Accidental hypothermia is a core temperature below 35°C: cooling leaves an irritable myocardium that handling alone can tip into arrest, while protecting the brain enough that prolonged resuscitation still works.

In a nutshell

Hypothermia is a core temperature below 35°C, staged one to four on consciousness because the thermometer usually arrives late. Keep the patient horizontal, insulate, and rewarm by stage using fluid warmed to 38 to 42°C; in arrest, stop shocking after three shocks and withhold adrenaline below 30°C.

Classic presentation

An older person is found collapsed in an unheated flat, cold, confused and bradycardic, with J (Osborn) waves on the ECG and a core temperature of 29°C.

Key points

Swiss stages: one conscious and shivering, 35 to 32°C; two impaired consciousness, 32 to 28°C; three unconscious with vital signs, 28 to 24°C; four none, below 24°C.
Shivering generally suggests a core temperature above 30°C, where hypothermic cardiac arrest is unlikely.
Cooling sharply cuts the body's oxygen demand, which is why witnessed hypothermic arrest still has roughly 73% survival to hospital discharge.
The J (Osborn) wave is an extra positive deflection where the QRS meets the ST segment, largest in V2 to V5, taller the colder the patient.
Rescue collapse is arrest triggered by moving the patient, and it roughly doubles the risk of death in severe hypothermia.
Bradycardia and hypotension in moderate or severe hypothermia match the temperature: they need rewarming, not vasopressors or pacing.
Unwitnessed arrest and asystole as the first rhythm do not exclude extracorporeal rewarming; normothermic arrest criteria do not apply.
Some patients still have vital signs below 24°C, which is why apparent death in a cold patient is checked rather than assumed.
The European guideline allows one 1 mg dose of adrenaline below 30°C if extracorporeal support is delayed; UK guidance says to hold it until 30°C.

First-line investigation

Core temperature on a low-reading thermometer (tympanic if breathing, oesophageal if intubated); if unavailable, stage on consciousness and check for signs of life for up to 1 minute.

Management

Protect, handle and stage

ABCDE (airway, breathing, circulation, disability, exposure). Cut wet clothing off, insulate from the ground, add a windproof layer, heat packs to chest and back not touching skin. Keep flat, move gently.^6,3
Do not rub the limbs and do not use a hot bath, hot water bottle or heat lamp: warming the periphery first causes afterdrop and can trigger arrest.^7,3
If unconscious, check for signs of life for up to 1 minute using examination, the monitor and ultrasound, before calling it cardiac arrest.^1,2
Core temperature: tympanic if breathing, oesophageal once intubated. With no low-reading thermometer, stage on consciousness using the Swiss stages one to four.¹

Triage the destination and take the first bloods

Straight to an extracorporeal cardiopulmonary resuscitation (ECPR) centre if arrested, heart rate below 45/min, systolic blood pressure below 90 mmHg, ventricular arrhythmia, or core temperature below 30°C.^1,2
Capillary glucose, U&E (potassium feeds the HOPE score), blood gas with lactate, FBC, creatine kinase, liver function and clotting. Repeat them through rewarming.^1,10
Use adhesive defibrillation pads rather than monitor electrodes, because shivering artefact hides the rhythm. Expect bradycardia, atrial fibrillation, and long PR, QRS and QT intervals.^3,9

Rewarm by stage and correct the glucose

Stage one: shelter, dry insulation and a warm sweet non-alcoholic drink if fully awake. Shivering does the rewarming.^7,3
Stage two or worse: active external rewarming with a forced-air blanket, heat packs to the trunk, and intravenous or intraosseous fluid warmed to 38 to 42°C.^3,2
Warmed fluid prevents further cooling but barely rewarms. If the temperature will not rise, critical care escalates to intravascular warming catheters or continuous renal replacement therapy.³
UK guidance sets no target rewarming rate in °C per hour: rewarm by stage and judge it on the response, not on a number.¹
Capillary glucose below 4.0 mmol/L: 15 to 20 g oral glucose if awake and swallowing, otherwise 15 to 20 g intravenously over 15 minutes as 10% or 20% glucose.^6,11

Cardiac arrest, modified for the cold

Standard advanced life support (ALS) with unchanged compression and ventilation rates, plus rewarming. Use mechanical compressions if transport is prolonged or the terrain is difficult.¹
Ventricular fibrillation persisting after three shocks: give no further shocks until the core temperature is above 30°C.^1,2
Amiodarone 300 mg once for a shockable rhythm, then no further doses until the core temperature is above 30°C.¹
Adrenaline 1 mg intravenously (10 mL of 1 in 10 000) is normally every 3 to 5 minutes: withhold below 30°C, every 6 to 10 minutes at 30 to 35°C, normal above 35°C.^1,8
Below 28°C, delayed or intermittent CPR is acceptable when continuous compressions are not feasible, for example during a technically difficult or dangerous rescue.^1,2

Extracorporeal rewarming

Rewarm arrested patients on veno-arterial extracorporeal membrane oxygenation (VA-ECMO): blood is drawn from a vein, warmed and oxygenated outside the body, and returned to an artery.^1,3
The HOPE score (Hypothermia Outcome Prediction after Extracorporeal life support rewarming) predicts survival from age, sex, asphyxia, CPR duration, potassium and temperature, and decides whether the centre offers it.^10,1
No such centre reachable within about 6 hours: rewarm by other means and keep resuscitating. Your job is to phone early, not to cannulate.^2,1

During and after rewarming

Watch for afterdrop, arrhythmia, electrolyte shifts, coagulopathy, rhabdomyolysis, acute kidney injury and aspiration. Repeat glucose, potassium, gas, creatine kinase, renal and liver function and clotting.^2,3
Treat the reason the patient became cold: sepsis, hypothyroidism, adrenal insufficiency, alcohol or sedatives, stroke, a long lie after a fall, or trauma.³

Deciding to stop

Do not start only for injuries incompatible with life: decapitation, truncal transection, a decomposed body, or a body frozen solid with a non-compressible chest wall.²
Otherwise a low temperature is never on its own a reason to stop; take that decision with the extracorporeal centre after rewarming.^1,2

Exam traps

Three shocks, then stop. Repeated defibrillation below 30°C does not work; the current guideline is not a single shock only.
Adrenaline is withheld below 30°C, the opposite of normal arrest practice, because the cold body cannot clear it.
Never rub cold limbs or warm the periphery first: it returns cold blood to the core and can stop the heart.
Serum potassium is one input to the HOPE score, not a stand-alone futility cut-off; prognostication is based on HOPE.
A low temperature alone never justifies stopping resuscitation, and no guideline sets a temperature to rewarm to first: prognosis is decided with the extracorporeal centre.
An older person found cold indoors has an illness until proven otherwise; the cold room is usually the consequence, not the cause.
Warmed intravenous fluid stops further cooling but hardly rewarms anyone. It is not the rewarming strategy.
Shivering is protective. Its absence in a cold patient means stage two or worse, not that they are improving.
Swiss stage three and four are set by vital signs, not by a number. The temperatures printed against them are estimates and differ between published versions of the table.

Illustrations

J (Osborn) waves of hypothermiaTwelve-lead ECG in hypothermia. After each QRS complex there is an extra positive deflection at the J point, the J (Osborn) wave. It is visible here across the precordial leads V2 to V6 and in the inferior leads. Its height grows as the core temperature falls. Hypothermia also slows conduction, prolonging the PR, QRS and QT intervals.WikiSysop, Wikimedia Commons · CC-BY-3.0

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.