Hypothermia

Hypothermia is the textbook illustration of a disease where the resuscitation is modified by a single number — the core temperature — and where every rule the Fellowship candidate has memorised is rewritten. A cold heart does not fibrillate in response to adrenaline, does not defibrillate at standard energy, and does not declare the patient dead; the candidate who runs a normothermic arrest algorithm on a core of 26 degrees will fail the station and kill the patient. The two papers the examiner expects by name are the Brown review in the New England Journal of Medicine (2012), which codifies the staging and the rewarming ladder, and the European Resuscitation Council 2021 guidelines on cardiac arrest in special circumstances, which set the modified algorithm for the hypothermic arrest.[1][3] The dictum that governs the whole topic — "no one is dead until warm and dead" — is not a slogan but a protocol, validated by the Danish case series of circulatory arrest survivors rewarmed on cardiopulmonary bypass.[4]

Definition and classification

Hypothermia is defined as a core body temperature below 35 degrees Celsius, measured at a central site with a low-reading thermometer. Two staging systems coexist and the candidate must know both, because the examiner may quote either. The standard medical classification divides hypothermia by core temperature into mild, moderate, severe, and profound, and each stage carries a characteristic clinical picture.[1]

The Swiss staging system (the HT or Durrer classification) is the field scale used by prehospital services, avalanche rescue, and the Wilderness Medical Society, and it grades the patient by the level of consciousness and the vital signs rather than by a thermometer that may be unavailable on a mountain. It maps directly onto the rewarming decision and is the staging the ERC and WMS guidelines cite.[1][2]

Hypothermia is further divided into primary (environmental exposure in an otherwise healthy person — the stranded walker, the immersion victim) and secondary (a patient whose thermoregulation has failed because of illness, drugs, or trauma — the septic, the myxoedematous, the intoxicated, the injured). The distinction matters because secondary hypothermia is a marker of the underlying disease, and the resuscitation must treat the cause in parallel with the temperature. [1]

Epidemiology and risk

Hypothermia is a disease of the vulnerable and the exposed. The classic ANZ and UK case is the elderly person found indoors on a cold floor, dehydrated, on sedatives, with a urinary tract infection or a fractured neck of femur — the hypothermia is secondary and the mortality is high. The outdoor cases are the stranded bushwalker, the immersion victim, the avalanche burial, and the alcohol-intoxicated young man asleep in the open. The risk factors cluster into three groups: increased heat loss (immersion, wet clothing, cold environment, burns, the large surface-area-to-mass ratio of the infant and the elderly), impaired thermogenesis (exhaustion, malnutrition, hypothyroidism, hypoadrenalism, sepsis), and impaired behaviour or awareness (alcohol, opioids, sedatives, dementia, head injury).[1]

Pathophysiology — the mechanism

Heat is lost by radiation (about 60 per cent in the resting adult), convection, conduction, evaporation from the skin, and the warming and humidification of inspired air. The healthy response to cold is peripheral vasoconstriction and shivering. Shivering increases metabolic heat production two- to five-fold but is energetically expensive and fails as the core falls, ceasing at about 30 to 32 degrees — a patient who has stopped shivering is by definition at least moderately hypothermic. As the core falls further the cardiovascular, neurological, respiratory, and metabolic systems fail in a predictable sequence.[1]

The myocardium becomes cold and irritable. The sinus node slows, the conduction system delays, and atrial fibrillation appears in the moderate range; below 28 degrees the risk of ventricular fibrillation rises sharply, and the cold ventricle will fibrillate in response to a seemingly trivial stimulus — rough movement, a central line, an endotracheal tube, a premature defibrillation. Once fibrillating, the cold heart is refractory to defibrillation and to antiarrhythmic drugs, which is the physiological basis of the modified ALS algorithm.[3] The central nervous system is progressively depressed — ataxia and confusion in the mild range, coma in the moderate, and areflexia and fixed dilated pupils in the severe. The candidate must remember that fixed dilated pupils are not a sign of brain death in hypothermia; the pupils can dilate at low temperature and still recover with rewarming.

Two biochemical consequences are examined. The coagulation cascade is temperature-dependent, so the hypothermic patient is functionally coagulopathic — but the laboratory measures the clotting at 37 degrees, warming the sample, and so the prothrombin time and the activated partial thromboplastin time read falsely normal. The Fellowship candidate who trusts a normal coagulation profile in a cold, bleeding trauma patient has fallen for the trap. The serum potassium rises as cells lyse in profound hypothermia, and a potassium at or above 10 mmol per litre reflects irreversible cell death and is incompatible with survival — it is a recognised criterion to cease resuscitation.[2][4]

Clinical presentation

The presentation tracks the core temperature. The mild case (32 to 35 degrees) is awake, cold, pale, and shivering vigorously, with tachycardia, tachypnoea, fine motor clumsiness, and apathy or poor judgement — the "umbles": stumbles (ataxia), mumbles (slurred speech), fumbles (clumsy hands), and grumbles (irritability or confusion). The moderate case (28 to 32 degrees) has stopped shivering, is drowsy or confused, bradycardic with a slow respiratory rate, hyporeflexic, and shows the Osborne J wave on the ECG. The severe case (below 28 degrees) is unconscious, profoundly bradycardic, hypotensive or in shock, may have fixed and dilated pupils and absent reflexes, and is at imminent risk of ventricular fibrillation or asystole.[1]

The history must establish the likely core temperature (the environment, the duration, the wet clothing), the mechanism (an immersion, an avalanche, a fall, a collapse indoors), and the potential precipitants and mimics — alcohol and drugs, a thyroid or adrenal failure, sepsis, hypoglycaemia, and occult trauma. A buried avalanche victim raises the separate question of asphyxia against hypothermia: an airway packed with snow and a burial under 35 minutes points to asphyxia, while a free airway and a long burial point to hypothermia.[2]

Differential diagnosis

The differential of hypothermia is the differential of the cold, collapsed patient, and the candidate must distinguish the primary environmental case from the secondary causes that both mimic and coexist with it. Every hypothermic patient is searched for a precipitant, because the temperature is often the consequence, not the cause. [1]

Bedside assessment

The assessment runs in parallel with the resuscitation, and it begins with the low-reading thermometer — the single most important and most often forgotten piece of equipment. A standard oral or tympanic device floors at about 34 degrees and will report a profoundly hypothermic patient as 34; the candidate who records "34.0" in a cold patient and is satisfied has missed the diagnosis. A core temperature is measured at the rectum, the bladder, or the oesophagus with a probe that reads to at least 25 degrees (and ideally lower), and it is the number that drives every subsequent decision.[1][2]

TheABCDE is applied with the constant proviso of gentle handling. Airway and breathing — assess the airway and the respiratory rate (a cold patient breathes slowly and may apnoeic); preoxygenate with warmed humidified oxygen, and intubate gently only if the airway is unprotected or the breathing is failing, accepting that intubation itself can provoke fibrillation. Circulation — apply the cardiac monitor (the J wave, the bradycardia, the atrial fibrillation), establish access, and check the pulse for a full minute because the cold bradycardia is slow and easily missed; in the severe case, feel for a pulse and confirm with bedside ultrasound before declaring an arrest, because a cold heart may have a faint output invisible to a hurried finger. Disability — document the Glasgow Coma Scale, reproduced because it is examined and because it is the bedside prognostic anchor: eye opening (4 spontaneous, 3 to speech, 2 to pain, 1 none), verbal response (5 oriented, 4 confused, 3 inappropriate words, 2 incomprehensible sounds, 1 none), and motor response (6 obeys commands, 5 localises pain, 4 withdraws, 3 abnormal flexion, 2 abnormal extension, 1 none), for a maximum of 15; check the pupils and the glucose, and remember that a fixed dilated pupil is not brain death in hypothermia. Exposure — remove the wet clothing, dry and insulate the patient, and complete the secondary survey for the precipitant and the trauma. [1]

Investigations

The investigations follow the resuscitation and never delay oxygen and rewarming. The electrocardiogram is the highest-yield single test and carries the eponymous sign of hypothermia — the Osborne wave, or J wave, a positive deflection at the junction of the QRS complex and the ST segment, seen below about 32 degrees and growing with the depth of hypothermia. The ECG also shows a sinus bradycardia, a prolonged PR and QT interval, atrial fibrillation in the moderate range, and, in the severe case, ventricular fibrillation or a slow, broad complex that can mimic a Brugada pattern. The J wave is the sign the examiner shows on a data-viva strip.[1]

The blood gas shows a metabolic and respiratory acidosis and is interpreted uncorrected — the "corrected for temperature" value is a mathematical fiction, and the candidate acts on the uncorrected value that reflects what the patient's tissues actually see. The electrolytes and glucose are essential: hypoglycaemia is common and correctable, and a rising potassium is the central prognostic marker. The coagulation profile, the full blood count, the troponin, and the creatine kinase are sent in the moderate and severe case — the troponin for a myocardial injury that may have caused the collapse, the creatine kinase for a rhabdomyolysis, and the coagulation profile interpreted with the caveat above. A chest radiograph looks for the aspiration and the precipitant pneumonia; a computed tomography of the head is reserved for the patient who fails to recover consciousness after rewarming or who has a head-injury mechanism. An alcohol and drug screen identifies the coexistent overdose. In the cardiac arrest, the HOPE score (Hypothermia Outcome Prediction after Extracorporeal life support) integrates the age, the sex, the asystole versus VF, the serum potassium, the core temperature, and the CPR duration to estimate the probability of survival after extracorporeal rewarming, and supports the decision to commence or to cease ECMO.[5]

Resuscitation — the first ten minutes

The resuscitation rests on three principles that the candidate must state aloud in any hypothermia station: handle the patient gently, remove the ongoing heat loss, and rewarm in a staged fashion matched to the severity. Rough handling — dragging a patient, a vigorous roll onto the spinal board, a forceful intubation — can precipitate ventricular fibrillation in a cold, irritable myocardium, and the patient is moved smoothly and with the minimum of disturbance. The wet clothing is removed (cut off, to avoid movement), the patient is dried and insulated in warm blankets, and the resuscitation bay is warmed.[1][2]

Oxygen is given warmed and humidified, the cardiac monitor is attached from the outset, and intravenous or intraosseous access is established. A capillary glucose is checked immediately and hypoglycaemia corrected with glucose 25 g intravenously (50 mL of 50 per cent). In the malnourished or the alcoholic patient a thiamine 100 to 300 mg intravenously is given before the glucose, and a naloxone trial is considered in the comatose patient with a respiratory depression. The patient who is shivering and cardiovascularly stable (mild hypothermia) needs no more than passive rewarming; the patient who has stopped shivering, who is bradycardic, or who is unconscious is escalated to active rewarming and continuous monitoring. The precipitant is sought and treated in parallel — the sepsis with antibiotics, the myxoedema with levothyroxine and hydrocortisone, the overdose with the antidote. [1]

Definitive management — the rewarming ladder

The rewarming method is chosen by the severity and is built on a four-rung ladder, from the least to the most invasive. The rate of rewarming is matched to the patient: too fast in the severe case can precipitate rewarming shock and arrhythmia, and too slow in the arrest is futile.[1][2]

Passive external rewarming is the foundation for the mild case: remove the wet clothing, dry and insulate the patient in warm blankets in a warm room, and let the shivering generate the heat. It is effective only in the patient who can shiver and who is cardiovascularly stable, and it achieves a rise of 0.5 to 2 degrees per hour. Active external rewarming adds the Bair Hugger (forced warm air at 43 degrees), warmed blankets, and a radiant heater, and is the standard for the moderate case; the trunk is warmed before the limbs to limit the afterdrop — the continued fall in core temperature as cold, acidotic peripheral blood returns to the circulation when the limbs are rewarmed first. Active internal rewarming delivers heat directly: warmed intravenous crystalloid at 40 to 42 degrees (each litre of room-temperature fluid lowers the core by about 0.25 degrees, so all fluids are warmed), warm humidified inspired gases, and body-cavity lavage through bladder, gastric, pleural, or peritoneal catheters; it is indicated in the severe, unconscious patient and in any patient in or near arrest. Extracorporeal rewarming with veno-arterial ECMO or cardiopulmonary bypass is the gold standard for the hypothermic cardiac arrest: it rewarms the blood directly to 37 to 38 degrees, it provides circulatory support, and it is the method with the best survival in arrest, validated in the Danish and the subsequent European series.[4]

Adjunctive drug doses follow the clinical picture. Morphine 2.5 to 5 mg intravenously is titrated for the pain and the distress of vigorous shivering (shivering is exhausting and painful); ondansetron 4 mg intravenously for the nausea of rewarming; antibiotics for a confirmed aspiration or pneumonia — but not prophylactically; levothyroxine and hydrocortisone 100 mg intravenously for the suspected myxoedema coma with coexistent adrenal insufficiency; and glucose for the hypoglycaemia. In the trauma patient every unit of blood and every fluid is warmed, and the coagulopathy is anticipated and treated with warm blood products, not with a normal-looking coagulation profile.[1]

Modified advanced life support

The hypothermic cardiac arrest is the single most modified algorithm in emergency medicine, and the candidate who runs a standard ALS loop on a core below 30 degrees fails the station. The principles, codified in the ERC 2021 and the WMS guidelines, rest on the physiology of the cold heart: it is refractory to defibrillation, it is unresponsive to adrenaline and amiodarone, and it tolerates ischaemia far longer than the warm heart — which is why prolonged cardiopulmonary resuscitation is appropriate and why the patient is not declared dead until rewarmed.[2][3]

The sequence is as follows. Confirm the arrest carefully (a full minute for the pulse, and bedside ultrasound), because chest compressions on a cold but perfusing heart can precipitate fibrillation. Begin high-quality chest compressions and ventilate with warmed humidified oxygen. If the rhythm is ventricular fibrillation, deliver a single biphasic shock at 150 to 200 joules; if fibrillation persists, defer further shocks and withhold adrenaline and amiodarone until the core temperature is above 30 degrees, because below that the drugs are ineffective and accumulate. Once the core is above 30 degrees, resume standard ALS with adrenaline 1 mg every 3 to 5 minutes (some guidelines extend the interval to 6 to 10 minutes to limit accumulation) and amiodarone 300 mg after the third shock. Continue prolonged cardiopulmonary resuscitation — the hypothermic myocardium tolerates ischaemia for hours, and CPR for several hours is appropriate where rewarming is under way or in transit to ECMO. Activate extracorporeal rewarming early — the patient in arrest is transferred to an ECMO-capable centre, with CPR maintained in transit, because ECMO rewarming is the intervention that changes survival. Do not pronounce death until the core is at least 32 degrees and the patient is unresponsive to the full algorithm — the rule of "no one is dead until warm and dead".[3][4]

Subtypes and scenarios

The Fellowship case is often a scenario. The elderly person found indoors — secondary hypothermia on a background of sepsis, a fracture, or a stroke, with comorbidity and polypharmacy; rewarm passively or with active external, treat the precipitant, and prognosticate cautiously because the mortality is high. The immersion victim — a drowning overlay, with the primary respiratory failure of aspiration and the hypothermia of cold water; resuscitate toward oxygenation first and rewarm in parallel, and apply the modified ALS. The avalanche burial — distinguish asphyxial death (a packed airway, a short burial under 35 minutes) from hypothermic arrest (a free airway, a long burial, a cold core), because the asphyxial case is not salvageable and the hypothermic case may be. The trauma patient — hypothermia as the first leg of the trauma triad of death; warm the bay, warm the fluids and blood, and treat the temperature as an injury. The intoxicated young person — alcohol and opioids cause both the exposure and the hypothermia, and the overdose is treated alongside the rewarming.[1][2]

Complications and pitfalls

The complications of hypothermia and its rewarming are predictable and examined. The afterdrop is the continued fall in core temperature after removal from the cold, as cold peripheral blood returns to the core; it is limited by warming the trunk first and by gentle handling. Rewarming shock is the vasodilation and the relative hypovolaemia of rewarming, requiring fluid support and vasopressors in the severe case. Ventricular fibrillation precipitated by movement or by premature defibrillation is the feared event, and it is the reason for gentle handling and the single-shock rule. Rhabdomyolysis with an acute kidney injury, and a disseminated intravascular coagulation, are recognised late complications. Aspiration pneumonia complicates the comatose patient.[1]

The pitfalls are the inverse of the management. Using a standard thermometer and missing the severe case is the commonest error. Rough handling that precipitates fibrillation is the second. Running a standard ALS algorithm — multiple shocks, early adrenaline, early cessation of CPR — on a core below 30 degrees is the third and the one the examiner most often tests. Trusting the normal coagulation profile in a bleeding trauma patient is the fourth. Treating the corrected blood gas rather than the patient is the fifth. Calling death too early, before rewarming to at least 32 degrees, is the sixth — the failure that the "warm and dead" rule exists to prevent. Forgetting the precipitant — the sepsis, the myxoedema, the adrenal crisis, the overdose — is the discharge pitfall. [1]

Prognosis and disposition

The prognosis of hypothermia spans the spectrum. The mild case recovers fully with passive rewarming and is discharged once normothermic and the precipitant is addressed. The moderate case is admitted to a monitored bed for active rewarming and observation. The severe case and any arrest are admitted to the intensive care unit, with extracorporeal rewarming for the arrest in an ECMO-capable centre. The two prognostic tools the examiner expects are the HOPE score, which estimates the survival probability after extracorporeal rewarming from the age, the sex, the rhythm, the potassium, the core temperature, and the CPR duration, and the serum potassium — a level at or above 10 mmol per litre in accidental hypothermia reflects massive cell lysis and is incompatible with survival, a recognised criterion to cease resuscitation.[4][5]

The rule that overrides every other is that no one is dead until warm and dead. A patient in asystole at a core of 24 degrees may recover neurologically intact after prolonged CPR and extracorporeal rewarming, and the candidate resuscitates fully, uses the cluster of prognostic factors (the HOPE score, the potassium, the mechanism, the downtime), and does not cease until the core is at least 32 degrees and the algorithm has failed.[4]

Special populations

The elderly patient is the classic ANZ case — secondary hypothermia, comorbidity, polypharmacy, and a high mortality; the precipitant is sought and treated, and the rewarming is gentle. The child has a large surface-area-to-mass ratio (losing heat fast) but also a robust diving reflex and a tolerance of hypothermia that makes the paediatric submersion the archetype of "dead until warm and dead"; resuscitate fully and prognosticate late. The avalanche victim is triaged by the asphyxia-versus-hypothermia distinction. The pregnant patient is resuscitated as the non-pregnant, with manual left uterine displacement and a perimortem caesarean at four minutes of arrest beyond 20 weeks; the fetal survival tracks the maternal rewarming. The trauma patient is warmed aggressively as part of damage control, with warmed fluids and blood and a warmed bay, because hypothermia worsens the coagulopathy and the bleeding. [1]

Evidence and regional guidelines

The evidence base for hypothermia is built on the Brown review (NEJM 2012) — the definitive contemporary reference, codifying the definition, the staging, the rewarming ladder, and the modified ALS; the Wilderness Medical Society 2019 guidelines (Dow et al), which set the out-of-hospital evaluation and treatment and the staging-to-rewarming mapping; the European Resuscitation Council 2021 guidelines (Lott et al), which set the modified ALS for the hypothermic arrest; the Danish Praesto Fjord series (Wanscher et al, 2012), which demonstrated that circulatory arrest from hypothermia is survivable with extracorporeal rewarming; and the HOPE score (Pasquier et al, 2019), which gave a validated prognostic tool for the hypothermic arrest after ECMO.[1][2][3][4][5]

Exam-exhaustive expansion — staging, the ECG, arrhythmia drugs, and the rewarming ladder

The Fellowship station turns on a small number of discriminating facts that the examiner probes by name: the Swiss HT staging and its rewarming mapping, the temperature-mapped ECG, the antiarrhythmic drugs that work and the ones that do not, and the ED approach as a protocol. This section assembles them for the viva.[1][2][3]

The Swiss staging in operational detail (HT I to HT V)

The Swiss (Durrer) scale is the field staging the candidate must recite cold — it maps the level of consciousness and the vital signs to a rewarming strategy without a thermometer, and it is the staging the ERC and the WMS guidelines cite.[2]

The ECG of hypothermia — temperature-mapped

The ECG tracks the falling core in a predictable sequence, and the candidate must be able to map each change to a temperature band.[1]

Arrhythmia management in the hypothermic heart

The cold myocardium is electrically irritable but pharmacologically refractory. The standard antiarrhythmic algorithm fails, and the candidate must know which drugs work, which do not, and which have been retired.[2][3]

The ED approach — the first ten minutes as a protocol

The ED approach to the hypothermic patient is the ABCDE with three modifications: a gentle handling to avoid VF, a warmed everything, and a low-reading core temperature from the first minute.[1][2]

Rewarming strategies — the methods compared

The four rungs of the ladder differ in the rate, the indication, the equipment, and the principal pitfall.[1][2]

The modified ALS — step by step

The hypothermic arrest is the most modified algorithm in the resuscitation syllabus, and the candidate must be able to walk the examiner through it.[3][6]

Prognostication — the HOPE score and the potassium

Landmark evidence

More clinical pearls — the high-yield cluster

Exam pearls

• Definition: hypothermia is a core temperature below 35 degrees Celsius; measure it with a low-reading thermometer.
• Staging: know both the standard medical (mild 32 to 35, moderate 28 to 32, severe 24 to 28, profound below 24) and the Swiss HT I to V; the cessation of shivering marks the transition to moderate.
• The J wave (Osborne wave) is the ECG hallmark, a positive deflection at the J point below about 32 degrees.
• Handle gently — the cold, irritable myocardium fibrillates with rough movement, intubation, and central lines.
• Modified ALS: one shock, then defer shocks and drugs until the core is above 30 degrees; prolonged CPR; rewarm to at least 32 degrees before calling death; ECMO for the arrest.
• The coagulation profile is falsely normal (the lab warms the sample); the patient is coagulopathic in vivo.
• A potassium at or above 10 mmol per litre is incompatible with survival — a criterion to cease resuscitation.
• Interpret the blood gas uncorrected; treat the patient, not the corrected number.
• No one is dead until warm and dead — resuscitate fully and prognosticate from the cluster of factors, not a single time cutoff. [1]

Red flags