Hypothermia

• Vasoconstriction and Metabolic Suppression: As core temperature drops below 37°C, the hypothalamus triggers sympathetic-mediated peripheral vasoconstriction to shunt blood to vital organs (brain, heart, kidneys), thereby preserving core temperature at the expense of peripheral tissues. Simultaneously, metabolic rate decreases approximately 7-10% for each 1°C drop in core temperature, reducing oxygen consumption and carbon dioxide production. This metabolic suppression is protective, allowing tissues to tolerate prolonged ischemia and anoxia that would otherwise be devastating. The combination of vasoconstriction and decreased metabolic demand creates the characteristic "mammalian dive reflex," wherein dramatically reduced oxygen needs may allow remarkable neurological recovery after prolonged circulatory arrest.

• Electrical Instability and the "Afterdrop" Phenomenon: Hypothermia causes profound electrolyte disturbances and altered cellular membrane potential, leading to a characteristic cardiac electrophysiology pattern. As temperature falls below 32°C, the ECG develops the pathognomonic Osborn wave (also called J wave), a distinctive deflection at the junction between the QRS complex and ST segment that increases in prominence with decreasing temperature. The myocardium becomes extraordinarily irritable, with even minor physical disturbances capable of precipitating ventricular fibrillation (VF)—this is the basis for the dictum that severely hypothermic patients should be handled "gently." Critical to understanding hypothermia management is the phenomenon of afterdrop: after removal from the cold environment, core temperature may paradoxically continue to fall (typically 1-2°C, sometimes more) due to continued peripheral vasoconstriction with resumption of circulation in the cold periphery, returning cold blood to the core. This mechanism explains why passive external rewarming is contraindicated in severe hypothermia and why central rewarming techniques are mandatory.

• Coagulation Dysfunction and Metabolic Acidosis: Hypothermia produces a consumptive coagulopathy through multiple mechanisms: decreased platelet function and production, reduced enzymatic activity of coagulation factors (temperature-dependent), and shortened red blood cell survival with resultant hemolysis. The result is paradoxical bleeding despite low temperatures that should theoretically preserve hemostasis. Simultaneously, the profound reduction in metabolic rate causes accumulation of metabolic byproducts, producing mixed metabolic acidosis. Paradoxically, despite severely reduced PaCO2 (due to decreased CO2 production), the base deficit becomes increasingly negative, reflecting severe metabolic derangement. This acidosis further depresses cardiac irritability but is generally reversible with rewarming.

• Neurological Effects and CNS Depression: Core temperatures below 28°C produce profound CNS depression, with markedly reduced cerebral metabolism and blood flow. Brainstem reflexes diminish and may disappear entirely, creating the clinical scenario of apparent death. However, the brain's reduced metabolic demands mean it can tolerate extraordinary periods of circulatory arrest—cases of complete neurological recovery have been documented after 6+ hours of cardiac arrest in profound hypothermia. This neuroprotection is the basis for the extended resuscitation attempts recommended in hypothermia management, contrasting sharply with the limited resuscitation periods in normothermic cardiac arrest.

• Fluid and Electrolyte Derangements: Hypothermia causes a "cold diuresis," where peripheral vasoconstriction and increased blood pressure trigger atrial natriuretic peptide release, resulting in substantial fluid and electrolyte losses. Hyperkalemia develops as a result of reduced cellular uptake of potassium (Na-K-ATPase function decreases dramatically at low temperatures), reduced renal clearance, and release from hemolyzed cells. Serum potassium levels serve as a powerful prognostic indicator—values >12 mmol/L in the severely hypothermic patient are associated with death and guide decisions about resuscitation intensity.

• Primary Hypothermia (Environmental Exposure): This represents hypothermia from exposure to environmental cold without underlying disease. It occurs in healthy individuals subjected to acute cold exposure (immersion accidents, avalanche burial, wilderness exposure) or subacute exposure in inadequately clothed individuals. Immersion hypothermia is particularly dangerous because heat loss occurs approximately 25 times faster in water than in air at the same temperature; persons immersed in near-freezing water can develop severe hypothermia in minutes. Wind exposure dramatically increases heat loss through increased convective heat transfer (wind chill). Risk increases with poor cold-weather preparation, inadequate clothing, fatigue, and dehydration.

• Secondary Hypothermia (Underlying Disease): This develops as a consequence of medical, psychiatric, or toxicological conditions impairing thermoregulation. Elderly patients with sepsis, stroke, or myocardial infarction frequently develop hypothermia despite mild environmental temperatures—the underlying illness impairs the hypothalamic thermoregulatory center or produces severe metabolic derangement. Hypothyroidism causes reduced basal metabolic rate and impaired thermogenesis. Severe hypoglycemia impairs CNS thermoregulation and reduces shivering thermogenesis. Psychiatric patients with schizophrenia may have altered temperature perception and poor cold-avoidance behaviors. Alcohol intoxication impairs judgment (leading to inadequate cold protection), causes peripheral vasodilation (increasing heat loss), and directly depresses the hypothalamus; this combination makes alcoholic patients extraordinarily susceptible to hypothermia even in mild cold. Opioid overdose impairs ventilation and consciousness, preventing cold-avoidance responses. Spinal cord injury eliminates the ability to generate heat through shivering below the level of injury.

• Medications Increasing Hypothermia Risk: Antipsychotics impair thermoregulation and cause neuroleptic malignant syndrome (paradoxically producing hyperthermia, but underlying mechanism involves thermoregulatory dysfunction). Benzodiazepines and barbiturates depress the CNS and impair thermoregulatory responses. Beta-blockers reduce cardiac output and shivering response. Tricyclic antidepressants and phenothiazines impair temperature perception and thermoregulation.

• Additional Risk Factors: Advanced age (>65 years) with declining thermoregulatory capacity; homeless status with inadequate shelter; poverty; malnutrition; immobility from any cause (allowing environmental heat exchange); and layering of wet clothing (which dramatically increases conductive heat loss).

• Mild Hypothermia (32-35°C): Patients remain conscious and exhibit shivering—the body's primary heat-generating mechanism, mediated by sympathetic activation of skeletal muscle. Tachycardia and tachypnea occur as initial sympathetic responses. Patients may experience confusion, irritability, and paradoxical undressing—a bizarre and characteristic finding where severely hypothermic patients remove their clothing despite the cold environment, likely representing a misinterpretation of the severe peripheral vasoconstriction (creating a subjective feeling of warmth) or dysfunction of the prefrontal cortex. Slurred speech, ataxia, and poor judgment are prominent. Peripheral vasoconstriction produces pallor and peripheral cyanosis. Muscle rigidity may be present. This stage is the most responsive to intervention.

• Moderate Hypothermia (28-32°C): Shivering ceases—a critical prognostic sign indicating severe physiologic derangement; this is sometimes called the "shivering limit." Consciousness becomes impaired with progressive altered mental status, lethargy, and potentially loss of consciousness. Muscle rigidity increases, sometimes to the point of a "frozen" or "wooden" appearance. The patient becomes profoundly bradycardic (heart rates of 30-40 bpm are common) and may be severely hypotensive. Atrial fibrillation with a slow ventricular rate is characteristic; the Osborn wave becomes visible on ECG. Respiratory depression occurs with shallow, slow breathing. Pupillary responses and corneal reflexes may be diminished but typically remain present.

• Severe Hypothermia (<28°C): The patient appears nearly dead, with unconsciousness, absent shivering, profound bradycardia (rates <30 bpm), markedly reduced blood pressure (sometimes undetectable), and severely depressed respiratory effort. Pupils become dilated and may appear fixed, creating the impression of irreversible death. However, pupils fixed and dilated in severe hypothermia does not indicate death—neuroprotection from profound cold may still allow recovery. The patient may be in apparent cardiac arrest with no palpable pulse. Blood becomes markedly viscous due to cold-induced hemoconcentration, potentially making central line placement and resuscitation drug administration difficult. The severe reduction in metabolic rate (75-90% of normal) means that prolonged circulatory arrest can be survived.

• Physical Examination Findings:
• Osborn wave on ECG: Small positive deflection at the QRS-ST junction; increases in magnitude with decreasing temperature
• Atrial fibrillation: Often with a slow, regular ventricular response; may degenerate to VF with manipulation or rewarming
• Pulmonary edema: Manifested as pink, frothy sputum (sometimes called "pulmonary edema of the drowned" even in non-submersion hypothermia), resulting from fluid transudation secondary to profound capillary injury and increased pulmonary vascular resistance
• Fixed, dilated pupils: Described above; not indicative of death in hypothermia
• Extremity findings: Peripheral tissues may appear waxy, pale, or erythematous; eschar formation may develop in severe cases with prolonged exposure

• Important Clinical Variants:
• "Rescue collapse": Sudden death occurring immediately after rescue of a hypothermic person, potentially triggered by sudden reversal of peripheral vasoconstriction with acute afterload reduction or by handling-induced VF
• Immersion hypothermia: May produce rapid onset of severe hypothermia (within minutes in very cold water) and carry risk of aspiration
• Submersion/near-drowning with hypothermia: Combination dramatically improves prognosis because the mammalian dive reflex (bradycardia, peripheral vasoconstriction, reduced metabolism) is potentiated by cold; children have better outcomes due to more robust dive reflex

• Core Temperature Measurement: Esophageal temperature is the gold standard for core temperature assessment in the emergency setting, as it accurately reflects myocardial temperature and avoids the artifact of cold peripheral blood returning to the core. Rectal temperature is commonly used but may underestimate core temperature due to the presence of cold blood in the peritoneal cavity and portal blood. Bladder temperature reflects core temperature reasonably well. Tympanic membrane temperature may be inaccurate. Standard thermometers are inadequate as they typically only measure to 34-35°C; low-reading thermometers capable of measuring to 25°C or below must be used. In severe hypothermia, attempt should be made to obtain an accurate core temperature, but resuscitation should not be delayed pending temperature confirmation if the patient appears severely hypothermic.

• ECG Findings and Severity Classification: The Osborn wave (J wave) appears as temperatures drop below 32°C and is pathognomonic for hypothermia, though its presence does not correlate with prognosis. Progressive ECG changes with decreasing temperature include:
• 32-35°C: Osborn wave, bradycardia, atrial fibrillation
• 28-32°C: Atrial fibrillation becomes more regular, PR and QT prolongation
• <28°C: Profound bradycardia, may mimic asystole
• The presence of atrial fibrillation is common (50-80% of severely hypothermic patients) but is usually stable and typically converts with rewarming alone

• Laboratory Studies:
• Serum potassium: Highly predictive of outcome; values >12 mmol/L associated with death, particularly if accompanied by asystole
• Arterial blood gas: Reveals combined respiratory and metabolic acidosis; PaCO2 is typically reduced (due to decreased CO2 production) despite the acidosis, creating a characteristic picture
• Complete blood count: May show hemoconcentration or hemolysis
• Coagulation studies: Show coagulopathy with prolonged PT/INR, aPTT, and low fibrinogen; however, do not use these results to withhold transfusion if bleeding is present
• Renal function: Elevation of creatinine reflects both cold-induced injury and dehydration from cold diuresis
• Blood glucose: May be low (due to impaired hepatic gluconeogenesis) or elevated (due to catecholamine release)
• Troponin and cardiac enzymes: May be elevated from direct cold-induced myocardial injury

• Diagnostic Criteria for Severity: The Swiss Staging System categorizes hypothermia severity based on clinical presentation (though core temperature is also used):
• Stage I (HT I): Conscious, shivering
• Stage II (HT II): Impaired consciousness, no shivering
• Stage III (HT III): Unconscious, no shivering, reflexes intact
• Stage IV (HT IV): Unconscious, no reflexes, pulse present
• Stage V (HT V): Unconscious, no reflexes, pulseless (cardiac arrest)

• Differential Diagnosis Considerations: Distinguish hypothermia from:
• Sepsis: Hypothermia can be a presenting sign of sepsis in elderly patients; look for infectious source
• Myxedema coma: Severe hypothyroidism presenting with hypothermia, bradycardia, and altered mental status; thyroid function tests will clarify
• Hypoglycemia: May coexist with hypothermia; check serum glucose
• Intracranial hemorrhage or stroke: Can cause secondary hypothermia; neuroimaging may be indicated after stabilization
• Drug intoxication: Alcohol, opioids, and other substances can cause or contribute to hypothermia

• Gentle Handling and Prevention of Afterdrop: All hypothermic patients require gentle, minimal handling to avoid triggering dysrhythmias; rough movement or aggressive rewarming can precipitate "rescue collapse" with sudden VF in the severely hypothermic heart. Avoid passive external rewarming (blankets, heat lamps) in severe hypothermia because it causes peripheral vasodilation and increases afterdrop. For mild hypothermia (>32°C), passive external rewarming plus active external rewarming (blankets, warm beverages if conscious) is appropriate.

• Active External Rewarming: For moderate hypothermia (28-32°C), active external rewarming is appropriate, including heating blankets, warm water immersion of extremities, and heated intravenous fluids. However, avoid rewarming the trunk before the extremities because this prevents afterdrop from returning cold peripheral blood to the core.

• Active Core Rewarming (For Severe Hypothermia <28°C and Cardiac Arrest): This is the definitive treatment for severely hypothermic patients and those in cardiac arrest:
• Extracorporeal membrane oxygenation (ECMO) or extracorporeal rewarming (ECR): This is the gold standard for severely hypothermic

Cardiac — treat as immediate emergencies

• Ventricular fibrillation / pulseless VT: the cold myocardium has prolonged repolarization and heterogeneous conduction, so mechanical stimulation (rough transfer, intubation, central line guidewire) can trigger degeneration of bradycardia or atrial fibrillation into VF. Signaled by abrupt pulselessness with a chaotic monitor tracing. The profoundly cold heart is frequently refractory to countershock and to vasopressors; the AHA 2020 Guidelines for CPR and ECC advise performing standard ACLS defibrillation and drug administration concurrent with active rewarming rather than withholding them.
• Rescue collapse: sudden arrest during or just after extrication, from loss of the hydrostatic squeeze on removal from water, afterdrop, and catecholamine-primed myocardial irritability. Emergency; horizontal extrication and minimal handling are the preventive measures emphasized by the Wilderness Medical Society accidental hypothermia guidelines.
• Rewarming shock: peripheral vasodilation unmasks the volume deficit created by cold diuresis, producing hypotension as the patient warms. Requires warmed crystalloid.

Metabolic and hematologic

• Hyperkalemia: cell lysis plus failed Na-K-ATPase; peaked T waves may be obscured by Osborn waves. A markedly elevated potassium in a hypothermic arrest patient is the standard marker of a non-survivable, pre-arrest asphyxial insult.
• Coagulopathy and clinically significant bleeding: clotting factor enzymes are temperature-dependent, yet laboratory PT/aPTT are run at 37°C and therefore underestimate the defect — treat visible bleeding, not the number.
• Rhabdomyolysis with acute kidney injury: prolonged immobility, shivering, and cold muscle injury; suspect with dark urine and rising CK.
• Hypoglycemia and pancreatitis: glycogen exhaustion and cold-induced acinar injury; check a fingerstick glucose in every patient.

Tissue and treatment-related

• Frostbite, nonfreezing cold injury, compartment syndrome: signaled by demarcation, tense compartments, or disproportionate pain on rewarming.
• Aspiration pneumonia and noncardiogenic pulmonary edema: depressed airway reflexes plus capillary leak.
• Iatrogenic thermal burns from heating pads applied to vasoconstricted, insensate skin, and ECMO/ECLS-related bleeding and cannulation vascular injury from required anticoagulation.

• "No one is dead until they are warm and dead": the single most tested principle. Fixed dilated pupils, absent brainstem reflexes, rigidity, and an unrecordable blood pressure do not establish death in a cold patient — cerebral metabolic suppression is neuroprotective. Continue CPR and rewarming; the AHA 2020 Guidelines support prolonged resuscitation in hypothermic arrest.
• Osborn (J) wave: the buzzword. A positive deflection at the J point (QRS–ST junction) that grows as temperature falls. It is a marker of hypothermia, not of prognosis, and needs no specific treatment.
• Best next step in hypothermic cardiac arrest: transfer for extracorporeal rewarming (ECMO/cardiopulmonary bypass) while CPR continues. If a stem offers "warm blankets" or "warm oral fluids" for a pulseless or profoundly cold patient, that is the distractor.
• The shockable pair is VF / pulseless VT: shock per ACLS, but recognize that the cold heart is often refractory until warmed. Do not abandon resuscitation because of failed initial shocks.
• Atrial fibrillation with a slow ventricular response is a distractor: it is expected, usually hemodynamically tolerated, and converts with rewarming. Do not anticoagulate or cardiovert it as the answer.
• Handle gently and extricate horizontally: mechanical stimulation precipitates VF, and vertical extrication from water precipitates rescue collapse — a favorite mechanism question.
• The one association examiners test: hypothermia in an elderly or normally housed patient is secondary until proven otherwise — look for sepsis, myxedema coma, hypoglycemia, adrenal insufficiency, or alcohol/sedative intoxication. Concomitant paradoxical undressing points to primary environmental exposure.
• Always check a glucose and a potassium: hypoglycemia is treatable and mimics the depressed sensorium; a markedly elevated potassium in an arrested hypothermic patient argues the insult was asphyxial (drowning, avalanche burial) rather than isolated cold, and predicts non-survival.
• Use a low-reading esophageal probe: a standard thermometer that "bottoms out" at 34°C is the classic reason a severely hypothermic patient is under-triaged.