Hypothermia

Hypothermia is an involuntary fall in core body temperature below 35°C, occurring whenever the body loses heat faster than it can generate or conserve it.^[1] It is staged by core temperature into mild, moderate, and severe categories — detailed fully in Recognize the Condition — but the single most important framing point in this entire chapter is not a specific number on a thermometer. It is this: a severely hypothermic patient can appear clinically dead — unresponsive, with an absent pulse and unmeasurable blood pressure — and still be genuinely resuscitable. Reduced metabolic demand at low core temperature allows the brain and other organs to tolerate a period of markedly reduced or absent circulation for far longer than under normal body temperature, and meaningful neurologic recovery has been documented even after prolonged cardiac arrest and CPR in a hypothermic patient.^[1][2] This is the basis for the well-known clinical teaching phrase "no one is dead until warm and dead" — fixed, dilated pupils, apparent rigor, dependent lividity, and absent reflexes are, on their own, not valid reasons to withhold or terminate resuscitation in a hypothermic patient.^[1][2]

As with the Heat Exhaustion and Heatstroke chapter elsewhere in this platform, pharmacotherapy plays a genuinely secondary role here — the single most important intervention is matching the right rewarming method to severity, not reaching for a drug. What belongs squarely to the pharmacist is upstream of the emergency: a long and often underappreciated list of common medications — antipsychotics most prominently and most seriously, alongside sedative-hypnotics and benzodiazepines, opioids, alcohol, and beta-blockers — meaningfully impair the body's ability to generate, conserve, or behaviourally respond to cold, and this risk is concentrated, predictably, in exactly the patients a pharmacist is most likely to be reviewing medications for: institutionalized or cognitively impaired older adults, and patients recently started on, or with a recently increased dose of, an antipsychotic (see Risk Factors).^[3]

Hypothermia is also a genuinely Canadian-winter and social-determinants issue rather than a wilderness-medicine curiosity. Most cases of injury and death from cold among people experiencing homelessness in Toronto occur at temperatures considerably warmer than the thresholds that trigger a formal extreme-cold-weather alert — a finding worth internalizing before it is covered in full in Special Populations.^[4] This chapter covers the staging system, the practical assessment pearls (a low-reading thermometer, and a full 60-second signs-of-life check before concluding a pulse is absent), the staged rewarming approach from passive insulation through to specialist extracorporeal rewarming, the critical gentle-handling safety point, the deliberately limited and cautious role of pharmacotherapy, and the extensive medication-risk-factor content that makes this a genuinely high-yield chapter for community and institutional pharmacy practice alike.

Staging by core temperature — the Swiss/HT I-IV system

^[1] Some sources, including StatPearls and the Wilderness Medical Society, use a slightly different mild/moderate/severe temperature-band system (mild 35-32°C, moderate 32-28°C, severe <28°C, with some further describing a "profound" category below 24°C) built primarily around a single temperature cut rather than the clinical staging above — the exact boundary matters less than recognizing that the two systems describe the same underlying continuum, and that clinical trajectory (worsening consciousness, disappearing shivering, developing arrhythmia) should drive urgency at least as much as any single number.^[5][2]

Clinical features by severity

In mild hypothermia, the body is still actively defending its temperature: shivering, tachycardia, tachypnea, and mild confusion or ataxia are typical, alongside pale, cool skin.^[5][1] As hypothermia progresses into the moderate range, the picture changes in a genuinely important way — shivering, the body's own heat-generating defence, itself begins to fail as glycogen stores are depleted, even as the patient continues to cool. Consciousness becomes progressively more impaired, reflexes and pupillary response diminish, bradycardia and hypotension develop, and atrial fibrillation is a recognized finding at this stage.^[5][1] A genuinely curious and clinically important sign sometimes seen in this range is paradoxical undressing — a confused, hypothermic patient removing their own clothing despite continuing cold exposure, thought to reflect a combination of a failing central thermoregulatory response and peripheral vasodilation producing a paradoxical sensation of warmth; recognizing this as a hypothermia sign, not simply confused or erratic behaviour, matters for anyone assessing a patient found this way outdoors.^[5] In severe hypothermia, the patient is unconscious with severely depressed vital signs — blood pressure, heart rate, and cardiac output all continue to fall — and the myocardium becomes genuinely irritable and prone to ventricular fibrillation; a patient at this stage can present with an unmeasurable blood pressure and an undetectable pulse and still, as above, be resuscitable.^[5][1]

Osborn (J) waves and other cardiac findings

The classic — though not universal — ECG finding of significant hypothermia is the Osborn wave (J wave), a positive deflection at the junction of the QRS complex and the ST segment, best seen in the inferior and lateral precordial leads; its amplitude is roughly proportional to the degree of hypothermia. Other expected ECG findings include sinus bradycardia and prolongation of the PR, QRS, and QT intervals, with atrial fibrillation the most commonly reported dysrhythmia.^[5]

Differential diagnosis

• Sepsis — can itself cause hypothermia rather than fever, particularly in older or immunocompromised patients, independent of environmental cold exposure.
• Hypothyroidism / myxedema coma — impaired thermogenesis can produce or worsen hypothermia, especially in an older patient with an unclear precipitant.
• Adrenal insufficiency — impaired stress response and hypotension can accompany or mimic hypothermia's presentation.
• Hypoglycemia — can independently cause altered mental status and is frequently either a cause or a consequence of hypothermia; always check (see Assessment/Diagnosis).
• Carbon monoxide poisoning or another toxic/metabolic intoxication — consider alongside hypothermia rather than instead of it in a patient found unresponsive in a cold environment.
• A primary neurologic event (stroke, seizure) — particularly where the degree of altered mental status seems disproportionate to the measured temperature.

Complications

• Concurrent cold injury — frostbite is common alongside significant hypothermia and should be assessed for separately (see the Frostbite chapter elsewhere in this platform).
• Rhabdomyolysis and acute kidney injury.
• Coagulopathy — genuinely easy to miss, since a cold blood sample analyzed at standard laboratory temperature can mask a real coagulopathy; samples should be warmed to 37°C before coagulation testing for an accurate result.^[1]
• Pulmonary edema, aspiration, and pancreatitis.
• Cardiac arrhythmia, up to and including ventricular fibrillation and cardiac arrest.
• Rewarming-specific complications: afterdrop (a further fall in core temperature as cold peripheral blood returns centrally), rewarming shock (hypotension from peripheral vasodilation), Takotsubo cardiomyopathy, infection, electrolyte abnormalities, and bleeding.

Hypothermia risk is rarely a single factor in isolation — environmental exposure, physiological vulnerability, and, very often, medication effects interact, and patients frequently have more than one of these at once.

Medications that increase hypothermia risk — genuinely high-yield pharmacist content

Exactly as with the equivalent medication list in this platform's Heat Exhaustion and Heatstroke chapter, the correct response to identifying one of these medications on a patient's list is very rarely simply "stop it" — abrupt discontinuation of an antipsychotic, a benzodiazepine, or a beta-blocker carries its own real risk of destabilizing the condition it is treating. A proactive, lower-risk approach — reinforcing cold-avoidance behaviour, structured temperature monitoring during the highest-risk window, and involving the prescriber in any actual dose change — is generally the safer course (see Choose Initial Treatment).^[3]

Other risk factors

• Cold environmental exposure, worsened by wind, wet or damp conditions, and inadequate shelter or clothing — genuinely relevant to Canadian winters.^[6]
• Homelessness and inadequate or unstable housing — a major and disproportionate driver of hypothermia risk, not a peripheral detail; covered in full in Special Populations.^[4]
• Extremes of age — infants have impaired thermoregulation and a larger surface-area-to-mass ratio than adults; older adults have a blunted thermoregulatory (vasoconstriction) response and are more likely to have a relevant comorbidity or reduced mobility limiting self-protective behaviour, such as adding clothing or moving to warmth.^[5]
• Alcohol intoxication (see medication table above).
• Malnutrition and low body fat — reduced substrate for shivering thermogenesis and less insulation against heat loss.^[5]
• Endocrine and metabolic causes — hypothyroidism, hypoglycemia, and adrenal insufficiency (see Recognize the Condition for these as differential diagnoses as well as risk factors).^[5]
• Reduced mobility — stroke, spinal cord injury, significant frailty, or advanced osteoarthritis limiting the ability to move to warmth or add clothing.^[5]
• Cognitive impairment, dementia, or psychiatric illness — reduced ability to recognize cold or dress appropriately, and very often combined with one or more of the medication risk factors above.^[3]
• Trauma, or immersion/near-drowning, in a cold environment.^[2]
• Skin conditions, burns, or excessive vasodilation from another cause, increasing heat loss through the skin.^[5]

History

Characterize the exposure — ambient temperature, duration, wet versus dry conditions, and how the patient was found. Ask specifically about alcohol and substance use, and take a focused medication history targeting every drug class in Risk Factors; for a patient on an antipsychotic specifically, ask whether it was started or the dose increased within the last 1-2 weeks.^[3] Ask about comorbidities relevant to the differential — known thyroid disease, diabetes, and cardiovascular disease.

Examination — the low-reading thermometer, a genuine practical pearl

Standard clinical thermometers frequently fail to register accurately — or produce a falsely reassuring "normal" reading — at hypothermic core temperatures, so a low-reading thermometer is required for an accurate diagnosis. Use an esophageal probe placed in the lower third of the esophagus for an intubated patient, or a rectal probe (approximately 15 cm deep) or bladder probe for a non-intubated patient.^[1][5]

Assess responsiveness and signs of life for a full 60 seconds — using clinical examination together with ECG, end-tidal CO2, or point-of-care ultrasound where available — before concluding that a pulse or breathing is absent; do not rush this assessment in a hypothermic patient, since a genuinely present but very slow pulse or respiratory rate can otherwise be missed.^[1]

Cardiac monitoring

Obtain continuous ECG monitoring for HT II-III (moderate-to-severe) hypothermia, watching specifically for bradyarrhythmia, atrial fibrillation, and Osborn (J) waves, whose amplitude roughly tracks the degree of hypothermia and whose presence has been correlated with a higher risk of ventricular fibrillation and cardiac arrest (see Recognize the Condition).^[5][1]

Investigations

• Point-of-care glucose in every hypothermic patient — hypoglycemia may be either a cause or a consequence of hypothermia, and is easy to miss without specifically checking.^[1]
• Complete blood count, electrolytes, and renal function.
• Coagulation studies — genuinely easy to get a falsely reassuring result here: a cold blood sample analyzed at standard laboratory temperature can mask a real underlying coagulopathy, so samples should be warmed to 37°C before testing.^[1]
• Lactate, creatine kinase, and troponin, particularly for moderate-to-severe presentations, given the rhabdomyolysis and cardiac-injury risk described in Recognize the Condition.^[5]
• A toxicology screen where clinically indicated given the frequency of alcohol and substance-use contribution.

Prognostication (specialist/hospital level)

For a hypothermic cardiac-arrest patient, a receiving centre may use a prognostication tool such as the HOPE score (incorporating age, sex, core temperature, serum potassium, asphyxiation, and CPR duration) to help decide on candidacy for extracorporeal rewarming — this is not a community-pharmacy decision, but understanding it explains why complete, accurate information about how a patient was found genuinely changes specialist decision-making.^[1]

Differential diagnosis

See Recognize the Condition for sepsis, hypothyroidism/myxedema coma, adrenal insufficiency, hypoglycemia, carbon monoxide poisoning or another intoxication, and a primary neurologic event as the principal alternatives — or genuine co-existing contributors — to actively consider.

• Safely rewarm the patient, matching the intensity of the rewarming method to severity rather than defaulting to the most aggressive option available.^[1]
• Prevent and manage cardiac arrhythmia, recognizing that standard cardiac medications and defibrillation are frequently ineffective until the core temperature has risen further.^[1][5]
• Avoid iatrogenic complications of rewarming itself — afterdrop, rewarming shock, and rewarming-related electrolyte shifts (see Recognize the Condition and Monitoring/Follow-up).
• Never assume death based on clinical appearance alone in a significantly hypothermic patient — continue resuscitation until the patient has been adequately rewarmed or a valid termination criterion is met (see Choose Initial Treatment).^[1][2]
• Identify and address the underlying or contributing cause — environmental exposure, alcohol, a thermoregulation-impairing medication, or an endocrine/metabolic driver.
• Recognize and, where appropriate, address modifiable medication risk factors before a cold-weather episode occurs — a genuinely concrete, pharmacist-actionable preventive intervention.^[3]

Rewarming by severity

Hypothermic cardiac arrest (HT IV) — continue resuscitation

A pulseless, apparently lifeless hypothermic patient is not necessarily dead — see Recognize the Condition for why. Continue CPR; epinephrine may be given per a modified protocol, with up to three doses a commonly cited approach and further dosing guided by response (the supporting evidence here is of low quality), while standard antiarrhythmic therapy and repeated defibrillation attempts are frequently ineffective until the core temperature has risen further — the priority throughout hypothermic cardiac arrest is rewarming, not repeated drug or shock administration.^[1] Two criteria are used, in an in-hospital, specialist setting, to help decide when resuscitation may reasonably be terminated: a serum potassium above 12 mmol/L, or the patient having been rewarmed to 32°C or higher with persistent asystole and no other identifiable reversible cause.^[1] Outside of this specific, specialist context, the default in a hypothermic patient is to continue resuscitation and rewarming rather than to conclude the situation is hopeless based on appearance alone.

Pharmacotherapy's limited and cautious role

Outside of rewarming itself, this chapter's pharmacologic footprint is deliberately narrow. The single most effective and time-critical intervention in significant hypothermia is rewarming, not a drug, and standard cardiac medications and defibrillation attempts are often ineffective at low core temperatures — the priority is always matching the rewarming method to severity rather than reaching for pharmacotherapy as a substitute.^[5][1] Where medications are used, dosing is genuinely complicated by significantly altered pharmacokinetics at low core temperature: hepatic and renal clearance both slow, drug effect and accumulation become less predictable, and standard dosing intervals may not reliably apply until the patient has been substantially rewarmed.^[5]

The pharmacist's preventive action: medication review before and during cold weather

Before and during cold-weather periods proactively review the medication list of any patient with one or more risk factors above — particularly an older or institutionalized patient — for antipsychotics, sedative-hypnotics/benzodiazepines, opioids, and beta-blockers, and involve the prescriber in any actual change rather than defaulting to discontinuation. For a patient recently started on, or with a recently increased dose of, an antipsychotic specifically, consider structured temperature monitoring for the first 7-10 days — the period during which antipsychotic-related hypothermia is disproportionately reported — especially where additional risk factors such as advanced age, benzodiazepine co-use, or cold exposure are also present.^[3] This is a genuinely concrete, pharmacist-actionable preventive step, directly analogous to the medication-review action described in this platform's Heat Exhaustion and Heatstroke chapter — and, as there, the goal is very rarely outright discontinuation.

• Remove wet clothing and insulate with dry layers; use a vapor barrier and wind protection to prevent further heat loss.^[1]
• Handle gently throughout — minimize movement and jostling of a significantly hypothermic patient (see Choose Initial Treatment for why this matters).^[1][2]
• Position and transport horizontally rather than upright where feasible, to reduce cardiovascular strain and afterdrop risk.^[2]
• For an alert, shivering patient with mild hypothermia, encourage active movement and caloric/carbohydrate intake to support endogenous heat production through shivering; avoid alcohol and caffeine.^[2]
• Provide a warm, dry, wind-protected environment or shelter as quickly as possible.
• A structured insulation/vapor-barrier wrap (sometimes called a "hypothermia wrap" or "hypothermia burrito") is a genuinely practical field technique to limit ongoing heat loss during transport.^[1]
• For a patient at risk of recurrence, counsel on layered, wind- and moisture-resistant clothing, awareness of wind chill, and avoiding alcohol before or during cold exposure.^[6]
• Encourage regular check-ins on older, cognitively impaired, or socially isolated neighbours and patients during cold weather — physical and social isolation delays recognition and help-seeking, exactly as in the Heat Exhaustion and Heatstroke chapter.^[4]
• Connect patients who are homeless or otherwise without reliable access to warm shelter with local outreach and warming-centre resources — a genuine, concrete, actionable intervention, and one that matters even in weather that does not feel "extreme" (see Special Populations).^[7][4]

Homelessness and outdoor-exposure populations

This is not a minor demographic footnote. In Toronto, 72% of hypothermia cases among people experiencing homelessness occur when temperatures are warmer than -15°C — considerably milder than the temperature most alert systems, and most people, associate with genuine cold-weather danger — and people experiencing homelessness account for roughly a quarter of all hypothermic injuries and a fifth of hypothermic deaths despite representing a small fraction of the overall population; precipitation and dampness meaningfully amplify the risk beyond temperature alone.^[4] Apply a lower threshold for concern even during "moderate" winter weather, not only during a formally declared extreme-cold-weather alert, and build any discharge plan around where the patient will actually be able to stay warm and dry, not around an assumption of stable, heated housing. Connect proactively with local outreach and warming-centre resources — Toronto's Streets to Homes program, for example, provides enhanced 24/7 street outreach and additional warming-centre capacity during declared alerts, and a wellness check for someone in apparent distress outdoors can be requested through the city's non-emergency line, with 911 reserved for an immediate medical emergency.^[7] Community pharmacists are often a trusted, low-barrier point of contact for this population and can play a genuine, concrete role in this outreach connection.^[4]

Infants

Infants have impaired thermoregulation and a higher surface-area-to-mass ratio than adults, cannot communicate cold or numbness, and are entirely dependent on a caregiver to notice environmental risk and respond — apply a lower threshold to bring a cold-exposed infant in for assessment even before frank hypothermia is obvious.^[5]

Older adults

Older adults have a blunted thermoregulatory vasoconstriction response even at baseline, a higher prevalence of the comorbidities and thermoregulation-impairing medications discussed in Risk Factors — particularly antipsychotics used for behavioural symptoms of dementia, benzodiazepines, and beta-blockers for cardiovascular disease — and often reduced mobility limiting self-protective behaviour. This is a genuinely compounded risk, and older, often institutionalized, patients are the population in which most reported antipsychotic-related hypothermia occurs.^[3][5] Apply a lower threshold for both clinical concern and a proactive medication review.

Pregnancy and breastfeeding

Evidence specific to hypothermia management in pregnancy is limited. Manage per the same staged-rewarming principles above, involve obstetric input for any pregnant patient with moderate-to-severe hypothermia, and confirm medication safety in pregnancy or lactation against each drug's own Drug Database entry before use.

Renal and hepatic impairment

Drug clearance is already unpredictable at low core temperature (see Choose Initial Treatment); baseline renal or hepatic impairment compounds this further, and doses given during the acute episode may need reassessment as clearance normalizes with rewarming.

• Monitor core temperature continuously during rewarming using the same low-reading thermometer/probe used for diagnosis.^[1]
• Maintain continuous cardiac rhythm monitoring through HT II-IV rewarming, given the ongoing arrhythmia risk described in Recognize the Condition.^[1]
• Reassess mental status as core temperature normalizes; persistent altered mental status despite adequate rewarming warrants reassessment for another or co-existing cause rather than an assumption that rewarming alone will resolve it (see Assessment/Diagnosis).^[5]
• Recheck point-of-care glucose and electrolytes as rewarming progresses — meaningful shifts can occur during this window.
• Watch specifically for afterdrop and rewarming shock (worsening hypotension or a new arrhythmia) during active external and active internal rewarming.^[5]
• For a patient recently started on, or with a recently increased dose of, an antipsychotic, continue structured temperature monitoring for the full 7-10 day higher-risk window, not only the first day or two.^[3]
• Before discharge, confirm the patient actually has a safe, warm place to go — rewarming a patient and returning them to the same cold-exposure conditions does not resolve the underlying problem.^[4]
• Complete or arrange the prescriber-involved medication review discussed in Choose Initial Treatment; do not let the acute episode resolve without addressing the risk factor that contributed to it.^[3]

1. Core temperature not rising despite active external rewarming: escalate to active internal/core rewarming methods rather than continuing the same approach for longer.
2. Persistent hemodynamic instability, arrhythmia, or failure to rewarm despite active internal rewarming, particularly in HT III-IV hypothermia: arrange transport to, or consult, a centre capable of extracorporeal (ECMO/cardiopulmonary bypass) rewarming.
3. Hypothermic cardiac arrest not responding to initial CPR and modified resuscitation measures: continue resuscitation and rewarming-focused care rather than terminating early — a fixed-pupil or rigor-like appearance is not, on its own, a valid reason to stop; escalate for extracorporeal support where feasible.
4. Persistent altered mental status despite adequate rewarming: reassess for hypoglycemia, myxedema coma, adrenal insufficiency, sepsis, or another cause rather than assuming rewarming alone will resolve it.
5. A recurrent hypothermia episode in a patient on an antipsychotic, sedative-hypnotic, opioid, or beta-blocker: this is a clear signal, not a coincidence — trigger a formal, prescriber-involved medication review, echoing the same principle used in the Heat Exhaustion and Heatstroke chapter.
6. A homeless or socially isolated patient being discharged back into the same cold-exposure conditions that caused the episode: escalate the outreach/social-support connection rather than repeating the same counseling and discharging without a safe-shelter plan.

• Assuming a pulseless, unresponsive, cold patient is dead based on appearance alone — fixed pupils, apparent rigor, and absent reflexes are not reliable indicators of death in a hypothermic patient.^[1][2]
• Using a standard, non-low-reading thermometer and being falsely reassured by a "normal" reading.^[1]
• Handling a significantly hypothermic patient roughly, or moving/transporting them upright rather than gently and horizontally, risking a fatal arrhythmia.^[1][2]
• Applying aggressive rewarming to the extremities in a moderately-to-severely hypothermic patient, risking afterdrop and rewarming shock.
• Giving repeated defibrillation attempts or stacking cardiac medication doses without allowing time for further rewarming.^[1]
• Reflexively stopping an antipsychotic, sedative-hypnotic, opioid, or beta-blocker without prescriber involvement, rather than taking the safer step of structured monitoring and cold-avoidance counselling.^[3]
• Treating only formally declared extreme-cold-weather-alert conditions as dangerous — most hypothermia cases and deaths among people experiencing homelessness in Toronto occur at temperatures warmer than -15°C, well above typical alert thresholds.^[4]
• Not checking point-of-care glucose in a hypothermic patient.^[1]
• Missing a secondary or contributing cause — hypothyroidism, sepsis, hypoglycemia, adrenal insufficiency, or an implicated medication — when hypothermia does not have an obvious environmental explanation.
• Overlooking the medication-review opportunity during a routine cold-weather visit, especially for an older or institutionalized patient recently started on an antipsychotic.^[3]

How to use this table: Doses are adult doses and require patient-specific confirmation (renal/hepatic function, weight, allergy history). Pediatric dosing is weight-based per the current product monograph unless stated. Drug pricing is intentionally omitted. Select any drug name to open its complete Pharmasist Drug Database page.

Warmed IV Fluid Support — Active Internal (Core) Rewarming

Always warm crystalloid before administration — cold (unwarmed) fluid itself worsens hypothermia.

Correcting Hypoglycemia

Check point-of-care glucose in every hypothermic patient — hypoglycemia may be either a cause or a consequence, and is easy to miss without specifically testing for it.

Hypothermic Cardiac Arrest (HT IV) — Modified Resuscitation Approach, Hospital/Specialized Protocol Only

Standard antiarrhythmic therapy and repeated defibrillation are frequently ineffective until the core temperature has risen further — the priority throughout is rewarming, not repeated drug or shock administration.

Suspected Concurrent Opioid Toxicity

Opioid intoxication is a recognized contributor to hypothermia (see Risk Factors) and can independently cause reduced responsiveness and respiratory depression.