Hypothermia and cold-related injury
Introduction
In the context of MCCQE1 preparation, understanding hypothermia and cold-related injuries is paramount. Canada’s geography and climate make these conditions a common presentation in emergency departments, from urban centers to remote rural nursing stations.
For the Medical Council of Canada Qualifying Examination Part I, candidates must demonstrate the Medical Expert role by identifying stages of hypothermia, initiating appropriate rewarming protocols, and managing complications like frostbite. Furthermore, the Health Advocate role is relevant when addressing vulnerable populations (e.g., individuals experiencing homelessness, elderly, and indigenous populations in remote areas) who are disproportionately affected.
Canadian Context: In Canada, accidental hypothermia is a significant cause of morbidity and mortality. Knowledge of pre-hospital care and transport logistics (e.g., transport to a center with ECMO capabilities) is a key component of Canadian emergency medicine practice.
Pathophysiology of Heat Loss
To manage cold injuries effectively, one must understand how the body loses heat.
Accidental Hypothermia
Definition: Core body temperature <35°C (95°F).
Classification and Clinical Presentation
The Swiss Staging System is widely used in Canadian protocols to guide management based on clinical presentation rather than precise temperature measurement, which can be difficult in the field.
| Stage | Temp Range | Clinical Findings | Risk of Cardiac Arrest |
|---|---|---|---|
| HT I (Mild) | 35°C – 32°C | Conscious, shivering vigorously. | Low |
| HT II (Moderate) | 32°C – 28°C | Impaired consciousness, shivering stops, ataxia, paradoxical undressing. | Moderate |
| HT III (Severe) | 28°C – 24°C | Unconscious, vital signs present but depressed (bradycardia, hypoventilation). | High |
| HT IV (Profound) | <24°C | Apparent death, no vital signs, asystole or VFib. | Very High |
ECG Findings in Hypothermia
The hallmark ECG finding in hypothermia is the Osborn Wave (J wave)—a positive deflection at the J point. Other findings include:
- Sinus bradycardia (physiologic response)
- Prolongation of PR, QRS, and QT intervals
- Atrial fibrillation (common below 32°C)
- Ventricular fibrillation (common below 28°C)
Management of Hypothermia
Management is dictated by the severity of the hypothermia and the patient’s hemodynamic stability.
Step 1: Initial Stabilization and Assessment
- Handle Gently: The cold myocardium is irritable; rough handling can precipitate Ventricular Fibrillation (VF).
- ABC vs CAB: Assess pulse for up to 60 seconds before starting CPR. Peripheral pulses may be difficult to detect due to profound vasoconstriction.
- Remove from Cold: Remove wet clothing and insulate.
Step 2: Rewarming Strategy
Selection of rewarming method depends on the stage of hypothermia.
Passive External
Indication: Mild Hypothermia (HT I) with intact shivering reflex.
Method:
- Remove wet clothing.
- Warm blankets/insulation.
- Warm sweet drinks (if airway is protected).
- Mechanism: Relies on the patient’s endogenous heat production (shivering).
Step 3: Cardiac Arrest Management Modifications
- Defibrillation: If VF persists after 3 shocks, defer further attempts until temp >30°C.
- Medications: Withhold epinephrine and antiarrhythmics if temp <30°C.
- Between 30°C and 35°C, double the dosing intervals.
- Metabolism of drugs is significantly reduced.
Step 4: Post-Resuscitation Care
Monitor for complications such as pulmonary edema, hyperkalemia, rhabdomyolysis, and compartment syndrome.
Local Cold Injuries
Frostbite
Freezing of tissue leading to ice crystal formation, microvascular thrombosis, and tissue necrosis.
Classification
- Superficial (First/Second degree): Skin is pale/waxy. Blisters contain clear fluid. Minimal tissue loss expected.
- Deep (Third/Fourth degree): Skin is hard/frozen. Blisters are hemorrhagic or absent. High risk of tissue loss.
Management Checklist
- Stabilize: Address systemic hypothermia first. Life over limb.
- Refreeze Risk: Do NOT thaw if there is a risk of refreezing (e.g., patient still in wilderness). Refreezing causes worse damage.
- Rapid Rewarming: Immerse part in circulating water at 37°C – 39°C.
- Continue until tissue is red/purple and pliable (usually 30 mins).
- Note: Avoid dry heat (fire, heater) as numb tissue burns easily.
- Pain Control: NSAIDs (Ibuprofen) are crucial (anti-inflammatory and analgesic) + Opioids.
- Adjuvants: Tetanus prophylaxis. Consider tPA (thrombolysis) for deep frostbite within 24 hours (specialist consultation required).
Surgical Intervention: Early surgery is generally contraindicated. “Wait for demarcation” (often weeks to months) unless there is wet gangrene or sepsis.
Trench Foot (Immersion Foot)
Non-freezing cold injury caused by prolonged exposure to wet, cold conditions (0°C – 15°C).
- Symptoms: Pale, anesthetic foot initially, followed by hyperemia and severe pain upon warming.
- Prevention: Keep feet dry, change socks frequently.
Canadian Guidelines & Special Considerations
”Not Dead Until Warm and Dead”
In Canada, resuscitation of a hypothermic cardiac arrest patient should generally continue until the core temperature is raised to >32°C.
Exceptions (When to stop/not start):
- Obvious fatal injuries (decapitation).
- Chest wall is frozen solid (cannot compress).
- Burial in an avalanche for >35 minutes with airway packed with snow (asphyxia).
- Serum Potassium >12 mmol/L (indicates cell lysis and irreversible death).
Transfer Logistics
For severe hypothermia with cardiac instability, transfer to a center with ECMO/Bypass capabilities is the priority. Canadian EMS protocols often bypass smaller hospitals to reach these centers if transport time is feasible.
Key Points to Remember for MCCQE1
- J Waves: Pathognomonic for hypothermia.
- Shivering: Stops below 32°C (transition from mild to moderate).
- Pulse Check: Assess for at least 60 seconds.
- Rewarming Shock: Peripheral vasodilation during rewarming can cause hypotension; requires aggressive fluid resuscitation.
- Frostbite Thawing: Rapid rewarming in water (37-39°C) is the standard. Never rub snow on the wound.
- Drug Metabolism: Withhold ACLS drugs below 30°C.
Sample Question
Clinical Scenario
A 45-year-old male is brought to the Emergency Department by EMS after being found unconscious in a snowbank in Winnipeg. The ambient temperature was -25°C.
Vitals:
- Temperature (rectal): 27°C
- BP: 80/40 mmHg
- HR: 38 bpm
- RR: 6/min
- O2 Sat: Cannot obtain trace
Physical Exam: The patient is comatose and does not shivering. His skin is cold and pale. Pupils are sluggish.
ECG: Shows sinus bradycardia with prominent J (Osborn) waves and prolonged QT interval.
Which one of the following is the most appropriate initial management step for this patient’s rewarming?
Options
Detailed Explanation:
- Diagnosis: The patient has Severe Hypothermia (Temp < 28°C). Key features are the loss of consciousness, cessation of shivering, and significant vital sign depression.
- Reasoning:
- Option C is correct: Patients with severe hypothermia and hemodynamic instability (hypotension, bradycardia) require active internal (core) rewarming. This minimizes the risk of rewarming collapse (rewarming shock) and afterdrop. While ECMO would be ideal if in arrest or periarrest, lavage and warm fluids are the immediate first-line active internal measures available in most EDs.
- Option A (Passive) is insufficient because the patient has lost the shivering reflex and cannot generate endogenous heat.
- Option B (Active External) alone in severe hypothermia can cause significant peripheral vasodilation, leading to “rewarming shock” (worsening hypotension) and “afterdrop” (cold blood returning to the heart), potentially precipitating cardiac arrest. It is usually combined with internal methods.
- Option D (Atropine) is ineffective in hypothermia-induced bradycardia because the bradycardia is caused by decreased metabolic rate, not vagal tone.
- Option E (Pacing) is generally avoided as the cold myocardium is resistant to pacing, and the mechanical irritation of a pacing wire can precipitate ventricular fibrillation. The bradycardia is physiologic at this temperature.
References
- Tintinalli’s Emergency Medicine: A Comprehensive Study Guide, 9th Edition. Chapter on Hypothermia and Frostbite.
- Rosen’s Emergency Medicine: Concepts and Clinical Practice, 9th Edition.
- Brown, D. J., et al. (2012). Accidental Hypothermia. New England Journal of Medicine.
- Wilderness Medical Society Clinical Practice Guidelines for the Out-of-Hospital Evaluation and Treatment of Accidental Hypothermia (2019 Update).
- American Heart Association (AHA) Guidelines for Cardiopulmonary Resuscitation and Emergency Cardiovascular Care. Part 8: Cardiac Arrest in Special Situations (2020).
- Government of Canada. Occupational Health and Safety – Cold Environments – Working in the Cold.