8.2 Environmental Emergencies (Hypothermia, Hyperthermia, Frostbite)
Key Takeaways
- Accidental hypothermia is classified into Mild (32-35°C), Moderate (28-32°C), and Severe (< 28°C); Swiss Hypothermia Staging provides rapid field categorization based on clinical signs.
- The hypothermic myocardium is hyperexcitable and refractory: limit defibrillation attempts to 3 shocks and withhold all ACLS medications when core temperature is below 30°C.
- Rewarming strategies progress from passive external to active internal (warmed IV fluids 40-42°C, warm humidified O2, pleural/peritoneal lavage, ECMO/CPB).
- Heat stroke is defined by core body temperature > 40°C (104°F) accompanied by central nervous system dysfunction; immediate cold-water immersion to a target core temperature < 39°C is mandatory.
- Frostbite management requires field preservation of frozen tissue without rewarming if any risk of refreezing exists, followed by rapid immersion in a 37-39°C water bath.
Environmental Emergencies (Hypothermia, Hyperthermia, Frostbite)
Environmental thermal emergencies represent systemic thermoregulatory breakdown caused by extremes of ambient temperature, physical exertion, or impaired physiological compensation. Critical care transport clinicians must understand the cellular pathophysiology of accidental hypothermia, heat stroke, and frostbite, and apply modified resuscitation algorithms specific to temperature-altered physiology.
Accidental Hypothermia Staging & Pathophysiology
Accidental hypothermia is an involuntary drop in core body temperature below 35°C (95°F). Thermoregulation fails as heat loss outpaces endogenous metabolic heat production.
Hypothermia Classification & Swiss Staging System
In field and transport environments where accurate core temperature measurement (esophageal or low-reading rectal probe) may be delayed, the Swiss Hypothermia Staging System correlates clinical findings with temperature ranges:
| Hypothermia Stage | Core Temperature | Swiss Stage | Clinical Manifestations & Physiological Status |
|---|---|---|---|
| Mild | 32°C to 35°C (89.6°F to 95.0°F) | Stage I | Patient conscious, intense shivering, dysarthria, tachypnea, tachycardia, cold diuresis, normal blood pressure |
| Moderate | 28°C to 32°C (82.4°F to 89.6°F) | Stage II | Impaired consciousness, lethargy, loss of shivering reflex (at ~31°C), hypoventilation, bradycardia, atrial fibrillation, progressive hypotension |
| Severe | 24°C to 28°C (75.2°F to 82.4°F) | Stage III | Unconscious, lost protective airway reflexes, severe hypotension, pulmonary edema, extreme risk of spontaneous ventricular fibrillation or asystole |
| Deep | < 24°C (< 75.2°F) | Stage IV | Apparent death, absent pulse and respirations, unmeasurable blood pressure, flatline ECG, fixed dilated pupils |
Core Temp < 35°C ──► Stage I (32-35°C): Shivering, Conscious
──► Stage II (28-32°C): No Shivering, Lethargic
──► Stage III (24-28°C): Unconscious, Spontaneous VF Risk
──► Stage IV (< 24°C): Apparent Death, Asystole / VF
Electrocardiography: Osborn (J) Waves
As core temperature drops below 32°C, cardiac conduction slows across the Sinoatrial (SA) node, Atrioventricular (AV) node, and Purkinje system. Electrocardiogram changes include:
- Osborn (J) Wave: A pathognomonic positive deflection at the junction of the QRS complex and ST segment (the J-point), best visualized in precordial leads V3–V6. The amplitude of the Osborn wave is directly proportional to the severity of hypothermia.
- Arrhythmias: Sinus bradycardia yields to atrial fibrillation with slow ventricular response below 32°C. Below 28°C, the ventricular fibrillation threshold drops drastically; mechanical stimulation (e.g., rough movement or endotracheal intubation) can precipitate VF.
- Conduction Intervals: Prolongation of PR, QRS, and QTc intervals, accompanied by baseline somatic tremor artifact from muscle shivering.
Hypothermia Resuscitation & Transport Protocols
Rewarming Modalities
Rewarming techniques must match the stage of hypothermia and hemodynamic stability:
- Passive External Rewarming (PER): Removal of wet garments, drying the skin, and insulating with dry blankets in a warm environment. Relies on the patient's endogenous shivering thermogenesis. Indicated for Mild Hypothermia (Stage I) only.
- Active External Rewarming (AER): Application of external heat sources (forced-air warming blankets such as Bair Hugger, warm water bottles, radiant heaters) applied to the trunk, axillae, and groin.
- Critical Warning: Avoid heating the extremities first. Peripheral vasodilation in cold limbs causes "afterdrop"—cold, acidotic, lactate-rich peripheral blood returns to the core, causing a secondary drop in core temperature and precipitating lethal cardiac dysrhythmias and severe rewarming shock.
- Active Internal (Core) Rewarming (AIR): Direct delivery of heat to core organs.
- Warmed IV Crystalloids: Infuse 0.9% Normal Saline or Lactated Ringer's warmed strictly to 40°C to 42°C (104°F to 107.6°F).
- Humidified Warm Oxygen: Administer warmed, humidified oxygen at 42°C to 46°C via ventilator circuit or mask.
- Cavity Lavage: Pleural or peritoneal lavage with warmed isotonic saline (40°C to 42°C) via closed chest tubes or diagnostic peritoneal catheters in severe refractory cases.
- Extracorporeal Life Support (ECLS): Veno-Arterial Extracorporeal Membrane Oxygenation (VA-ECMO) or Cardiopulmonary Bypass (CPB). Gold standard for severe hypothermia (< 28°C) with cardiac arrest or hemodynamic collapse, providing rapid rewarming rates (6°C to 10°C per hour) while maintaining organ perfusion.
Resuscitation & ACLS Modifications (< 30°C Core Temp)
The hypothermic heart is resistant to defibrillation and pharmacological cardioversion, and hepatic/renal clearance of drugs is near zero, causing administered medications to accumulate to toxic levels upon rewarming.
Core Temp < 30°C ──► Withhold ALL ACLS Meds ──► Limit Defibrillation to Max 3 Shocks ──► Focus on CPR & AIR/ECLS
Core Temp 30-35°C ──► Double Dosing Intervals ──► Defibrillate per Standard ACLS ──► Active Core Rewarming
- Defibrillation Limit: If Ventricular Fibrillation or Pulseless Ventricular Tachycardia is detected, attempt defibrillation up to a maximum of 3 shocks (at maximum joule setting). If VF/pVT persists, defer further shocks until the core temperature is warmed above 30°C.
- ACLS Medication Withholding: Withhold IV epinephrine, vasopressin, and antiarrhythmics (amiodarone/lidocaine) when core temperature is < 30°C (86°F).
- ACLS Dosing Modifications (30°C to 35°C): When core temperature is between 30°C and 35°C, administer ACLS medications, but double the dosing intervals (e.g., epinephrine every 6 to 10 minutes instead of every 3 to 5 minutes).
- Termination of Resuscitation Rule: "A patient is not dead until they are warm and dead." Continue CPR and core rewarming until core temperature reaches 32°C to 35°C before declaring death, unless there are obvious lethal injuries (decapitation, truncal transection) or the body is frozen solid (chest non-compressible).
Hyperthermic Emergencies & Heat Stroke Protocols
Heat-Related Illness Spectrum
Hyperthermia results from failure of heat loss mechanisms when ambient heat and metabolic heat load exceed cooling capacity. Unlike fever, hyperthermia is not mediated by pyrogens or hypothalamic set-point changes.
- Heat Exhaustion: Core temperature < 40°C (104°F). Intact neurological status. Characterized by profuse diaphoresis, headache, nausea, weakness, tachycardia, orthostatic hypotension, and fluid depletion.
- Heat Stroke: A life-threatening medical emergency defined by a core body temperature > 40°C (104°F) accompanied by central nervous system (CNS) dysfunction (encephalopathy, delirium, seizures, ataxia, or coma).
| Feature | Exertional Heat Stroke (EHS) | Non-Exertional / Classic Heat Stroke (NEHS) |
|---|---|---|
| Patient Population | Young athletes, military personnel, industrial workers | Elderly, infants, chronically ill, psychiatric patients |
| Mechanism | Strenuous physical activity in warm/humid environments | High ambient heat exposure with impaired thermoregulation |
| Onset | Sudden, acute onset (minutes to hours) | Gradual onset over several days |
| Skin Condition | Diaphoresis often present (50% of cases) | Hot, red, dry skin (anhidrosis common) |
| Complications | Severe Rhabdomyolysis, Acute Kidney Injury, DIC | High mortality rate, heart failure, pulmonary edema |
Critical Cooling Protocol for Heat Stroke
- Gold-Standard Cooling: Cold-Water Immersion (CWI): Submerge the patient up to the neck in a cold water bath (1°C to 14°C / 34°F to 57°F) with continuous water agitation. Provides rapid cooling rates exceeding 0.2°C per minute (up to 1°C every 5 minutes).
- Cooling Endpoint: Discontinue active cooling when core temperature reaches 38.5°C to 39.0°C (101.3°F to 102.2°F) to prevent overshoot hypothermia and shivering.
- Alternative Cooling: If CWI is unavailable during transport, perform Evaporative and Convective Cooling by continuously spraying warm-to-cool water mist over the exposed skin while directing high-speed fans across the patient.
- Fluid Resuscitation & Rhabdomyolysis: Administer isotonic 0.9% Normal Saline. In Exertional Heat Stroke, titrate fluid boluses to maintain urine output at 1.0 to 2.0 mL/kg/hr to mitigate acute tubular necrosis from myoglobinuria.
- Antipyretic Contraindication: Acetaminophen, Aspirin, and NSAIDs are strictly contraindicated. Heat stroke is not hypothalamic-mediated, and antipyretics exacerbate hepatic necrosis and coagulopathy.
Frostbite Pathology & Field Management
Frostbite is localized freezing of tissue resulting from cold exposure, causing intracellular ice crystal formation, cellular dehydration, microvascular thrombosis, and reperfusion injury.
Frostbite Classification
- 1st Degree (Superficial): Erythema, edema, localized anesthesia, central pallor; no vesicles or tissue loss.
- 2nd Degree (Full-Thickness Skin): Clear or milky fluid-filled blisters (vesicles) within 24 hours, surrounding erythema and edema; intact sensation to pinprick.
- 3rd Degree (Subcutaneous Tissue): Hemorrhagic blisters, blue-purple skin discoloration; loss of sensation.
- 4th Degree (Deep Structure): Freezing extends to muscle, tendon, and bone; dry, black eschar formation, gangrene, and non-viable tissue.
Field & Transport Rewarming Protocol
- Refreezing Hazard Assessment: Do NOT attempt field rewarming if there is ANY risk of refreezing before reaching definitive care. Walking on thawed frozen feet or exposing rewarmed tissue to sub-zero temperatures causes catastrophic tissue destruction far worse than leaving the tissue frozen during transport.
- Rapid Water-Bath Rewarming: Immerse the affected extremity in a circulating water bath maintained strictly between 37°C and 39°C (98.6°F to 102.2°F) for 15 to 30 minutes until the tissue feels soft, pliable, and regains an erythematous flush. Do not use dry heat or hot water (> 40°C), which causes thermal burns.
- Analgesia: Rewarming causes intense reperfusion pain. Administer IV parenteral opioids (fentanyl 50-100 mcg IV) prior to initiating the water bath.
- Post-Rewarming Dressing: Elevate the extremity. Place sterile dry gauze between digits to prevent maceration. Aspirate clear fluid blisters to reduce prostaglandin/thromboxane tissue toxicity, but leave hemorrhagic blisters intact to prevent secondary infection.
A critical care transport team is performing CPR on a 42-year-old hypothermic patient retrieved from a snowdrift. The esophageal core temperature probe reads 26°C (78.8°F). Monitor shows Ventricular Fibrillation. The team has delivered 3 defibrillation shocks at 360 Joules without conversion. What is the correct next step in resuscitation?
A marathon runner collapses during a mid-summer race. On arrival, the patient is obtunded, responding only to noxious stimuli with purposeful withdrawal. Core rectal temperature is 40.8°C (105.4°F), blood pressure is 92/56 mmHg, heart rate is 148 bpm, and skin is moist. What is the primary priority intervention?
During a wilderness transport mission, a paramedic evaluates a mountaineer with deep 2nd-degree frostbite on both feet. The team is 3 hours away from the transport helicopter landing zone in sub-zero ambient conditions. How should the paramedic manage the frozen feet?