Three temperature and circulation emergencies have specific field protocols with different treatment logic: hypothermia requires gentle rewarming, heat stroke requires aggressive cooling, and shock requires positioning and fluid management. Confusing the treatments makes outcomes worse. This article covers all three; the broader first aid context is in wilderness first aid: 15 field-treatable emergencies.
Hypothermia: Recognizing the Four Stages
Hypothermia is classified by core temperature using the Swiss staging system (HT-I through HT-IV). Core temperature assessment in the field: a tympanic (ear) thermometer provides a reasonable estimate; an axillary (armpit) thermometer reads approximately 1°F lower than core temperature. Most field thermometers do not read below 93°F (34°C) — if the thermometer reads at its lowest limit and the patient appears ill, assume severe hypothermia.
| Stage | Core temp | Clinical signs | Field treatment |
|---|---|---|---|
| HT-I (Mild) | 98.6–91°F (37–33°C) | Shivering, cold extremities, impaired fine motor | Remove wet clothing, insulate, warm shelter, warm beverages if conscious |
| HT-II (Moderate) | 91–82°F (33–28°C) | Shivering stops, muscle stiffness, drowsiness, poor judgment | Horizontal position, no exercise, insulate all surfaces, heat packs to axilla and groin |
| HT-III (Severe) | 82–68°F (28–20°C) | Unconscious, minimal vital signs, rigid muscles | Gentle handling (arrhythmia risk), CPR if no pulse, insulate from all sides, evacuation |
| HT-IV (Death) | Below 68°F (20°C) | No vital signs, maximum rigidity | Continue resuscitation — “not dead until warm and dead” |
The HT-II to HT-III transition is the most dangerous because shivering — the body’s primary heat-generation mechanism — stops. A patient who stops shivering in cold conditions is not warming up; they are entering moderate-to-severe hypothermia. Do not interpret cessation of shivering as improvement.
Hypothermia Rewarming: Active vs Passive
Passive rewarming — removing the patient from the cold environment, removing wet clothing, adding insulation, and allowing the body to self-rewarm — is appropriate for HT-I and HT-II. The patient’s own metabolic heat generation, when preserved by insulation, is sufficient to reverse mild and moderate hypothermia in most healthy individuals.
Active external rewarming — chemical heat packs, warm water bottles, or body-to-body contact with a warm person — accelerates rewarming. Apply heat packs to the axilla, groin, and neck — high-flow blood vessel areas where surface heat transfers efficiently to the core circulation. Do not apply heat directly to hands and feet in HT-II or HT-III — cold peripheral blood returning to the core can cause “afterdrop” (further core temperature decrease) and cardiac arrhythmia.
Warm beverages are appropriate only for HT-I patients who are alert and able to swallow without risk. The physiological rewarming benefit of warm beverages is modest — approximately 50kcal per cup of hot tea — but the psychological benefit of a warm drink is real.
Heat Stroke: The 104°F Line and Aggressive Cooling
Heat stroke is a medical emergency defined by two criteria: core temperature above 104°F (40°C) AND altered mental status (confusion, agitation, bizarre behavior, or unconsciousness). The combination of both — not heat alone — defines heat stroke. High temperature without mental status change is heat exhaustion, treated with rest and hydration.
The goal in heat stroke: reduce core temperature below 102°F (39°C) as rapidly as possible. Above 106°F (41.1°C), protein denaturation begins in the brain — permanent neurological damage occurs in minutes. Time is the critical variable.
Cooling methods by effectiveness:
- Cold water immersion: Most effective — reduces core temperature at approximately 0.35°F per minute. Immerse the patient in the coldest available water up to the neck. This is the treatment of choice for exertional heat stroke in young, healthy patients.
- Ice packs to neck, axilla, and groin: Approximately 60% as effective as immersion. Practical when immersion is not possible. Apply ice packs (chemical cold packs or improvised ice in cloth) to all three locations simultaneously.
- Evaporative cooling (wet skin + fanning): Spray or wet the skin and fan vigorously. Effective in low-humidity environments; less effective in high humidity (sweat does not evaporate).
Stop active cooling when the patient’s mental status returns to normal or when temperature reaches 102°F — do not continue cooling to lower temperatures as overshoot hypothermia can occur. Do not give oral fluids to a patient with altered mental status (aspiration risk).
Hypovolemic Shock: Recognizing and Responding
Hypovolemic shock occurs when blood or fluid volume is insufficient to maintain adequate tissue perfusion. Most common field causes: severe hemorrhage, severe dehydration, and burns. Progression is predictable:
| Class | Blood loss | Heart rate | Mental status | Skin |
|---|---|---|---|---|
| Class I | <750mL (<15%) | Normal | Normal | Normal |
| Class II | 750–1500mL (15–30%) | >100 bpm | Anxious | Pale, cool |
| Class III | 1500–2000mL (30–40%) | >120 bpm | Confused | Pale, cold, clammy |
| Class IV | >2000mL (>40%) | >140 bpm | Lethargic | Mottled |
Field treatment for hemorrhagic shock: control the source of bleeding first. Once bleeding is controlled:
- Positioning: Supine with legs elevated 6–12 inches (modified Trendelenburg) — increases venous return to the central circulation. Exception: leg elevation is contraindicated with suspected spinal injury or fractures of the legs or pelvis.
- Hypothermia prevention: Hypovolemic patients lose core temperature rapidly — blanket and insulate from all sides. Hypothermia + shock together are significantly more lethal than either alone.
- Oral fluids: In conscious, alert patients with Class I or II shock (heart rate under 120), controlled oral hydration is beneficial. Do not over-hydrate — aggressive oral fluids in a patient who may need surgery can complicate care.
Class III and IV shock require IV fluid resuscitation — a field-impossible intervention in most scenarios. The correct response is immediate evacuation while maintaining the patient warm, supine with legs elevated, and controlling any controllable bleeding.
Where to Go Next
The complete wilderness first aid reference covering bleeding control, wound care, fractures, anaphylaxis, and more is in wilderness first aid: 15 field-treatable emergencies. Wound irrigation and closure technique are in field wound care and infection prevention. Fracture splinting and neurovascular assessment are in improvised splints and fracture management in the field. For preventing hypothermia before it becomes an emergency, see preventing hypothermia without shelter: layering and heat retention.
