Hypothermia staging and rewarming

Hypothermia kills in ways that deceive the rescuer. The patient looks dead — cold, pale, unresponsive, with barely detectable vital signs — but is not necessarily dead. Hypothermia is the one condition where the axiom "not dead until warm and dead" is medically accurate. Patients have survived core temperatures as low as 56.7°F (13.7°C) with intact neurological function after hospital rewarming.

The cardinal error in field management is giving up too early or moving the patient too aggressively and triggering the cardiac arrhythmia that finally does kill them. This page gives you the system to stage, stabilize, and safely rewarm without crossing either failure mode.

Educational use only

This page provides general educational information for emergency preparedness scenarios when professional medical care is unavailable. It is not a substitute for professional medical advice, diagnosis, or treatment. Always seek the advice of a qualified healthcare provider. Use this information at your own risk.

The Swiss staging system

The Swiss staging system (also called the Swiss clinical staging system, revised by ICAR MedCom) is the standard field classification for accidental hypothermia. It is organized by clinical signs — not thermometer readings — because in field conditions a thermometer may be absent or unreliable, and clinical findings are sufficient to guide treatment decisions.

Stage	Name	Core temp range	Clinical signs	Rewarming approach
HT I	Mild hypothermia	90–95°F (32–35°C)	Shivering, alert and oriented, can walk	Passive rewarming; active external if shivering insufficient
HT II	Moderate hypothermia	82–90°F (28–32°C)	No shivering, altered mental status, can sit but cannot walk	Active external rewarming; hospital transfer
HT III	Severe hypothermia	75–82°F (24–28°C)	Unconscious, has a pulse and respirations (may be very slow)	Active external + internal rewarming; urgent hospital ECMO
HT IV	Cardiac arrest	Below 75°F (24°C)	No vital signs detectable	cardiopulmonary resuscitation (CPR); ECMO at hospital; do not stop resuscitation in the field

The most important clinical distinction is between HT I and HT II. The sign that separates them is shivering. A patient who is shivering has an active metabolic response to cold and a core temperature above approximately 90°F (32°C). A patient who has stopped shivering despite being cold has crossed into moderate hypothermia — shivering has been suppressed by the deepening cold. Absence of shivering in a cold, confused patient is a red flag, not reassurance.

Field staging without a thermometer

When no thermometer is available, use this clinical sequence:

Can the patient shiver? — Yes → HT I. No → continue to step 2.
Can the patient walk? — Yes, mostly → HT I. Stumbling/cannot → HT II.
Is the patient conscious? — Yes, confused → HT II. Unconscious → HT III.
Does the patient have detectable vital signs? — Yes → HT III. None detectable after 60-second check → HT IV.

This four-question sequence allows rapid staging without equipment and drives the right rewarming decision.

Field note

Wet 50°F (10°C) conditions with wind can kill a healthy adult faster than dry 20°F (-7°C) calm conditions. Water conducts heat away from the body 25 times faster than still air. Fishing accidents, canoe swamps, and hiking in rain without wind protection account for a substantial fraction of hypothermia fatalities — none of them occurring at "obvious" cold temperatures. Do not wait for snow to take cold injury seriously.

Why hypothermia kills — the physiology

Understanding the mechanism tells you why every step of the protocol exists.

Cold suppresses everything: Metabolic rate drops approximately 6% per 1°C drop in core temperature. Cardiac output falls. Heart rate and respiratory rate slow. Consciousness dims. Shivering — the body's primary heat generation mechanism — is maximal around 95°F (35°C) and stops around 86–90°F (30–32°C) when the shiver centers themselves are too cold to fire.

Afterdrop: When rewarming begins, peripheral blood vessels dilate, releasing cold blood from the extremities back into the core. This causes a transient further drop in core temperature even as the surface warms. Afterdrop of up to 5–6°C has been documented. This means a patient at HT II/III can drop into cardiac arrest during what appears to be adequate rewarming — particularly if the periphery (limbs) is warmed before the core. This is why the rewarming protocol focuses on core heating first.

Ventricular fibrillation risk: Below 86°F (30°C) core temperature, the myocardium becomes electrically unstable. Mechanical stimulation — rough handling, sudden position changes, chest percussion — can trigger ventricular fibrillation (VF). VF in a hypothermic patient is resistant to defibrillation below 86°F (30°C) — the cold myocardium simply will not respond to shocks. The patient must be rewarmed first, then defibrillated. This is why HT II–IV patients are handled with deliberate gentleness.

Rescue (circumrescue) collapse: Even after the patient is removed from cold water or cold environment, catecholamine (adrenaline) levels drop as the rescue stress resolves, peripheral vessels dilate, and core temperature can plummet. Life-threatening hypotension or sudden cardiac arrest in the first 30 minutes after rescue is well documented. This is why HT II–IV patients are kept horizontal and monitored intensively immediately after extraction.

Stopping further heat loss

Before active warming, stop the cold exposure. This step is consistently under-executed because rescuers rush to apply heat. Incomplete insulation delivers heat from one source while losing it from three others.

Field extraction

Assess the environment before touching the patient — ice that holds a 150-pound (68 kg) person may not hold a 150-pound person plus a rescuer plus the mechanical forces of extraction. Plan the rescue path before committing to it.
Extract the patient in a horizontal position whenever possible. Do not stand them up. Even in HT II, the heart is under significant autonomic stress; standing a hypothermic patient causes blood to pool in the legs (orthostatic stress), which can precipitate cardiac arrest.
In suspected HT II–IV: do not allow the patient to walk even if they insist they can. Exertion drives cold blood from muscles into the core, can precipitate VF, and is a recognized cause of afterdrop-triggered arrest.

Removing wet clothing

Wet clothing must come off — it conducts heat away from the body even after the patient is out of cold water. However, the method matters.

Cut clothing off when possible rather than pulling, rolling, or peeling. Rolling a HT II–IV patient to remove a jacket requires significant body movement that risks mechanical VF induction.
Use trauma shears or any scissors to cut garments along seams. Cut up the midline and peel the halves away without lifting.
Cut footwear off if needed — do not have the patient pull their legs up to remove boots.
Protect wet areas by immediately replacing removed clothing with dry insulation before moving to the next item.

The vapor barrier principle

Evaporative cooling from the skin surface continues even when the ambient temperature is warm, as long as the skin is wet and exposed to air. A vapor barrier — any waterproof layer placed directly over dry insulation — stops this mechanism.

Suitable vapor barriers: large plastic garbage bags, space blankets (reflective mylar), plastic sheeting, tarp material, rain ponchos. The vapor barrier goes between the patient's skin (or thin base layer if available) and the insulation.

The hypothermia wrap — four-layer system:

Starting from the patient outward: 1. Vapor barrier: Plastic sheeting or large garbage bag against the skin. Tuck around all surfaces including under the patient. 2. Insulation layer 1: Sleeping bag, blankets, or any insulative material covering the entire body. 3. Insulation layer 2: Second sleeping bag, additional blankets, clothing. 4. Outer wind and water barrier: Tarp, bivy sack, or second plastic sheeting layer to block wind and rain.

Ground insulation is critical: More heat is lost downward to cold ground than in any other direction. Insulate the patient from ground contact with at least 4 inches (10 cm) of insulation underneath. A sleeping pad, folded blankets, backpacks, or any dead-air space material. Do not skip ground insulation — it is more important than the top layers.

Cover the head: The head represents approximately 10% of body surface area and loses heat disproportionately. Cover it with a hat or wrap inside the system. Do not leave the head uncovered while the body is wrapped.

Active rewarming procedures

Active rewarming by stage.

HT I — mild hypothermia

Shivering is the treatment. The patient's own metabolic heat production (shivering can increase metabolic rate 2–5 times above resting) is sufficient to rewarm HT I if fuel is available and heat loss is stopped.

Move to shelter out of wind and precipitation.
Apply the hypothermia wrap.
Give warm sweet beverages by mouth: hot tea, broth, hot cocoa. Sweet fluids provide glucose fuel for continued shivering. Give 300–500 mL (10–17 oz) over 20–30 minutes if the patient can swallow safely.
High-calorie foods (chocolate, nuts, energy bars) support shivering — the metabolic cost of sustained shivering is approximately 200–300 calories per hour.
Place the patient in a sleeping bag. Body-to-body contact from a normothermic rescuer sharing the bag (a rescuer with a dry base layer) adds conducted heat.
Allow 30–60 minutes in a fully insulated system before reassessing. If shivering has not improved mental status by 60 minutes, reassess for HT II.

Do not give alcohol. Alcohol causes peripheral vasodilation, increases heat loss, impairs shivering, and creates a false sensation of warmth.

HT II — moderate hypothermia

At HT II, the patient is no longer generating adequate self-heat. External heat sources must be applied to core vascular areas.

Apply the full hypothermia wrap as above.
Chemical heat packs or warm water bottles: Place at bilateral axillae (armpits), neck/carotid, and bilateral groin (femoral vessels). These locations have superficial high-flow blood vessels that carry heat rapidly to the core.
Critical: Always wrap heat sources in cloth before application. A chemical hand warmer at maximum output can cause contact burns to hypothermic skin, which has reduced sensation and reduced protective blood flow. Minimum barrier: one layer of dry cloth.
If IV access is available and equipment permits, give warm IV saline at 104–107.6°F (40–42°C) at 200 mL/hr. Warmed fluids provide internal heat directly to the circulating blood volume. Do not give room-temperature or cold IV fluids — they accelerate core cooling.
Do not rewarm the limbs first. Applying heat to the arms and legs (instead of the core) drives cold peripheral blood into the core (afterdrop mechanism) and can precipitate cardiac arrest. Always heat the core vascular zones — armpits, neck, groin — not the extremities.
Keep the patient horizontal. Arrange evacuation while rewarming is in progress.
Monitor for afterdrop: temperature may fall 1–3°C during the first 20–30 minutes of rewarming as cold peripheral blood reaches the core. This is expected. Continue rewarming. If the patient deteriorates (loses pulse, stops breathing), move to HT III/IV protocol.

HT III — severe hypothermia

HT III patients are unconscious but alive. They require the same insulation and external core rewarming as HT II, with additional airway and vital sign management.

Airway first: position the patient on their side (recovery position) if no suspected spinal injury. Unconscious patients cannot protect their own airway.
Apply the full hypothermia wrap with core heat sources.
If trained: place a supraglottic airway (iGel, LMA) or nasopharyngeal airway to maintain the airway. Hypothermic patients have significantly reduced oxygen consumption but still need a patent airway.
Vital signs check — full 60 seconds: Hypothermic patients may have a heart rate of 6–10 beats per minute and respiratory rate of 2–4 breaths per minute. These are easy to miss in a 10-second check. Count for a full 60 seconds at the wrist or carotid before concluding no pulse. A Doppler device, if available, is more reliable than finger palpation at low heart rates.
If pulse is present, do not begin chest compressions — compressions on a beating hypothermic heart can induce VF.
Handle with absolute gentleness: No jolting, bumping, or rapid repositioning. Any mechanical stimulus to the cold myocardium below 86°F (30°C) can trigger VF. Move slowly, deliberately, and in a coordinated team.
Urgent evacuation to a facility with extracorporeal membrane oxygenation (ECMO) rewarming capability. ECMO is the definitive rewarming method for HT III–IV — it rewarns blood externally at a controlled rate and supports cardiac function simultaneously. Without ECMO, HT III survival in cardiac arrest drops substantially.

HT IV — cardiac arrest

A patient with no detectable vital signs in a hypothermic context may be alive with a heartbeat too faint and slow to detect, or may be in true cardiac arrest.

Check for vital signs for a full 60 seconds using carotid pulse palpation. If any doubt: assume cardiac activity is present and do not begin compressions.
If no pulse confirmed after 60 seconds: begin CPR at the standard rate (100–120 compressions per minute at 2 inches / 5 cm depth for adults).
Continue CPR without stopping until:
Core temperature has been rewarmed to above 86°F (30°C) and the patient remains pulseless, OR
ECMO or hospital capability is reached, OR
Physical exhaustion of rescuers forces cessation and no relief is available
Defibrillation: if a patient has a shockable rhythm (VF or pulseless VT on automated external defibrillator (AED) or monitor), attempt one defibrillation shock. If the AED recommends a shock, deliver it. If the patient remains in VF after one shock, continue CPR and transport — additional shocks before rewarming above 86°F (30°C) are not expected to convert VF in severe hypothermia. Most defibrillation guidelines recommend withholding repeated shocks until core temperature is above 86°F (30°C).
Hypothermia is neuroprotective — the cold brain tolerates prolonged cardiac arrest far better than a normothermic brain. Never declare death in a hypothermic patient in the field. "Not dead until warm and dead."

Afterdrop: prevention in practice

The afterdrop mechanism causes preventable deaths in apparently stable hypothermic patients. It is not inevitable — deliberate technique avoids it.

What causes afterdrop: - Warming the periphery (limbs) before the core: cold peripheral blood returns centrally, drops core temperature - Exertion during extraction: muscle contraction drives cold blood from muscles into core - Upright positioning: hydrostatic pressure shifts blood distribution, including cold peripheral blood - Rapid or aggressive massage of cold limbs: forces cold venous blood toward the heart

What prevents afterdrop: - Warm the core first, always — armpits, neck, groin. Not the hands and feet. - Keep the patient horizontal throughout rescue and transport. - Do not allow exertion — carry, do not walk, a HT II–IV patient. - Add active external warming to passive insulation — active warming reduces afterdrop compared to passive-only rewarming in non-shivering patients. - Administer warm IV fluids rather than room-temperature fluids, which act as internal cold loading.

Fluids and nutrition

Oral fluids (HT I only)

Warm, sweet beverages provide two benefits: thermal input and glucose for shivering fuel. Temperature: hot but not scalding — comfortable to hold. Target: 300–500 mL (10–17 oz) over the first 30 minutes.

Acceptable options: hot tea with sugar, hot cocoa, broth, warm sports drink. Not acceptable: alcohol (vasodilates and accelerates heat loss), caffeine in large amounts (diuretic), plain cold water.

Give oral fluids only to patients who are alert, oriented, and swallowing normally (HT I). Any altered mental status eliminates oral administration — the patient cannot protect their airway.

Warm IV fluids (HT II–IV)

Warm normal saline (0.9% NaCl) at 104–107.6°F (40–42°C) is the evidence-based IV warming adjunct. It does not provide large amounts of thermal energy (the heat capacity of IV fluids is modest compared to core warming methods), but it prevents the additional cold loading from room-temperature or cold IV fluids and provides modest direct warming to central blood.

Practical field warming: place IV fluid bags in the patient's hypothermia wrap near a heat source for 10–15 minutes before use. Do not heat above 108°F (42°C) — hemolysis risk. If no way to warm fluids, withhold IV access until hospital — room temperature saline in HT III–IV does more harm than good.

Special scenarios

Cold water submersion

Cold water removes heat 25 times faster than still air at the same temperature. Water at 59°F (15°C) produces unconsciousness in approximately 30 minutes in a lightly clothed adult. Water at 32°F (0°C) can incapacitate before the person can swim.

Survival after cold water submersion depends heavily on whether the patient went into cardiac arrest before or after rescue. Patients who are pulseless at rescue after prolonged cold water immersion should receive CPR and rapid evacuation to ECMO. Survival rates with ECMO are substantially higher than without.

Circumrescue collapse risk is highest in cold water rescues. The transition from horizontal cold water to vertical upright extraction drops blood pressure dramatically. Extract horizontal when possible.

Avalanche burial

Avalanche victims have an additional consideration: asphyxia vs. hypothermia. In avalanche burial, asphyxia (snow blocking the airway) kills faster than hypothermia. For burials under 35 minutes, suspect primarily asphyxia. For burials over 35 minutes, hypothermia becomes the dominant survival factor.

Patients buried under snow with a packed airway are not hypothermia survivors — they are asphyxia fatalities. Patients found with an air pocket around the face after prolonged burial may be genuine hypothermia arrest candidates.

Always check for an air pocket before assuming asphyxia. Clear the airway before beginning resuscitation.

Frostbite concurrent with hypothermia

Treat hypothermia first. Rewarming frostbitten tissue while the patient is still systemically hypothermic wastes body heat, causes pain that may induce movement, and the tissue will re-freeze if systemic warming is not achieved first. See burns and cold injury for the frostbite rewarming protocol once the patient is normothermic.

Prevention and risk recognition

The conditions that produce hypothermia are predictable. Wet, wind, and fatigue are the triad.

Wet: Rain, sweat, immersion. Replace wet insulation — wet down loses essentially all insulating value.
Wind: Wind chill drives convective heat loss. A 20 mph (32 kph) wind at 40°F (4°C) produces a wind chill equivalent to 18°F (-8°C). Treat wind as a multiplier, not a minor inconvenience.
Fatigue: Exhausted people stop generating shivering heat and stop making good decisions about adding layers, seeking shelter, or recognizing their own decline. Set a strict turnaround time, not a turnaround feeling.

Insulation planning: the shelter insulation page covers sleeping bag ratings, R-values, and layering systems for cold environments. Adequate fuel (firewood) for active warming is covered in the energy section.

Hypothermia response checklist

Hypothermia is a cold exposure emergency, but the downstream consequences extend to cardiac and neurological injury that require the same systematic approach as trauma. For managing frostbite alongside systemic hypothermia, and for burns from improvised rewarming devices, see burn assessment and treatment. For shelter construction principles that prevent cold exposure in the first place — insulation ratings, vapor barriers, and wind protection — see shelter insulation.