Hypothermia Stages and Frostbite Stages: Field Care, and Hyperthermia vs Hypothermia

Hypothermia is a core body temperature below 35 °C (95 °F). It is not only a mountain or winter problem. It happens to an older adult in a cold house, a person who is intoxicated and lies outside overnight, a trauma patient exposed on a wet road, and someone pulled from cold water in summer. Frostbite is a separate injury, the freezing of tissue, usually the fingers, toes, nose, ears and cheeks, and the two often arrive together. This guide covers how hypothermia is staged, why a cold heart needs gentle handling, how rewarming works in principle, how frostbite is graded and cared for, and how hypothermia differs from hyperthermia.

Hypothermia stages by core temperature

Most EMS and emergency medicine texts stage hypothermia by core temperature into three bands. The bands are approximate, the body does not change abruptly at any of them, and references draw the lines in slightly different places. Some references draw the severe line at 30 °C (86 °F), and some texts add a fourth, profound stage below about 24 °C (75 °F).

Shivering is the body's main way of making heat, and it is a useful sign but not a reliable one. It usually fades somewhere around 32 °C (90 °F), but exhaustion, low blood sugar, alcohol and some medications can stop it at a higher temperature. A cold patient who has stopped shivering may be getting colder, not warmer. Many programs teach the mental status changes of mild and moderate hypothermia as the umbles: the patient stumbles, mumbles, fumbles and grumbles.

The Swiss staging system: staging without a thermometer

In the field you often cannot measure a core temperature at all. The Swiss staging system, published by ICAR MedCom (the medical commission of the international alpine rescue body) and used in the WMS clinical practice guidelines for accidental hypothermia, stages the patient by what you can see instead: whether they are conscious and whether they are shivering. Each stage carries an approximate temperature, but the findings come first. A later revision drops shivering as a criterion and stages by level of consciousness alone.

Score and trend the level of consciousness with the Glasgow Coma Scale

Why the thermometer matters

Many standard clinical thermometers do not read below about 34 °C (93 °F), so a seriously cold patient can show a reading that is merely low, or an error, rather than the true number. Oral, tympanic, forehead and armpit readings also under-read the core in a cold patient: a cold ear canal, a shivering mouth and cold skin all give a number that has more to do with the surface than with the heart. Where the temperature will change your management, it needs a low-reading thermometer and a core site, as your protocol allows. If you cannot get a trustworthy number, stage the patient by consciousness and shivering, and say in your handoff that the reading was peripheral or could not be obtained rather than reporting a falsely reassuring figure.

Gentle handling and the cold heart

A cold heart is irritable. As the core temperature falls the heart slows, the rhythm becomes unstable, and atrial fibrillation is common in moderate hypothermia. A classic ECG finding is the J wave, also called the Osborn wave, a positive hump at the end of the QRS complex. In severe hypothermia the heart can go into ventricular fibrillation with very little provocation, and rough handling is one of the provocations.

• Handle the patient gently. Avoid jolting, dragging and unnecessary movement during extrication, packaging and loading.
• Keep the patient horizontal where you can. Do not walk a moderately or severely hypothermic patient out, and do not sit them up suddenly.
• Do not rub or massage the arms and legs. It does not usefully rewarm and the movement can trigger an arrhythmia.
• Take longer than usual over the pulse check. A slow, faint pulse in a cold patient is easy to miss, and your protocol sets how long to feel for one.
• Put the patient on a monitor early, and expect a slow heart rate. In a cold patient a slow rate is part of the hypothermia, not by itself a reason to speed it up.
• Check a blood glucose level, and look for anything else that explains the picture: trauma, intoxication, an overdose, a stroke, sepsis, or an older adult who fell and could not get up.

Rewarming: the principles

Stopping further heat loss comes first, and it is something every crew can do well. Rewarming is described in three tiers. How far a crew goes along them, and with what equipment, is your protocol's.

Warm the trunk, not the limbs. Warming the arms and legs opens up their blood vessels and lets cold, acidotic blood that has been sitting in the periphery return to the core. That can drop the core temperature further after rewarming starts, which is called afterdrop, and it can unsettle a cold heart. Insulate the limbs and put the heat on the chest, the armpits and the back.

A fully alert patient with mild hypothermia who can swallow safely may be given a warm, sweet drink, which provides fuel for shivering more than heat. A patient who is drowsy, confused or not shivering gets nothing by mouth.

Hypothermic cardiac arrest

Cold protects the brain. Full neurological recovery has followed very long hypothermic arrests, which is why a cold patient in arrest is handled differently from a patient who arrested at a normal temperature. Start CPR as you would for any arrest, and continue rewarming measures alongside it. A cold heart may respond poorly to defibrillation and handles drugs slowly. Resuscitation guidelines differ on whether a cold heart changes how shocks and medications are used: the European (ERC) guidelines limit shocks and withhold epinephrine until the patient is warmer, while AHA advanced life support guidance treats further shocks and a vasopressor under the standard algorithm as reasonable alongside rewarming. Which applies is your protocol's.

A commonly quoted teaching point is that a patient is not dead until they are warm and dead. It expresses the idea that cold can make a living patient look dead and that a hypothermic arrest can justify a long resuscitation. It is a teaching point, not a field rule. Whether, when and how a resuscitation is ended, and where a hypothermic patient is taken, are decided by your protocol and your medical direction.

Frostbite stages

Frostbite is tissue that has frozen. Ice forms in and around the cells, and the small blood vessels are damaged, so more tissue can be lost after thawing than was obviously frozen. Most EMS texts grade it as frostnip, superficial frostbite and deep frostbite. Some references use four degrees instead, much like burn depth: first degree (numbness and redness without blisters), second degree (clear blisters), third degree (blood-filled blisters and full-thickness skin injury) and fourth degree (into muscle and bone). Either way, the true depth often is not clear for days or weeks, so a field grade is a description, not a prediction.

Frostbite field care

• Treat hypothermia first. A cold core is the threat to life; frozen fingers are a threat to tissue.
• Get the patient out of the cold and remove wet clothing, rings, watches and anything tight on the affected part before it swells.
• Do not rub the area, and never rub it with snow. Rubbing damages frozen tissue and does not rewarm it.
• Do not use dry heat such as a fire, a heater, an engine or a heating pad. Numb tissue cannot feel a burn.
• Do not thaw a frozen part if there is any chance it will refreeze. Thawing and refreezing does more damage than staying frozen until thawing can be done once and kept warm.
• Protect the part: pad it, splint it, and place dry, bulky dressings between frozen fingers and toes.
• Leave blisters intact and protect them from rupture.
• Avoid walking on a frozen foot once it has thawed. Where walking out on a frozen foot is the only way to reach help, that is a decision for the situation and your protocol, not a reason to thaw it first.

Where thawing is done, the WMS clinical practice guidelines for frostbite describe rapid rewarming in warm water held at about 37 to 39 °C (99 to 102 °F) until the tissue is soft and color returns. Most EMS crews will not do this in the field, because it needs controlled water and a guarantee against refreezing, and many systems leave it to the hospital. Thawing is very painful, and pain management is your protocol's.

Hyperthermia vs hypothermia

The two are opposite failures of the same system: the body's balance between making heat and losing it. Hypothermia is a core temperature that has fallen below normal because heat loss has outrun heat production. Hyperthermia is a core temperature that has risen because heat gain, from the environment or from exertion, has outrun the body's ability to shed it. Hyperthermia is not the same as a fever: in a fever the brain has raised its own temperature set point, while in hyperthermia the set point is normal and the cooling system is overwhelmed.

Our Heat Stroke vs Heat Exhaustion guide covers the hot end of the spectrum: how to tell heat exhaustion from heat stroke, why mental status is the dividing line, and how to cool. It also explains why aggressive cooling is stopped short of normal, so that a hot patient does not overshoot into hypothermia.

Organize your handoff, including how the temperature was measured

The short version

• Hypothermia is a core temperature below 35 °C (95 °F). Commonly published stages are mild 32 to 35 °C (90 to 95 °F), moderate 28 to 32 °C (82 to 90 °F) and severe below 28 °C (82 °F), and references draw the lines differently.
• Without a core temperature, stage by consciousness and shivering, as the Swiss system does.
• Shivering usually stops as hypothermia deepens. A cold patient who has stopped shivering may be getting worse.
• Standard and peripheral thermometers under-read a cold patient. Use a low-reading thermometer and a core site where your protocol allows, or say the number could not be trusted.
• Handle a cold patient gently and keep them horizontal: rough movement can trigger ventricular fibrillation.
• Remove wet clothing, insulate, stop heat loss, and warm the trunk rather than the limbs.
• In a hypothermic arrest, start CPR and keep rewarming; shocks and drugs may be modified by protocol. 'Not dead until warm and dead' is a teaching point, and ending a resuscitation is your protocol's decision.
• Frostbite runs from frostnip through superficial to deep. Do not rub it, do not use dry heat, do not thaw it if it may refreeze, and protect the blisters.
• Hyperthermia is the opposite failure: the patient needs cooling, and mental status separates heat exhaustion from heat stroke.