Hypothermia: Understanding and Prevention

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By Minnesota Sea Grant Director John A. Downing.

It was minus 8 F as I worked in my driveway to change the battery in my truck after several nights of temperatures below minus 20 F. Part of that time I had to work bare-handed to loosen corroded nuts. My hands became numb enough that they did not work well and bringing them back to my normal body temperature was very painful.

Our bodies are made to work well only within a narrow range of temperatures. Pushing them much above that or below it can cause impairment, serious damage, and death. In northern climates, accidental hypothermia (low body temperature) is more common than accidental hyperthermia (high body temperature). It is particularly common during winter recreation, especially when water immersion is possible.

As a Minnesotan and a lake scientist who takes pleasure in being outside in all seasons and frequently in, on, and around water, I created this guide to serve us all.

 

History of Hypothermia

The symptoms of hypothermia have been recognized for 2,000 years.7 The human body works best within a very narrow range of body temperature and decline in body temperature leads to a spiral of declining physical and mental ability that can exacerbate cooling, leading to incapacitation and death (see Table 1).

The human heart can stop at a body temperature of about 64 F, which is about like a room with cold air conditioning. At a body temperature of 84 F, the heart and breathing slow and at 91 F brain function begins to decline. 5,6


A Common Danger

Accidental hypothermia affects and kills people every year. In 2019, more than 1,000 died of hypothermia in the United States with almost half of those deaths occurring in the Great Lakes states of Illinois, Indiana, Michigan, Minnesota, Ohio, Pennsylvania, and Wisconsin.1 In the Great Lakes states, 35% more people die of hypothermia than drown1 and many of those drowning deaths are due to exhaustion from hypothermia. Paradoxically, the per-capita death rate from hypothermia is highest in areas with a normally mild climate. These mild-climate areas can have rapid changes in temperature (e.g., North and South Carolina) and large day-to-night temperature swings (e.g., Arizona).2 The majority of hypothermia deaths occur in people over the age of 55 in rural environments3 and about two-thirds of the deaths occur in males.4 In the Great Lakes region, hypothermia is the greatest cause of unintentional death occurring in natural environments, not including transportation accidents.1

Accidental hypothermia occurs with substantial frequency in all 50 states, in all seasons, and is the result of not only intensely cold ambient or air temperatures. One reason for this is that the human heart will stop at a body temperature of about 64 F. This corresponds to a room with cold air conditioning. A person's heart and breathing slows at a body temperature of 84 F. Brain function begins declining at a body temperature of 91 F. 5,6 These symptoms of hypothermia have been recognized for about 2,000 years.7 The human body works best within a very narrow range of body temperature and decline in body temperature leads to a spiral of declining physical and mental ability that can exacerbate cooling, leading to incapacitation and death (see Table 1).


An Ounce of Prevention

One of the best ways to prevent hypothermia is to dress in appropriate outdoor clothing, use heat packs or other body warmers as necessary, keep as little skin exposed as possible, stay dry, and move to a warm environment when there are signs of body cooling (e.g., numbness, instability, confusion).

Children are much more susceptible to hypothermia because they typically have less insulating fat and a higher ratio of skin surface to body volume. Infants have what is called brown fat or brown adipose tissue around their necks, chests, backs, and bottoms that keep them warm, so while they shiver little, they are still at risk of hypothermia.8

An adult over 50 years who has low or high body mass, problems with circulation, certain medical conditions (e.g., diabetes, hypothyroidism, Addison’s disease, kidney disease), takes particular medications (e.g., sedatives, antipsychotics, beta-blockers, diabetes meds), or has experienced frostbite or hypothermia in the past, may need more protection from the cold.9

Everyone should avoid alcohol and recreational stimulant drugs when they are going to be in a cold environment. Many cases of hypothermia occur in people with elevated blood alcohol levels and combinations of alcohol and recreational drugs can cause hypothermia on their own.10

Alcohol use opens up blood vessels near the skin and on extremities, which increases heat loss from the body.7 Because alcohol can produce a feeling of warmth, it tends to decrease a person’s perception of cold, delays the onset of shivering and reduces its duration, and can impede the body’s ability to regulate its temperature. If you are going to be on or around ice on water bodies, you should wear an approved personal flotation device (PFD) and clothing that will insulate you in cold-water such as flotation suits and jackets.


How Long Have I Got?

It has long been known that exposure to cold water increases the rate at which the temperature of a human cools. In fact, water can cool a person at least 24-times faster than air because of water's high thermal conductivity.11

Table 1 shows just how serious heat loss from cold water can become in the case of accidental immersion in water. For example, a person who breaks through ice will lose the ability to make coordinated hand and finger movements in less than 2 minutes. They will be exhausted and unconscious in less than 15 minutes and will drown if they are not pulled out quickly or if they are not wearing flotation gear.

Thus, the trick of keeping a couple of big spikes or ice picks in your pocket to haul yourself out onto the ice in case you break through ice is only useful in the first minute of immersion. Your hands very quickly lose the ability to grasp and exhaustion will soon make arm muscles ineffective. Since it is important to get out of the water as fast as possible, sometimes swimming can be effective if the water is not around the freezing point. In water that is 32-40 degrees Fahrenheit, a person can typically swim for up to 7 minutes before exhaustion. In water that is around the freezing point, a person is likely to survive only 15 to 45 minutes with flotation and possibly up to an hour or so with flotation and protective gear before the brain and heart stop (Table 1).

The surface temperature of Lake Superior in early to mid-summer is about 40 to 50 F. At those temperatures, a person can lose the ability to make coordinated hand and finger movements in less than 5 minutes, lose consciousness in 30 to 60 minutes, and can likely swim only 7 to 40 minutes before exhaustion and die 1 to 3 hours even with flotation.

The average surface water temperatures in inland lakes in summertime is about 60 to 70 F. At those temperatures, a person can lose the ability to make coordinated hand and finger movements in as little as 40 minutes, can become exhausted in as little as 2 hours, and can sustain swimming for less than 2 hours.

The estimates above depend on the physical characteristics of an individual. Overweight people may survive longer due to their body fat, which acts as insulation. Younger people and individuals who are healthy, well-nourished, and hydrated may also survive longer than these estimates.


Types of Rescues

The most important elements of a cold-water rescue include:

  1. Ensuring the safety of the rescuer
  2. Removing the person from the cold water as soon as possible
  3. Treating the person gently and with care not to jostle them12
  4. Keeping the person as horizontal as possible
  5. Moving the person to warm shelter or medical treatment as soon as possible13

A person’s core body temperature can continue to decline after they are removed from cold water. This is called the after-drop effect and can cause more serious decline, so gentle speed is important.14 Even responsive people removed from cold water or excessive cold should not be expected to help themselves. Their muscle rigidity, uncoordinated grasping ability, and decreased heart function may make it dangerous for them to move, walk, assist in rescue, or survive anything but horizontal transport to safety.15

Self-Rescue

Unless a rescuer is close-by, self-rescue may be the best means of survival so action in the first 15 minutes of cold water immersion is critical.16 When a person initially falls into cold water two phases of incapacitation occur prior to hypothermia. The two phases are cold shock with nearly uncontrollable hyperventilation (usually within 2 to 3 minutes) followed by a gradual incapacitation (usually within 20 to 30 minutes) (Table 1).12

During cold shock it is important to keep as calm as possible. During the incapacitation phase there is a rapid cooling of skin, nerves, and muscles that results in a rapid loss of dexterity, sense of touch, muscle strength and speed. Once cold shock has dissipated, swimming with or without a PFD may be a viable means of self-rescue. Table 1 shows that swimming times can be substantial, even in cold water. In 40 F water, for example, once cold shock has dissipated, a good swimmer may be able to cover a distance of about 300 feet (the length of a football field) in about seven minutes before exhaustion.17

If you have been pulled from shore by a rip current, then it is especially important to swim parallel to shore and perpendicular to the rip current so you are not swimming against the outgoing rip current.

If you fall into the water unexpectedly and are unable to self-rescue, and ideally if there are rescuers nearby, you can help retain body heat by curling into a ball and tucking your arms and legs and covering your face with your forearms.10,18 Because clothing helps retain body heat, it should not be removed in the water unless the clothing is excessive weight that is pulling the person under the water. The time a person spends removing their clothing might be better spent swimming to self-rescue.

As soon as practical, you should attempt to swim to the place where you fell in because, up until the breaking point, the ice there had supported your weight. This can be done by getting into a horizontal position in the water and kicking toward that place. Once there, try to haul yourself onto the ice using your hands or ice picks. Once on the ice surface, try to roll toward more solid ice and then crawl to safer ice.19 If you cannot get fully free from the water, hoist as much of your body out of the water as possible. This will help reduce the heat lost from your body. Try holding your arms outstretched and flat on the ice surface. Your arms may freeze to the ice surface, but this can help you avoid drowning even if unconsciousness occurs.19

If a rescuer approaches you never attempt to grab onto them but allow yourself to be towed by them even if the splashing of water on your face is uncomfortable. When you are removed from the water, you should sit or lie down to avoid sudden lost blood pressure.18

Rescue from a Vehicle in Water

When ice anglers take vehicles onto lake ice early or late in the season there is a risk of the vehicle breaking through the ice and becoming submerged. As soon as it seems that a vehicle is about to break through the ice, the occupants should unfasten their seat belts, unlock doors, and open electric windows (manual windows should be kept closed). Escaping while the vehicle is still on the surface is best.18

Generally, automobiles float for less than two minutes so making a fast escape is critical.20 If occupants are unable to escape while the vehicle is on the surface of the ice, they should attempt to escape through a door or a side window broken out at the corner with a glass-breaking hammer or hand axe. Under water, the electric door locks and windows of most modern vehicles will malfunction, plastic laminated safety glass is hard to break, air bags can make the interior disorienting. If occupants are unable to escape the vehicle, they should turn on the dome and other interior lights to make the vehicle easier to find underwater.

Fast and concerted action by passengers is essential to survival.21 In the unlikely event that a submerged vehicle retains air inside, occupants can die from exhaustion and drowning within 15-20 minutes at the temperature at the bottom of most temperate zone lakes in winter.

Protect the Rescuer

In all ice rescues, it is paramount to protect the rescuer. Too often, well-intentioned rescuers become victims themselves. Ice is not a predictable material, especially in early and late winter. Anyone involved in ice-related activities like ice-fishing, snowmobiling, skating, skiing, or walking on ice, runs the risk of finding themselves immersed in cold water. Everyone should be prepared to save themselves or assist in the rescue of others.19 In a 2006 review of ice rescue techniques in the book "Handbook on Drowning: Prevention, Rescue, Treatment," section authors Carla St. Germain and Andrea Zaferas19 make the following recommendations to all individuals working or recreating on or around ice:

  • Wear thermally protective buoyant clothing or wear a personal flotation device
  • Carry ice picks, a rope, waterproof matches and fire starters
  • Stay on marked areas and obey warning signs
  • Consider carrying materials for managing hypothermia.

The U.S. Navy developed a compact hypothermia prevention management kit (HPMK) that is commercially available.13 People recreating or working around cold water and ice should consider carrying such a kit. They cost about $100 and consist of a heat-reflective shell of technically advanced material to cover a hypothermic person and a self-heating blanket for rewarming. Other body heat management approaches are discussed below.

If you are close by a person who goes through the ice, you want to prevent yourself from falling in too. To do that, lie down on the ice to spread out your weight to lessen the chances of breaking through the ice. Then roll or crawl away from the place where the person went through the ice, put on a PFD or other flotation equipment immediately, and summon as much assistance as possible as fast as possible. The safest way to rescue someone is to talk them through self-rescue. Any available floatable object should be thrown to the person in the water. A rope with a large loop knotted at the end could allow a person in the water to slip it around their body so that bystanders could pull them free of the water. Extending a tree branch to a person in the water might provide some flotation and a means of pulling them free if they can grip it. It is important to remember that the ice near where a person breaks through has been weakened and other people will likely break through faster.19 If a rescuer approaches a person in the water, that person should not grab onto the rescuer, but should allow the rescuer to tow them to safety.

Bystander Rescue

Bystanders should be aware of how fast death in cold water can occur, whether due to exhaustion and drowning or by hypothermia. Because the majority of hypothermia cases occur in rural or rugged environments,3 professional assistance may not arrive rapidly enough to save a person in icy water. Additionally, professional rescuers may not always be well-equipped, or well-practiced enough to rescue people who have fallen through ice.19


Stages of Hypothermia

Cold-Shiver-Stumble-Bumble-Sleep-Die

These are the serial characteristics to review to determine the severity of hypothermia. The reason accidental immersion causes lost body function and death is because the body's core temperature drops (Table 2). The human body works well only when it can maintain a fairly narrow range of internal temperature. Measuring internal body temperature cannot be done well with home thermometers. 22 The best substitute is to measure rectal temperature but that is difficult to obtain without undressing or stressing the person concerned. Therefore, when you're in the field and not in a professional medical facility determining what stage of hypothermia a person is in is often based on indirect observation of their behavior, sensation, and external signs.

The indicators5 are:

  1. The sensation of cold
  2. The onset and progression of a shivering response, which is one of the ways a body rewarms itself
  3. The loss of ability to do normal physical activities like zip zippers, walking without stumbling
  4. Impairment of mental ability as the brain cools like slurred speech, disorientation, sleepiness
  5. Degree of consciousness
  6. The decrease or absence of signs of life like breathing or heartbeat (Table 2).

Because a person suffering from hypothermia can sometimes be resuscitated due to the preservative action of cold, a person should not be presumed dead until they are pronounced dead by a medical professional after warming.14 Whether or not a person can be revived successfully after heartbeat has ceased depends on several factors including how long they have gone without a heartbeat and whether cooling of the brain was sufficient prior to lost circulation.8

Table 2, created from several sources but most notably a publication by a panel of Wilderness Medical Society Clinical professionals,5 can be used as a rough guide to the stages of hypothermia. In all cases where hypothermia is suspected, medical assistance should be sought because medical institutions typically have the knowledge, technology, and equipment to warm bodies safely and more quickly while monitoring their progress. The stages of body cooling include:

  1. Cold-stressed
  2. Mild hypothermia
  3. Moderate hypothermia
  4. Extreme or severe hypothermia, and finally
  5. Death or apparent death (Table 2)5,13

Cold Stressed

A person who is simply cold-stressed but not hypothermic will feel very cold and may be shivering, even quite vigorously, but is able to perform physical tasks and care for themselves. This means they are able to walk, zip zippers, or remove or put on their clothing, for example. Their mental state is seemingly normal. Their core body temperature is likely to be 96-98.6 F (not by oral thermometer).

Mildly Hypothermic

A person who is mildly hypothermic will be cold, shivering, and will show some signs of impaired physical ability like stumbling and physical inability to perform physical care for themselves. Their core body temperature is likely to be 91-95 F. They may show some signs of altered mental state like slurred speech. Their pulse may be quite rapid although it will decline with cooling.9

Moderately Hypothermic

A person who is moderately hypothermic will feel cold, but their shivering is weak or stopped. They will have little to no physical ability to care for themselves or perform physical tasks and have a substantially altered mental state (see more examples below Table 2). It will be difficult for them to speak, they may feel apathetic or lethargic, they may seem confused, make odd decisions, show excessive hunger, and their heartbeat and breathing may be irregular or breathing slow (less than 12 breaths per minute). If these signs are present, their core body temperature is likely to be 83-90 F.

Severe or Profound Hypothermia

If a person is suffering severe or profound hypothermia, they are likely to be unconscious, experience decreasing strength, decreasing pace and regularity of heartbeat, slow and irregular breathing, no nervous reflexes, and no pain responses. Their core body temperature is likely to be 75 to 82 F. Below a core body temperature of 74 F, signs of life continue to decline. This includes a lack of response of the eyes to light, decreasing heartbeat and eventually a loss of brain activity or heartbeat. Although signs of life may be apparently absent, such people may be revivable if the brain is cooled enough before their heartbeat becomes undetectable. If there is no other massive trauma to the body,14 a hypothermia victim should not be assumed to be dead until their rewarmed body cannot regain signs of life.23


Field Treatment of Hypothermia

There are many actions people can take to rewarm a hypothermic person without specialized equipment, but hospitals and clinics have methods and equipment that can work more quickly and efficiently to help people recover from hypothermia. If a cold, shivering or non-shivering person is showing altered mental abilities, sleepiness or unconsciousness, or slow or irregular life signs, it is important to seek medical assistance as soon as possible.

Cold-stressed, Not Hypothermic

According to a panel convened by the Wilderness Medical Society,5 the best treatment for a person who has become cold-stressed (shivering but lucid and not suffering physical incapacities) is to return them to shelter, reduce their loss of body heat, and increase their ability to generate body heat. Shivering is a muscle movement reaction to skin cooling that can increase the production of body heat three to five times.24 Shivering burns energy stores rapidly so sugary or high-calorie beverages and foods are helpful in keeping a shivering person’s energy levels replenished. Getting a person moving or exercising is also a means of increasing internal heat production. As with hypothermia cases, it is important to provide effective insulation, dry clothes, and shelter from wind and further cold.23 If a person’s feeling of cold is slow to subside, it is important to warm their body core or trunk more than their arms and legs. Heating pads and hot packs, not directly applied to the skin, but placed around the armpits, groin, neck and trunk can be effective.

Mild Hypothermia

According to the Wilderness Medical Society panel,5 a person suffering mild hypothermia should:

  1. Be handled carefully to prevent a serious medical decline and exacerbation of symptoms during transport and stabilization
  2. Sit or lie down for 30 minutes or more
  3. Be insulated against further heat loss
  4. Have heat applied to the upper body (see insulating and heating victims of cold, below)
  5. Be given high-calorie food or non-alcoholic drinks10 to replenish lost energy
  6. Be taken to medical care if conditions and symptoms (e.g., impaired movement, mental alertness, shivering) do not improve

Normally, getting the person to warm conditions and wrapping them in dry insulating materials should help enable them to shiver and rewarm themselves. Applying external heat sources can improve comfort.24 By combining dry, warm insulation and application of external heat, a person with mild hypothermia should be back to normal within an hour or so.

Moderate Hypothermia

According to the Wilderness Medical Society panel,5 a person suffering moderate hypothermia should:

  1. Be handled carefully to prevent a serious decline and exacerbation of symptoms during transport and stabilization
  2. Be kept lying down
  3. Not stand or walk as they are likely to be physically impaired
  4. Not be offered food or drink due to their lack of physical reliability
  5. Be insulated against further heat loss with use of a vapor barrier and other insulating materials
  6. Have heat applied to the upper body (see Insulating and heating victims of cold, below)
  7. Be evacuated gently and speedily to medical care

Because they are unable to reliably consume liquids or food they will become dehydrated and exhausted rapidly and likely will need intravenous fluids.

Severe Hypothermia

If a person is cold and unconscious, it should be assumed that they are suffering severe hypothermia5 unless there is catastrophic injury.14 They should:

  1. Be handled carefully to prevent a serious decline and exacerbation of symptoms during transport and stabilization
  2. Be kept lying down
  3. Not stand or walk as they are likely to be physically impaired and/or unconscious
  4. Not be offered food or drink
  5. Be insulated against further heat loss with use of a vapor barrier and other insulation
  6. Have heat applied to the upper body (see insulating and heating victims of cold below)
  7. Be evacuated gently and speedily to medical care

Persons suffering severe hypothermia may begin to show very weak or absent breathing and pulse. Apparent lack of pulse can be due to both the temperature of the person taking the pulse and weak pulse in the victim. Cold fingers have difficulty feeling a pulse in the wrist. Taking a pulse at the carotid artery in the neck is more reliable. The carotid pulse can be found in the groove on the sides of the windpipe. The pulse in the carotid artery should be checked for at least a minute before assuming there is none. If the heart has stopped then cardiopulmonary resuscitation by a trained responder may be needed5 understanding that it should not be performed if there are signs of life (pulse, breathing).25 Breathing, although slow, may be easier to detect than a pulse.25


How to Rewarm a Hypothermic Person

General Approaches to Reducing Heat Loss

In all cases of hypothermia, increased insulation of the person is very important. Use sleeping bags, blankets, bubble wrap, insulated sleeping pads, extra clothing or any other available insulation.5,13 If the person is wet and outdoors, wrapping them in a vapor barrier such a plastic blanket or a foil insulation blanket before wrapping them in insulating material will help to keep the loft and dead-air space in insulating materials working well.

If a hypothermic person can be brought indoors or to a warm and wind-free environment, the rewarming process is somewhat different than what you would do in the field or outside. If necessary, the hypothermic person's clothing could be removed or cut away before they are wrapped in dry insulating materials. Ensuring there is insulation both beneath the person and on top of them is important to reduce heat loss.

General Approaches to Warming the Body

Warming just arms and legs is a poor way to warm a hypothermic person and may have a negative effect on their well-being. Warming the person's body core or trunk is likely to have the best effect. This can be done with a variety of heat sources including chemical hot packs, hot water bottles, electric heating pads, or another safe heat source. A recent study26 showed that commercially available gel chemical heat packs (not the kind requiring heating in a microwave or the type made for in mittens and boots25) heat up faster and hotter than dry chemical heating pads and blankets but the latter sustain heat longest. Although not an endorsement, blankets like the TechTradeTM Ready Heat II and sturdy water bags filled with warm water (e.g., DromliteTM) have gained acceptance by the emergency medical community.24 Heat sources should not be placed on the skin but separated from it by cloth or another insulator to avoid burning the skin.

Cold, hypothermic skin should be protected because it can be burned by heat that seems mild to normal skin.5 The best places to put heat sources are the arm pits, groin (if practical), neck, lower back near the sides of the body, in the middle of the chest, and in the middle of the upper back. These are areas where blood flow is high and external heat can most speedily warm that blood. As a person's trunk warms, that warmth will be distributed to the legs, arms, feet, and hands. Keeping the head and neck insulated during warming is also very important.

Note: Do NOT use warm showers or baths to rewarm a hypothermic person as these could send blood to skin, legs, arms, hands and feet and away from the heart and brain which could cause a dangerous drop in blood pressure. Body-to-body warming of a shivering person in an insulating wrap may make the person more comfortable but will do little to rewarm them because the body-to-body contact may reduce heat-producing shivering in the hypothermic person. This arrangement may also impede evacuation.25

Combining Insulation and Heating

There is a great deal of variation in the heat retention ability of various wrapping systems27 and diverse heat sources have various advantages and disadvantages.28 A review of commercial and homemade combined wrapping/heating systems26 compared combined insulation and heating packages with a 10-fold range in cost. The least costly and most compact is a military-designed system consisting of a cocoon made of a heat-reflecting blanket with chemical heating packs. Although it allows more heat loss than some of the more costly systems, it is compact and easy to pack into most any location and may provide good protection during the first two to three hours of evacuation. The review suggests that a user-assembled hypothermia wrap system consisting of a three-season, mummy-type sleeping bag, a heat-reflective blanket, a small plastic tarp, and gel chemical heating packs provides more protection at only a small additional cost.

According to Dow et al.5 and bicorescue.com the order in which a rescuer uses the items is as follows:

  1. Lay the tarp on the ground
  2. Open the sleeping bag
  3. Lay the reflective blanket on top of the open sleeping bag
  4. Place the hypothermic person on the reflective blanket
  5. Place heating packs in the person's armpits, on their chest, etc.,
  6. Wrap the reflective blanket around the person
  7. Close the sleeping bag making sure the mummy bag fits close around the person's head and neck
  8. Wrap the tarp around the sleeping bag to seal out wind and moisture.

Dow et al.5 suggest that clothing be left intact if it is dry or damp, or removed before wrapping the hypothermic person if this can be done within 30 minutes in a sheltered environment (transport or warm shelter). Otherwise, the person should be wrapped in their wet clothing rather than risk further cooling resulting from clothing removal.


Assembling a Cold-Weather Safety Kit

If you are frequently outdoors in the winter, especially around water and ice, consider assembling and bringing with you a cold-safety readiness kit. With the exception of a sled, the kit components can be packed into a small watertight container or sealed bag. Some suggested items to include are:

  • Hand ice picks
  • Ice cleats for boots
  • A 20-30 foot length of strong, floating rope with a body-sized loop pre-tied in one end and knots in the other for gripping
  • Waterproof matches and fire-starters for building a warming fire if evacuation is slow
  • A three-season mummy-style sleeping bag
  • A foil-type thermal blanket
  • Small tarp for shelter from wind and precipitation
  • Gel chemical heating packs or dry chemical heating packs (five to 10). You want the kind made for rescue that heat up rapidly and not the kind for putting in boots and gloves.
  • Cloth bags to place between heating pack and bare skin
  • Rescue/trauma shears for speedy removal of wet clothing
  • Sugar-rich foods or beverages
  • A toboggan, a long sled, or other means of transporting a person in a horizontal position

Frostnip, Frostbite, Chilblain

Frostbite is often a problem when people are at risk of hypothermia, although treating hypothermia needs to be the priority.29 Frostbite is the actual freezing of skin and its underlying tissue. The degree of frostbite is determined by how deeply the freezing goes, whether there are blisters and what kind they are, and what kind of tissues are involved. Fourth degree frostbite (the worst) can cause death of muscle and bone. As tissues get colder, the circulation of blood decreases which speeds freezing and tissues may become very painful sometimes producing a "pins-and-needles" feeling. Damage is caused by ice crystals that form in skin and tissue cells or between cells. When skin and tissue freezes, thaws and then refreeze again, injury can be amplified.

The best way to avoid frostbite damage is to prevent it. Dressing appropriately for the weather is very important. It is important to recognize cold-induced numb sensations in hands and feet as a sign that frostbite is not far behind. This numbness occurs because nerves are approaching the freezing point and are no longer transmitting pain sensations to your brain.

If you are wearing proper cold weather gear, you can help yourself avoid frostbite by keeping your blood circulation as high as possible. Frostbite prevention includes:

  1. Keeping hydrated, preferably with water, as this helps blood circulation
  2. Maintaining an adequate core body temperature
  3. Avoiding the use of drugs and alcohol that impede circulation
  4. Regular exercising or movement
  5. Covering all of your skin including your head
  6. Avoiding constrictive clothing and footwear
  7. Keeping well nourished.

In temperatures near and below zero degrees, frostbite becomes difficult to avoid. At these temperatures, skin should be protected from moisture, wind, and cold, insulation should be increased, and hand and foot warmers may help. If your skin becomes numb or pale or you experience a loss of dexterity (frostnip), pay attention to these signs and rewarm yourself immediately.

Treatment of frostbite has been regularly reviewed by an expert panel convened by the Wilderness Medical Society 30,31 and their findings were largely based on the State of Alaska Cold Injuries Guidelines.29 Their recommendations are summarized below.

If frostbite occurs, frozen tissue should be protected from further damage by removing jewelry, watches, and other items that could reduce blood flow. Do not rub the affected body parts in ice and snow. It is important to safely bring tissue back from frostbite. However, it may be better to leave frozen tissue frozen until thawing can be done without the risk of exacerbated damage from refreezing. If a person's feet are frostbitten, they should not walk on them. Most frostbitten tissue will thaw by itself if returned to a warm environment and sheltered from weather. Rewarming hands and feet should be done with a gentle water bath if possible and provided the person is not hypothermic. Water should be about body temperature (98-102 F). If you do not have a thermometer, you or an uninjured person should place their hands in the water for about 30 seconds to ensure the water does not feel too hot or too cold. Moving the water around will help keep a more consistent temperature at the skin surface. Also, frozen body parts should be protected from touching the sides of the vessel holding the water so tender skin is not harmed. Rewarming should occur in 30 minutes or so but may take longer or shorter depending on the severity of the frostbite. Rewarming a frozen body part can be very painful so the Wilderness Medical Society panel recommends low doses of ibuprofen or other anti-inflammatory medications.31

Once the affected body part is thawed or warmed in water the skin should be air-dried or gently patted dry to avoid skin damage. If there are blisters or apparent lesions they can be covered with dry, bulky dressings during transport to medical care. Blisters imply deeper damage than first-degree frostbite and may require more complex care. Aloe vera ointment applied gently before dressings can be useful in speeding the healing of frostbite because it may decrease inflammation in surface tissues.30,31 A person with frostbite should keep the frostbitten body part elevated above their heart to avoid swelling through edema. In severe frostbite, the extent of full tissue death may not be known for one to three months. Usually, frostbitten tissue will continue to be highly sensitive to cold far into the future.2 Clearly, avoiding freezing parts of your body is a good idea!

Two other forms of cold injury that are common in outdoor sports are chilblain and trench foot. Both are caused by prolonged exposure to cold that is just above freezing (32 F).2 Chilblain or pernio results in reddish purple, painful and itchy raised pimple-like swelling or rash, flat discolorations, raised places, or lumps beneath the skin of the face, backs of hands and feet, and lower legs. As with frostbite, you should not rub the skin, but keep the affected part elevated above the heart to avoid swelling, and gradually rewarm. Trench foot or cold immersion foot comes from having wet and cold feet for long periods of time. It is common among sailors, anglers, and hunters. Trench foot causes numbness followed by the accumulation of excess blood in the vessels that leads to painful burning, prickling sensations, or aching. Feet can swell, turn red, bruise, and blister. Treatment should occur as soon as possible and should include drying, air warming, elevation, and attention for potential infection. Feet should not be soaked, massaged or warmed too quickly.2 Chilblain and trench foot can result in a person losing their resistance to cold.

Bibliography

  1. CDC.    (ed National Center for Health Statistics Centers for Disease Control and Prevention) (Centers for Disease Control and Prevention, 2020).
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Charts and graphs

Table 1. Cooling effects of immersion in cold water.

Water Temperature °C / °F Loss of Dexterity without protective clothing Exhausted or Unconscious (drowning without flotation) Maximum swimming time for self-rescue Likely Survival Time (with flotation) Maximum Survival Time (with flotation and protective gear)
0.3 / 32.5 <2 min <15 min. Nil <15-45 min 1.4 hrs
0.3 to 4.5 / 32.5 - 40 <3 min 15-30 min 0-7 min 2-3 hrs 1.5-5 hrs
4.5 to 10 / 40 - 50 <5 min 30-60 min 7-40 min 2-3 hrs 5-12 hrs
10 to 15.5 / 50 - 60 10-15 min 1-2 h 40-75 min 3-5 hrs 12-22 hrs
15.5 to 21 / 60 - 70 30-40 min 2-7 h 75-105 min 4-17 hrs 22-36 hrs
21 to 26.5 / 70 - 80 2 h 7-12 h 105-140 min >17 hrs 36-54 hrs
>26.5 / >80 2-12 h >12 h >140 min >17 hrs >54 hrs

after 17,32-34 and http://www.shipwrite.bc.ca/Chilling_truth.htm, https://www.ussartf.org/cold_water_survival.htm. Figures in the “Likely Survival Time (with flotation)” are modified from common tables from the equations published by Hayward et al. 32
Maximum swimming times are based on the author’s linear regression of data from Ducharme and Lounsbury 17

Table 2. Stages of Hypothermia

*Body core temperature cannot be measured with a standard oral thermometer. Normal body temperature is 37°C (98.6°F) orally, 38°C (100.4°F) rectally, but 38.5°C (101.5°F) inside the body2.

Impaired physical ability indicators: Difficulty walking, inability to zip zippers, difficulty removing own clothing, imbalance, slow reaction time, stumbling, excessive urination, fetal position or squatting, twitching

Altered mental state indicators: Apathy, inability to form sentences, irritability, confusion, garbled or unclear speech, poor decision making, lethargy, sleepiness, slurring, paradoxical undressing when cold, impaired memory, inattention, disorientation, seizure, slow reasoning, excessive hunger, derangement

Signs of life indicators: Rapid or slowing breathing normal adult breathing is 12-20 breaths per minute;, rapid or slowing heartbeat (normal adults have 60-100 beats per minute), pale skin

Sources: 5,6,9,36


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