Mountain Rescue Association

Hypothermia...The Most Common Killer of Backcountry Users 
On February 1, 1989, the temperature in Butte, Montana dropped from 42 degrees to -4 in one hour. Regardless of the season, a temperature drop of 30-40 degrees Fahrenheit in one hour is not uncommon in the mountains.  Add cold rain and wind and nature has mixed the perfect recipe for hypothermia. 

What is Hypothermia? 
Hypothermia is the rapid, progressive mental and physical collapse accompanying the chilling of the inner core of the body.  It is caused by exposure to cold, aggravated by wet, wind and exhaustion. Hypothermia has killed more unprepared backcountry users than any other malady.  In fact, the state with the most reported cases of hypothermia is, believe it or not, Florida!  The reason is simple. Floridians are generally unprepared for cold weather. 

There are a number of ways to avoid hypothermia.  The trick to staying warm is to gain more calories than you lose.  The body can burn as little as 50 calories per hour while sleeping or more than 1,000 an hour during heavy work.  Just as the body constantly produces heat, it constantly loses it.   

Your body may burn over 50% more fuel in winter than it would in summer.  This is because you are inhaling cold air, warming it and saturating it with water vapor.  In fact, as much as one-third of your body-heat loss can occur through breathing.  Breathing through a scarf or balaclava may help by "pre-heating" the inspired air.  The body also loses heat by perspiration and its subsequent evaporation from the skin.  In addition, 75% of the body heat can be radiated from an unprotected head, since the blood vessels in the scalp lie close to the 
skin. 

Creating Body Heat Clothing and shelter can only conserve body-heat, they can't create it.  Liquids and food are the only "internal" source of heat creation for the body.  This is because heat is produced in the body by chemical reactions through the metabolism of food, mainly oxidation of carbohydrates.  Muscular activity is a second source of heat, but uses food energy to generate the heat. 

The Body's Reaction to Cold 
The body's first reaction to cold is to shiver. Shivering is the first sign of hypothermia and is the body's way of forcing an isometric contraction and triggering a stored glycogen "dump" from the liver.  It is the body's attempt to generate heat by rapidly and rhythmically contracting muscles. Despite the fact that shivering is fatiguing, it generally helps keep us warm.  It diminishes with oxygen deficiency, breathing of carbon  monoxide or the taking of aspirin or alcohol.  The body's ability to maintain warmth is depressed by the lack of water, lack of food, fatigue and shock. After shivering stops, hypothermic victims are confused into thinking they are feeling warmer. THEY ARE NOT.  They are dying. 

Problems Which Increase Hypothermia Dangers 
Constipation retards efficient metabolism of food and reduces energy levels.  This is a dangerous situation in the winter, as the body can no longer take advantage of the energy provided by the "fuel" ingested. Despite what grandmother told you on those cold Wisconsin evenings, alcohol reduces the body's ability to fight cold.  It dilates peripheral blood vessels, blocking vasoconstriction and allowing warm blood to exit the body's core.  In addition, the alcohol may actually make the victim feel warm and more competent. The low temperatures will increase the intoxication because brain cell membranes are more fluid as a result of the increased metabolism.  Smoking or chewing tobacco constricts peripheral vessels, reducing circulation necessary to keep the skin warm.  Aspirin also dilates the vessels.  Such conditions are conducive to frostbite and hypothermia. In addition, sedatives, antidepressants and neurological problems common in the 
elderly will all increase the risk of hypothermia. 

Hypothermia is a killer in summer as well as winter.  It is more often triggered by a combination of wind, wet and cold than by cold alone. In fact, just plain dry cold, even at extremes of -30 degrees, is far more manageable and far more pleasant than 20-degree weather with wet snow and rain falling and a harsh wind blowing.  I'll take the 30 below any day. 

The Hypothermia Lab in Duluth Minnesota has studied this phenomenon for over a decade.  The lab discovered that the human body can adjust its metabolism to adapt to the cold.  Studies showed that Eskimos respond to cooling with an almost instant metabolic leap and with skin temperatures that remain remarkably high.  The "Ama", Korean pearl divers who once dived naked into icy waters in search of treasure, had high basal metabolic rates, more efficient tissue insulation and a higher threshold of tolerance before the onset of shivering.  One generation after they had started using wet-suits, they had completely lost their specialized responses to the cold. 

Hypothermia and the Mind 
The Hypothermia Lab also found that circulation can be increased by mind-power.  Subjects of experiments who thought about how much they wanted to get out of the cold suffered rapidly falling body temperatures.  On the other hand, shivering subjects, directed to perform a mental arithmetic task, stopped shivering for short periods.  In addition, when people get anxious, they have more problems with temperature 
regulation.   

One of hypothermia's strangest manifestations is "paradoxical undressing." People suffering severe hypothermia are often observed throwing off their clothes, as if they felt they were burning up.  This is believed to be because the hypothermic victim's body, which has been vasoconstricted to maintain core heat, may abruptly vasodilate, allowing warm blood to pump briefly through the body's peripheral areas. To the hypothermic victim, who is already mentally foggy, the vasodilation may produce a sense of extreme warmth. In addition, chemical changes occur in the body that can make the situation more dangerous.  First, epinephrine (adrenaline) is released into the bloodstream, which increases the heart rate.  This is healthy, since it increases the metabolism.  Other chemical changes, however, can cause hypothermic victims to experience vivid hallucinations very similar to those reported by schizophrenics.  This is believed to be caused by increased dopamine in the blood.  In addition, researchers have found that spinal and cerebral neurons become hypersensitive when they are cooled just three or four degrees below normal.  This can lead to neural misfiring and to seeing things that just aren't there. 

Believe the signs, not the victim.  
Team members should monitor each other carefully, even in temperatures of 50 degrees.  Any early sign of hypothermia is a serious warning. Take immediate action to correct the situation before it is too late. Most cases of hypothermia develop in temperatures between 30 and 50 degrees Fahrenheit.  Many novice backcountry users simply don't believe such temperatures are dangerous.  They fatally underestimate the dangers of being wet and/or poorly clothed at such temperatures. 

Environmental Conditions Contributing to Hypothermia 
When the body is wet, the evaporation of moisture from the skin has a very rapid cooling effect that can be extremely dangerous.  Water conducts heat 25 times faster than air.  Therefore, heat is lost much more quickly if evaporation is occurring.  A wet backcountry user must always change quickly into extra dry clothing as soon as possible.  Staying wet is an open invitation to the dangers of hypothermia. It is equally important to protect yourself from your own sweat.  Working up a sweat on the trail will result in wet clothes by the time a final destination or resting place is reached.  Wet clothes will chill the body significantly, especially in conditions of high wind where evaporation takes place much more quickly. 

The Body's Reaction to Hypothermia 
Mother often said and (for once) she was right:  "If you want to keep your feet warm, wear a hat."  Up to 75% of heat loss is through your head and neck, since the blood vessels are close to the surface.  If the head, or any other body part, is exposed to cold, the body chills and "shunting" can result.  When this happens, circulation to the extremities is sacrificed to assure that the remaining body heat is reserved for vital internal organs.  The result is that the hands and feet receive less warm blood. Shunting occurs as a result of vasoconstriction. Vasoconstriction cranks up your blood pressure as you chill.  As a result, cold can be dangerous for people with heart disease. 

The key to avoiding this dangerous situation is to be brave (and smart) enough to give up reaching the peak when the first signs of hypothermia present themselves. 

Have fun and stay safe out there!

For more information on backcountry safety, check out the Mountain Rescue Associations public education programs @ http://mra.org/training/public-education

Courage - Commitment - Compassion
     Mountain Rescue Association