Hypothermia: cold weather killer

  • Published
  • By Lt. Col. George J. Bifolchi
  • Directorate of Aerospace Safety
Note:  Long before the Air Force Safety Center stood up at Kirtland Air Force Base, N.M., on Jan. 1, 1996, Airmen were sharing their lessons learned in a variety of safety publications such as Aerospace Safety, Aerospace Maintenance Safety, Air Force Driver, among others.  During a year-long commemoration of the safety center's 20th anniversary, the Public Affairs Office will highlight previously-published articles and reprise historic Rex Riley cartoons to emphasize that long-standing safety practices and lessons learned remain relevant to the mishap prevention program of today's Air Force.

Weather conditions: Ceiling obscured, visibility one-half mile in ice fog, wind calm, temperature minus 50 degrees Fahrenheit. The cargo compartment of the tanker was unheated because the auxiliary power unit wasn't operating. For two hours the heavily clad pilots, navigator and boom operator struggled to keep warm in cockpit temperatures nearly as cold as outside. As the last of four spare aircraft providing air refueling support for an airborne reconnaissance mission, the crew did not expect to launch. Then the unexpected occurred . . . the crew completed their final checks, advanced power and released brakes. The heavy aircraft lumbered down the runway and slowly rotated into the black arctic night. A few minutes later the crew reported having a problem raising the gear. Thirty seconds passed . . . radio and radar contact were lost . . . and a huge fireball lit up the sky.

Most of us relate cold injuries with "exposure" to the elements; however, we usually expect sufficient warning to eliminate the problem before becoming incapacitated. Yet even knowledge of cold weather hazards with adequate warning is we fail to apply good judgment in a timely manner.

A significant factor in this accident was the overwhelming distraction caused by chilled extremities. Also suspected was a subtle but pernicious hypothermia resulting from a lowering of the body's inner core temperature through a loss of heat. In extreme cases the loss of
heat can result in uncontrollable shivering, increasing clumsiness and loss of judgment followed rapidly by unconsciousness and death.

Hypothermia has a well documented history. Although relatively rare as a threat to the flier, it constitutes a high risk for a traveler in mountainous terrain or a cold weather crash survivor. During World War II, it was a routine threat to wa gunners aboard unpressurized bombers flying at altitudes above 25,000 feet. Hypothermia has meant death to scores of mountain climbers suddenly beset by unplanned conditions ... it's known as a killer of the unprepared.

The body maintains thermal equilibrium by regulating the production and loss of heat. Body heat is produced through eating and muscle activity while external sources heat, such as the sun, a campfire or warm liquids, also contribute. The most immediate benefits of increased heat are realized through warm liquids or sweet foods that are quickly transformed into heat energy. Heavy physical heat production up to I times the exertion can increase body basal metabolic rate, while heat production drops to 80 percent of the basal rate when sleeping. Internal shivering produces heat equivalent to running at a slow pace (six times the basal rate). Body hormones can also produce heat when adrenalin increased or when body illnesses produce fever.

Heat loss occurs through the mechanics of cooling, respiration, radiation, evaporation, conduction and convection. Not much can be done to decrease heat loss through respiration - inhalation of cool air and exhalation of warm air. Radiation, on the other hand, is a leading cause of heat loss through an uncovered or unprotected head. At 5 degrees Fahrenheit, radiation can account for the loss of up to 75 percent of the total body heat produced. Evaporation losses occur through sweating; however, this process should be assisted by wearing loose fitting fabrics that "breathe" but still retain body heat. Conduction occurs when the skin transfers heat through contact with metal or stone surfaces. Convection heat losses occur when the warm air layers next to the body are removed by a brisk wind.

Two elements accelerate the loss of body heat: wind and water. Wind chill is a product of temperature and wind velocity. The chill factor at 40 degrees F with a wind blowing at 25 miles an hour is 15°, generally considered "very cold." At 0°F the same wind will produce a chill factor equal to -45 degrees. Water conducts heat 40 times faster than air. When clothing gets wet it no longer insulates by trapping warm air next to the body, but instead, rapidly dissipates the heat into the atmosphere. Experiments have shown that wet clothing retains only 10 percent of the heat retained by dry clothing. Moreover, a cold wind blowing against wet clothing can cause "waterchill" which will dissipate heat much quicker than the body can produce it.

Maintaining the body's thermal equilibrium seems simply a matter of balancing "calories lost" with "calories gained"; however, body heat loss through cooling is often compounded by heat loss through physical exertion. The thermal balance in cold wet conditions is maintained by a combination of shivering and increased work rate. In severe cold stress, the metabolic demand may be so great that only an individual in top condition can meet it over a sustained period.

The body's initial response to cold is constriction of the blood vessels of the skin and tissue beneath. This action decreases the amount of heat transported to the skin with a resulting decrease in the temperature of the skin. The skin and surface tissues then act as insulation for the body core which maintains a constant temperature of 99 degrees Fahrenheit.

As skin temperature drops, sense of touch and pain decrease, the muscles and their motor nerves are weakened. Shivering produces heat, but it also consumes energy and, if it is intense and prolonged, can result in exhaustion. Continued heat loss produces violent and uncontrollable shivering, difficulty in speaking, sluggish thinking and amnesia. Advanced heat loss results in muscular rigidity, erratic heartbeat and labored breathing, unconsciousness and, finally, death. Simple maintenance of heat equilibrium can become extremely difficult in a survival situation where a lack of resources, physical injury, or poor planning have rapid and disastrous consequences.

Field treatment for hypothermia involves two aspects: Preventing further body heat loss and increasing the existing level of heat. Several actions are essential:

· Obtain shelter from wind and rain.
· Remove wet clothing and replace with dry clothing.
· Insulate the victim from cold or dampness.
· Add heat by any method available.

Shivering is a good sign that the victim is able to provide self-warmth. When shivering stops, the individual is no longer able to warm himself and must be assisted by others.

A cold sleeping bag, regardless of rating, will not provide sufficient warmth to treat hypothermia. The sleeping bag should be prewarmed by another individual who has stripped down to his under garments in order to transfer maximum heat from his body to the bag. Conscious victims of hypothermia should be given warm fluids or sweetened foods which are most quickly converted to heat.

To prevent hypothermia you must plan for the unexpected, be alert to the causes and know how to treat it effectively. Your choice of survival clothes may well be limited to those you wear in flight. Will they keep you warm and protect you from the rain? Do you carry food in your flight suit for quick energy and heat? Injuries will affect your efforts to keep warm; therefore, avoid situations which lead to uncontrolled heat loss. Minimize the effects of wind and rain. Conserve your energy; exhaustion can produce a loss of heat as great as that caused by wet clothing. Be familiar with the symptoms of hypothermia and probable sources of heat loss . . . and remember hypothermia can subtly become a cold weather killer. (Reprinted from Aerospace Safety, February 1980)