Wilderness and Rescue Medicine 7th Edition Jeffrey Isaac, PA-C and David E. Johnson, MD

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Section V: Environmental Medicine

patient who is immobilized by injury will not be able to exercise or consciously act to retain heat. Drugs that cause vasodilatation of the skin result in greater heat loss. Chronic endocrine system problems like hypothyroidism or diabetes can impair the body’s ability to sense or respond to temperature changes. Elderly people have less muscle mass and a reduced ability to perceive and respond to heat loss. Children tend to have less body fat and a greater surface area to mass ratio, which also increases the rate of heat loss.

Hypothermia Cold response is a normal reaction to feeling cold and starts long before the body core tem- perature begins to fall with the onset of hypother- mia. Shell/core compensation reduces heat loss to the environment, while shivering increases heat production frommuscle activity. The discomfort you feel by being cold motivates your conscious effort to add layers of clothing and get out of the weather. If the system works normally and is not overwhelmed by an extreme challenge, normal core temperature and mental status is preserved. Nobody can mount an effective cold response when short on food and fluids. Shivering is a very efficient form of heat production but requires a tremendous amount of energy. Living outside in a cold environment can require more than 6,000 calories a day. Adequate glycogen stores and easily digested food must be available to maintain the effort. Normal body fluid volume is also required to generate and distribute heat effectively.

General Principles

Impaired Compensation

• Immobilized by injury, unable to generate heat • Multiple trauma, volume shock • Illness that impairs circulation, metabolism, sweat production, or temperature sense • Drugs that inhibit temperature regulation: cocaine, methamphetamine, diuretics, behavioral medications, pseudoephedrine. • Extremes of age • Sunburn

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General Principles

Reversing cold response requires insulation, protection, calories, and fluids. To reverse a cold response most effectively, you need to under- stand the physics of heat production, retention, and dissipation. Heat energy flows from warmer objects (like your patient) to colder objects (like the ground or litter). The mechanisms are conduc- tion, convection, evaporation, and radiation. In protecting and packaging your patient, you must consider the combined effect of all these forms of heat loss. Conduction is heat transfer between objects in contact. The denser the object, the faster heat energy is transferred. Your patient will lose heat more quickly to the cold hard ground he is lying on than to the low-density foam pad that you should have placed under him. Convection is heat transfer via moving fluids, including air and water. Although air is the least dense substance, there is an infinite supply of it. Heat lost to wind or even to the air billowing in and out of loose clothing can be considerable. Water works the same way, just 25 times faster.

Normal Body Compensation Heat Response: • Vasodilation • Sweat Cold Response: • Shivering • Vasoconstriction • Cold diuresis “You can watch these mechanisms work, but they are not under your direct control.”

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An anticipated problem associated with the cold response is cold diuresis. This is the tendency of the body to produce more urine when shell/ core compensation occurs. As blood is shunted from the shell into the core, the kidneys sense an increase in fluid volume in the central circulation and act to get rid of some of it. Cold diuresis and the logistics involved in obtaining fresh water in an extreme environment can lead to dehydration. Although cold response is normal and healthy, it carries the anticipated problem of hypother- mia. Several factors can accelerate the process. A