Like birds and other mammals, humans are endotherms; our core body temperature is independent of our environment and, in order for our organ systems to function properly, it must be maintained within a rather narrow range. Heat generation is through shivering, other muscular activity and our internal metabolism. Ninety percent of our heat loss is through the skin (sweating, radiation, conduction, convection)while 10% is via the lungs; heat loss via conduction increases dramatically with water immersion and convectional heat loss is augmented by wind. If supplied with adequate food, water and shelter, a naked human adult can maintain a normal core temperature in environments ranging from 55 to 130 degrees F; beyond this range, our compensatory mechanisms are inadequate and we become hypothermic or hyperthermic (both potentially fatal conditions).
As the present wave of Arctic air spreads across much of our country, some humans, tropical creatures that we are, will die of hypothermia; risk factors include advanced age, exposure to the environment, general debilitation, malnutrition, injury, substance abuse, poverty, chronic medical illness and certain medications. As mild hypothermia develops, peripheral vasoconstriction shunts blood to the vital organs, kidney perfusion increases, urine formation increases (known as cold diuresis), blood volume falls and our heart rate increases; shivering develops and we increase our metabolic rate to generate heat. As hypothermia progresses, the shivering reflex is lost, our heart rate slows, blood pressure falls and blood flow to our brain decreases, leading to confusion and paradoxical undressing. Severe hypothermia results in coma, depressed ventilation and, eventually, death (usually from ventricular arrhythmias or asystole).
Prevention of hypothermia includes the avoidance of unnecessary exposure, intake of high-caloric food, adequate hydration and the use of warm, layered clothing (including efforts to minimize heat loss from the head and extremities); layers should be adjusted to prevent sweating, which accelerates heat loss. Use of alcohol, which promotes peripheral dilation and augments heat loss, must be avoided. Persons found to be hypothermic should be warmed with multiple blankets (after removal of wet clothing) and should not be moved any more than is absolutely necessary (movement may trigger lethal arrhythmias in a hypothermic heart). Emergency medical assistance should be sought as soon as possible and CPR should be initiated if vital signs are lost; it is important to keep in mind that some individuals with extreme hypothermia have been successfully resuscitated, often with minimal neurologic sequelae (the record low core temperature for a surviving adult is 60.8 F or 16 C).
