Temperature Regulation

Temperature regulation is a complicated process that is altered by aging. Temperature is controlled primarily by the hypothalamus in the brain, but the body relies on a complex mechanism of neurological, circulatory, metabolic, and cardiac pathways to respond appropriately to heat and cold. Body temperature is maintained by balanced abilities to generate/maintain heat and to lose heat. These abilities are influenced by the body's basal metabolic rate, muscle activity, and effects of stress and thyroid hormones. In addition, many older adults have a diminished ability to recognize changes in the ambient temperature that necessitate changes needed to maintain body heat (e.g., changing location, changing amount/type of clothing). Cognitive impairment can further reduce the likelihood that older adults can initiate appropriate compensatory behaviors in hot or cold environments.

Hypothermia is the presence of a core temperature less than 35 degrees Celsius (95 degrees Fahrenheit). In older adults, it can be due to exposure to air just 15 degrees cooler than body temperature and can be complicated by physiological changes with aging or [Page 560]disease. Shivering, which serves to generate heat, can be less effective in older adults. In addition to a reduced basal metabolic rate, older adults might not be able to mediate thermogenesis via the β-adrenergic system. Disease processes can cause hypothermia (e.g., profound hypothyroidism, sepsis), but much more common risk factors are living alone, dementia, and use of alcohol and certain medications such as benzodiazepines, barbiturates, and phenothiazines. Social circumstances, including lack of adequate heating, are frequent contributors to hypothermia. Outdoor exposure to extreme cold is not necessary for hypothermia to develop in older persons; older persons trying to conserve on use of heating may become hypothermic in their own homes.

Clinically, hypothermic symptoms can be nonspecific and include weakness, confusion, fatigue, and cool skin. As core temperature decreases, consciousness is frequently lost and cyanosis, bradycardia, and hypotension can occur. The most significant complications are cardiac arrhythmias. Ventricular fibrillation is a common cause of death in hypothermic patients, and it can be refractory to treatment until the core temperature has been raised above 28 degrees Celcius. The most specific electrocardiogram (EKG) finding is a J wave (Osborne wave) following the QRS complex. Treatment includes rewarming and intensive care monitoring for multiorgan system dysfunction. Rewarming techniques include passive techniques for those with mild hypothermia (> 32 degrees Celcius) and core rewarming for those with more severe hypothermia. Passive maneuvers include removal from the cold environment and insulation with blankets and dry clothing. Active treatments include warmed intravenous fluids and peritoneal dialysis with warmed dialysate. More rapid warming is required in cases of more severe hypothermia and cardiac irritability.

Hyperthermia is the presence of a core temperature higher than 40.6 degrees Celcius. Older adults generally present with nonexertional heat stroke due to impaired homeostatic mechanisms. Older adults may have a decreased ability to sense an environment as too warm due to impaired function of thermoregulatory centers. With age, decreased or absent sweating may occur due in part to atrophy of skin and associated sweat glands, and a higher threshold is required for sweating to be initiated. Age-related increases in peripheral vascular resistance may impede cutaneous vasodilatation as a compensatory cooling mechanism. Physiological or medication-induced inability to increase cardiac output may also impair the ability to dissipate heat through the skin. Medications associated with reduced sweating include anticholinergics and phenothiazines. Persons with cognitive impairment or mental health problems might also not recognize the need to take steps to compensate for hot environments.

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