Thermoregulation

To be vigilant or aware of older adults at risk, it is important to understand the basis of thermal vulnerability. Neurosensory changes in thermoregulation delay or diminish the older person’s awareness of temperature changes and may impair behavioral and thermoregulatory response to dangerously high or low environmental temperatures (Chapter 4). Decline in thermoregulatory responsiveness to temperature extremes as one ages is well documented, yet these changes vary widely among individuals and are related more to general health than to age. The delicate equilibrium required to maintain thermal balance at any age involves the generation or replacement of body heat at about the same rate as heat is lost to the environment.

A number of physiological changes associated with aging affect heat generation, distribution, and conservation. The aging of the skin, with the loss of subcutaneous fat, is a factor in temperature regulation. Heat conservation is especially affected by changes in body density, water content, and insulation that accompany aging. Circulatory impairment and changes in vascular responsiveness affect the distribution of heat carried by blood. As a result, thermoregulatory sensitivity declines with age, and both cooling and warming responses appear to be blunted. Many of the drugs taken by older people affect thermoregulation by affecting the ability to vasoconstrict or vasodilate, both of which are thermoregulatory mechanisms. Other drugs inhibit neuromuscular activity (a significant source of kinetic heat production), suppress metabolic heat generation, or dull awareness (tranquilizers, pain medications). Alcohol is notorious for inhibiting thermoregulatory function by affecting vasomotor responses in either hot or cold weather.

Economic, behavioral, and environmental factors may combine to create a dangerous thermal environment in which older persons are subjected to temperature extremes from which they cannot escape or that they cannot change. Caretakers and family members should be aware that persons are vulnerable to environmental temperature extremes if they are unable to shiver, sweat, control blood supply to the skin, take in sufficient liquids, move about, add or remove clothing, adjust bedcovers, or adjust the room temperature. Economic conditions often play a role in this vulnerability, such as when an older person cannot afford air conditioning or adequate heating. During winter months, the older person may try using little or no room heat to either reduce or eliminate the high cost of fuel. Fear of unsafe neighborhoods in some urban areas prompts many elders to keep doors and windows bolted throughout the year. Although most of these problems occur in the home setting, older adults with multiple physical problems who reside in institutions may be especially vulnerable to temperature changes.

Hyperthermia

More older people die from excessive heat than from hurricanes, lightening, tornadoes, floods, and earthquakes combined (Centers for Disease Control and Prevention, 2006). When body temperature increases above normal ranges because of environmental or metabolic heat loads, a clinical condition called heat illness, or hyperthermia, develops. Heat illnesses tend to follow a continuum (Table 13-2), beginning with mild heat fatigue and ending with the potentially fatal heat stroke, so it is imperative to assess hyperthermia quickly and appropriately. Heat fatigue is usually caused by exposure to high outside temperatures or overexertion in a hot environment. It is characterized by pale or sweaty skin that is still moist and cool to touch, elevated heart rate, and feelings of exhaustion and weakness. Core body temperature remains normal (Ham et al., 2007). Diuretics and low intake of fluids exacerbate fluid loss and can precipitate the onset of hyperthermia in hot weather.

Heat stroke, the most serious form of heat illness, is a medical emergency that usually arises from failure of normal body-cooling mechanisms to cope with extremely high environmental heat or humidity. Heat stroke affects the hypothalamic thermoregulatory center and impairs the ability to sweat or lose heat by vasodilation. This condition can quickly lead to death unless treated, and rising core temperatures above 40° C (104° F) increase the likelihood of irreversible brain damage. Hyperthermia requires active cooling and fluid replacement. Because hyperthermia induces loss of thermoregulatory control, temperature must be closely monitored to avoid inducing hypothermia.

Box 13-1 presents interventions to prevent hyperthermia when the ambient temperature exceeds 90° F (32.2° C). Local governments and communities must coordinate response strategies to protect the older person when environmental temperatures rise. Strategies may include providing fans, opportunities to spend part of the day in air-conditioned buildings, and identification of high-risk older people.

Hypothermia

Nearly 50% of all deaths from hypothermia occur in older adults (Ham et al., 2007). Hypothermia is produced by exposure to cold environmental temperatures and is defined as a core temperature of less than 35° C (95° F). Hypothermia is categorized into mild, moderate, and severe, depending on the core temperature taken with a rectal probe thermometer. Hypothermia is a medical emergency requiring comprehensive assessment of neurological activity, oxygenation, renal function, and fluid and electrolyte balance.

During cold weather, two situations tend to produce hypothermia: (1) exposure involving a healthy individual in severely cold environmental conditions for a prolonged period; or (2) exposure involving a person with impaired thermoregulatory ability in room temperature without protection. The more severe the impairment or prolonged the exposure, the less able are thermoregulatory responses to defend against heat loss. Older adults are particularly predisposed to hypothermia because the opportunity for heat loss frequently coexists with the decline in heat generation and conservation responses. Such coexistence occurs frequently among persons who are homeless or cognitively impaired; persons who are injured in falls or from trauma; and persons with cardiovascular, adrenal, or thyroid dysfunction, and diabetes. Other risk factors include excessive alcohol use, exhaustion, poor nutrition, inadequate housing, as well as the use of sedatives, anxiolytics, phenothiazines, and tricyclic antidepressants.

Unfortunately, a dulling of awareness accompanies hypothermia, and persons experiencing the condition rarely recognize the problem or seek assistance. For the very old and frail, environmental temperatures below 65° F (18° C) may cause a serious drop in core body temperature to 95° F (35° C). Factors that increase the risk of hypothermia are numerous, as shown in Box 13-2.

Under normal temperature conditions, heat is produced in sufficient quantities by cellular metabolism of food, friction produced by contracting muscles, and the flow of blood. Paralyzed or immobile persons lack the ability to generate significant heat by muscle activity and become cold even in normal room temperatures. Persons who are emaciated and have poor nutrition lack insulation, as well as fuel for metabolic heat-generating processes, so they may be chronically mildly hypothermic. Box 13-3 lists factors that may induce low basal body temperatures in elders.

When exposed to cold temperatures, healthy persons conserve heat by vasoconstriction of superficial vessels, shunting circulation away from the skin where most heat is lost. Heat is generated by shivering and increased muscle activity, and a rise in oxygen consumption occurs to meet aerobic muscle requirements. Circulatory, cardiac, respiratory, or musculoskeletal impairments affect either the response to or function of thermoregulatory mechanisms. Older persons with some degree of thermoregulatory impairment, when exposed to cold temperatures, are at high risk for hypothermia if they undergo surgery, are injured in a fall or accident, or are lost or left unattended in a cool place.

All body systems are affected by hypothermia, although the most deadly consequences involve cardiac arrhythmias and suppression of respiratory function. Correctly conducted rewarming is the key to good management, and the guiding principle is to warm the core before the periphery and raise the core temperature 0.5° C to 2° C per hour. Heating blankets and specially designed heating vests are used in addition to warm humidified air by mask, warm IV boluses, and other measures depending on the severity of the hypothermia (Ham et al., 2007).

Recognition of clinical signs and severity of hypothermia is an important nursing responsibility. Nurses are responsible for keeping frail elders warm for comfort and prevention of problems. It is important to closely monitor body temperature in older people and pay particular attention to lower-than-normal readings compared with the person’s baseline. The potential risk of hypothermia and its associated cardiorespiratory and metabolic exertion makes prevention important and early recognition vital.

Detecting hypothermia among community-dwelling older adults is sometimes difficult, because unlike in the clinical setting, no one is measuring body temperature. For persons exposed to low temperatures in the home or the environment, confusion and disorientation may be the first overt signs. As judgment becomes clouded, a person may remove clothing or fail to seek shelter, and hypothermia can progress to profound levels. For this reason, regular contact with home-dwelling elders during cold weather is crucial. For those with preexisting alterations in thermoregulatory ability, this surveillance should include even mildly cool weather. Because heating costs are high in the United States, the Department of Health and Human Services provides funds to help low-income families pay their heating bills. Specific interventions to prevent hypothermia are shown in Box 13-4. Additional resources can be found on the Evolve website.

BOX 13-4   Nursing Interventions to Prevent Cold Discomfort and the Development of Accidental Hypothermia in Frail Elders

Desired Outcomes

• Hands and limbs warm

• Body relaxed, not curled

• Body temperature >97° F

• No shivering

• No complaints of cold

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