Cynthia A. Lien, MD
Associate Professor of Anesthesiology
New York Presbyterian Hospital-
Weill Cornell Medical College
Calien@med.cornell.edu

Normal autonomic responses to decreases in core body temperature include vasoconstriction and shivering. Of the two defense mechanisms, shunt vasoconstriction is the more efficient. Restriction of blood flow, especially to the fingers, toes, and nose, reduces heat loss to the environment. Shivering occurs at core body temperatures that are about 1° C lower than those required for activation of vasoconstriction. Although shivering can double metabolic rate, most of it occurs in the extremities. As heat is generated in the extremities, peripheral blood flow will increase due to local heating as well as to meet the increased muscle metabolic demand. The end result is that much of the heat produced by shivering is lost to the environment.

Elderly patients are unable to regulate their body temperature to the same degree as young adults because their responses to changes in body temperature are altered. They do not respond normally to hypothermic challenges. In general, geriatric patients neither vasoconstrict nor shiver in response to cold until their temperature has fallen to levels below that required for activation of these defense mechanisms in young adults. The relationship between impairment of thermoregulation and age is not linear and it does not occur in all aged patients. Rather, it is most common in patients over the age of 80. While younger patients will shiver at a temperature of 36.1°C, most patients over the age of 80 will not shiver until their core body temperature falls to 35.2°C, on average. Furthermore, the ability to vasoconstrict and reduce skin blood flow is reduced with age, making obligatory heat loss in a cold environment greater than in young adults. These alterations in the elderly patient's ability to regulate body temperature result in more frequent and severe hypothermia in this patient group.

Anesthetics alter thermoregulatory responses in all patients. In clinically useful doses, sedatives and general anesthetics impair thermoregulatory responses by preventing vasoconstriction or shivering until more extreme decreases in body temperature are achieved in comparison to the absence of the anesthetic agents. Consequently, the immobile, vasodilated patient in a cold operating room will tend to become relatively hypothermic unless active measures are taken to avoid heat loss. Geriatric patients are even more prone to intraoperative hypothermia, not only for the reasons cited in the previous paragraph, but also because the inhibition of thermoregulatory responses by anesthetics is greatly exaggerated in elderly patients. Body temperature must decrease to a lower level in the elderly before vasoconstriction or shivering is triggered. Because clearance of anesthetic agents tends to be reduced in the elderly, their effects in this patient population are prolonged. This renders the geriatric patient more susceptible to postoperative hypothermia as well.

Hypothermia, in addition to being more pronounced, lasts longer in geriatric patients than it does in young patients. Recovery from mild hypothermia is accompanied by shivering in elderly patients. The shivering that does occur, though, is milder than it is in young patients. In elderly patients who shiver, mean total body oxygen consumption only increases approximately 38% over nonshivering levels. Whether or not patients are shivering, the observed increase in their oxygen consumption is proportional to their degree of hypothermia. Recovery from even mild hypothermia is prolonged in the elderly because with their lower metabolic rate they will produce heat more slowly.

Elderly patients are not immune to the adverse effects of hypothermia, which include bleeding, decreased immune functions, and decreased wound strength. The increased bleeding is due to decreased platelet function and inhibition of the enzymes of the coagulation system. Decreases in temperature by as little as 2°C will increase intraoperative blood loss and increase the need for transfusion. The vasoconstriction that accompanies hypothermia causes relative tissue hypoxia as less oxygen rich blood is brought to the vasoconstricted areas and the hypoxia results in decreased wound strength.

Hypothermia may exacerbate the decreased clearance of drugs in the elderly. This diminished clearance, accompanied by a decreased MAC in the elderly, means that anesthetic effects may be both pronounced and prolonged.

Elderly patients are more prone to have coronary disease than are younger adults. Hypothermia causes an increased incidence of myocardial ischemia in geriatric patients that is not related to shivering. Instead, ischemia is likely due to hypertension and increased plasma concentrations of norepinephrine. Consequently, it is not too surprising to note that hypothermia is associated with an increased risk of perioperative myocardial infarction.

The last major complication of hypothermia is an increased risk of infection. In a randomized study of colorectal surgery, patients assigned to routine care were almost 2°C colder at the end of surgery than patients who received aggressive intraoperative warming. Despite achieving normothermia in both groups by 6 hours postoperatively, the subjects receiving routine care suffered three times as many wound infections (19% vs, 6%) and remained hospitalized an average of two days longer than the patients who were more aggressively warmed.

As temperature regulation is altered in elderly patients, extra care must be taken to maintain their body temperature. This can be done by several consecutive measures, which include: warming the operating room until the patient is covered with drapes and warming blankets, prepping preoperatively and cleaning postoperatively with warmed solutions, not infusing cold intravenous fluids, and covering the patient with warm blankets at the end of a surgical procedure for transport to the post anesthesia care unit. Maintenance of temperature is extremely important as the elderly are susceptible to all of the adverse effects of hypothermia, which may be more prolonged in this patient population.

References:

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Frank SM, Beattie C, Christopherson R, et al. Unintentional hypothermia is associated with postoperative myocardial ischemia. Anesthesiology 1993;78:468-476.

Frank SM, Fleisher LA, Breslow MJ, et al. Perioperative maintenance of normothermia reduces the incidence of morbid cardiac events. JAMA 1997;277:1127-1134

Kurz A, Sessler DI, Lenhardt R. Perioperative normothermia to reduce the incidence of surgical-wound infection and shorten hospitalization. New Engl J Med 1996;334:1209-1215.

MacMillan AL, Corbett JL, Johnson RH, et al. Temperature regulation in survivors of accidental hypothermia of the elderly. Lancet 1967;2:165-169.

Vassilieff N, Rosencher N, Sessler DI, et al. The shivering threshold during spinal anesthesia is reduced in the elderly. Anesthesiology 1995;83:1162-1166.

Vaughan MS, Vaughan RW, Cork RC. Postoperative hypothermia in adults: relationship of age, anesthesia, and shivering to rewarming. Anesth Analg 1981;60:746-751.