Complications

Coagulation is impaired by mild hypothermia. The most important factor appears to be a cold-induced defect in platelet function.^[102] Interestingly, the defect in platelet function is related to local temperature, not core temperature.^[103] Wound temperature, however, is largely determined by core temperature and will be distinctly higher in normothermic patients. Perhaps as important, hypothermia directly impairs enzymes of the coagulation cascade. This impairment will not be apparent during routine coagulation screening because the tests are performed at 37°C. When these tests are performed at hypothermic temperatures, however, the defect becomes apparent.^{[104] [105]} Consistent with these in vitro defects, prospective, randomized clinical trials indicate that mild hypothermia significantly increases blood loss during hip

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Wound infections are among the most common serious complication of anesthesia and surgery in that they probably cause more morbidity than all other anesthetic complications combined do.^{[109] [110]} Hypothermia can contribute to wound infections both by directly impairing immune function^[111] and by triggering thermoregulatory vasoconstriction, which in turn decreases wound oxygen delivery.^[112] It is well established that fever is protective and that infections are aggravated when naturally occurring fever is prevented.^{[113] [114]} Similarly, mild hypothermia, maintained only during anesthesia, impairs subsequent resistance to both Escherichia coli and Staphylococcus aureus dermal infections in guinea pigs.^{[115] [116]} As might be expected from these in vitro and animal data, a prospective, randomized clinical trial has indicated that mild intraoperative hypothermia triples the incidence of surgical wound infection in patients undergoing colon surgery. Furthermore, hypothermia delayed wound healing and prolonged the duration of hospitalization 20%, even in patients without infection.^[117] Consistent with poor wound healing, urinary nitrogen excretion remains elevated for several postoperative days in patients allowed to become hypothermic during surgery.^[118]

Thermal comfort is markedly impaired by postoperative hypothermia. ^[119] Patients, asked years after surgery, often identify feeling cold in the immediate postoperative period as the worst part of their hospitalization—sometimes rating it worse than surgical pain. Postoperative thermal discomfort is also physiologically stressful because it elevates blood pressure, heart rate, and plasma catecholamine concentrations.^{[120] [121]} These factors presumably contribute to what may be the most important consequence of mild peroperative hypothermia: morbid myocardial outcomes.^[122] Given that myocardial ischemia is among the leading causes of unanticipated perioperative death, the results of this prospective, randomized trial must be taken extremely seriously.

Drug metabolism is markedly decreased by perioperative hypothermia. The duration of action of vecuronium is more than doubled by a 2°C reduction in core temperature, and the prolongation is a pharmacokinetic effect, not a pharmacodynamic one.^[123] Atracurium's duration of action is less dependent on core temperature: a 3°C reduction in core temperature increases the duration of muscle relaxation by only 60%.^[124] With each drug, the recovery index (time for 25% to 75% twitch recovery) remains normal during hypothermia. Interestingly, core hypothermia per se decreases twitch strength 10% to 15%, even without muscle relaxants.^[125] The efficacy of neostigmine as an antagonist of vecuronium-induced neuromuscular blockade is not altered by mild hypothermia, although onset time is about 20% longer. ^[126]

During a constant infusion of propofol, the plasma concentration is approximately 30% greater than normal when individuals are 3°C hypothermic. ^[124] The effects of mild hypothermia on the metabolism and pharmacodynamics of most other drugs have yet to be reported. However, the results for muscle relaxants and propofol suggest that the effects are substantial. Hypothermia also alters the pharmacodynamics of the volatile anesthetics, with the minimum alveolar concentration being reduced about 5%/°C.^{[127] [128]} Consequently, no anesthesia whatsoever is required to prevent movement in response to skin incision at core temperatures below 20°C.^[129] As might be expected from the pharmacokinetic and pharmacodynamic effects of hypothermia, the duration of postanesthetic recovery is significantly prolonged—even when temperature is not a discharge criterion. When "fitness for discharge" and a core temperature exceeding 36°C are required (as in many postanesthesia care units), the duration of recovery is prolonged several hours.^[130] Table 40-1 lists the proven consequences of mild perioperative hypothermia.