Hypothermia is defined as a core temperature <35°C and may be classified according to severity based on temperatures below this reading.4 Perioperative hypothermia may produce a multitude of deleterious effects, which are summarized in Table 2.5
Table 2
Classifications of Hypothermia and Deleterious Effects4,5
Other than shivering, the most common complications associated with hypothermia are (1) a threefold increase in morbid myocardial events,6 (2) a threefold increase in the risk of surgical wound infection,7and (3) an increase in blood loss and transfusion requirements.8 Adverse cardiovascular events can follow intraoperative depression of cardiac output and heart rate, as well as a rebound during the postoperative period. Hypothermia during the postoperative period markedly impairs thermal comfort, and physiologic stress leads to increases in heart rate, blood pressure, and oxygen consumption.9Hypothermia most likely contributes to wound infection through impairment of immune function and through thermoregulatory vasoconstriction, which, in turn, diminishes oxygen delivery to surgical sites.3 Fever normally increases leukocyte mobilization, but this protective response is lost during hypothermia. Even mild hypothermia hampers blood clotting. The most significant factor is a cold-induced inhibition of platelet function, but the activities of enzymes that drive the coagulation cascade are also impaired.10
Drug metabolism can be markedly decreased by hypothermia. During a constant infusion of propofol, plasma concentration is increased by 30% in patients who are 3°C hypothermic.11 The pharmacodynamics and pharmacokinetics of muscle relaxants and volatile anesthetics are likewise altered. Minimum alveolar concentration (MAC) is reduced by 5% for each °C below normal.3
Although hypothermia is generally regarded as deleterious, it can be beneficial in some situations. Hypothermia decreases the overall metabolic rate by 8% per °C to about half the normal rate at 28°C.3Oxygen demand drops and those tissues that have high oxygen consumption normally, such as brain and heart, have a proportionally greater reduction of oxygen use. This allows aerobic metabolism to continue through greater periods of compromised oxygen supply, thereby reducing the production of anaerobic byproducts such as superoxide radicals and lactate. Additional protection can be attributed to decreased release of excitatory neurotransmitters, reduced synthesis and release of kinases and proinflammatory cytokines, and decreased apoptosis.12 In addition, hypothermia lowers intracranial pressures and cerebral perfusion pressure.3
Substantial protection against cerebral ischemia and hypoxia can be gained by providing a 1°C to 3°C reduction in core temperature. Therapeutic hypothermia is used in many neurosurgery cases and in other procedures such as coronary artery bypass surgery in which tissue ischemia can be anticipated. Therapeutic hypothermia has also been shown to improve outcome during recovery from cardiac arrest and recovery.13,14
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