The second subgroup of risk factors for perioperative hypothermia is related to anesthesia. The effect of general and regional anesthesia on perioperative core temperature is significantly different, both in terms of intraoperative thermoregulation and postoperative recovery.12 Intraoperatively, the core body temperature during the first 2 hours of general anesthesia decreases at a rate of 1.3°C per hour due to loss of thermoregulatory cutaneous and peripheral vasoconstrictive responses resulting in heat loss exceeding metabolic heat production. However, the core temperature remains virtually constant during the subsequent 3 hours due to the return of the thermoregulatory response, which causes cutaneous and peripheral vasoconstriction and increased metabolic heat production. Postoperative recovery from the hypothermia induced by general anesthesia is significantly faster than from that induced by regional anesthesia.13
The effect of regional hypothermia on core body temperature is more complex because it must be considered in addition to the effect of an associated procedure-related variable (ie, tourniquet application). If a tourniquet is not used during a surgery with regional anesthesia, a linear decrease in core temperature follows until recovery, due to increased blood flow from the loss of sympathetic peripheral vasoconstrictive response with resultant core-to-peripheral heat redistribution to the exposed operating limb. If a tourniquet is used during surgery with regional anesthesia, there will be no significant effect of the exposed operating limb on core temperature, as there is no blood flow between them. However, once the tourniquet is deflated, the core body temperature will be affected significantly as a result of core-to-peripheral distribution of heat to the operated limb with the return of blood flow. This fall in core body temperature after tourniquet deflation can be prevented by active forced-air warming initiated from the beginning of surgery.10 The extent and rate of development of peripheral/limb hypothermia during surgery (and its subsequent effect on core body temperature) depends on several factors, including the operating room ambient temperature, duration of tourniquet application, and temperature of the irrigation fluid. Postoperative recovery from the hypothermia induced by regional anesthesia takes longer than from that induced by general anesthesia because of the prolonged period of loss of vasoconstrictive response.
The third subgroup of risk factors associated with perioperative hypothermia is procedure related. Several procedure-specific risk factors for inadvertent perioperative hypothermia during arthroscopic surgery are identified, including prolonged operating time, low blood pressure during the procedure, and low temperature of the irrigation fluid.11 It is logical to extrapolate the importance of these risk factors to other orthopedic procedures which also require prolonged operating times, are performed under hypotension, or expose the patient to irrigation fluid that is at a low temperature. Understanding the importance of each of these procedure-related risk factors is the most important from the perspective of the orthopedic surgeon when compared to the rest of the subgroups of risk factors for inadvertent perioperative hypothermia as he/she is directly responsible for them.
The ambient operating room temperature has traditionally been considered a risk factor for inadvertent hypothermia in perioperative patients, but evidence is available to the contrary. The recommended ambient room temperature as per the clinical guideline published by the American Society of PeriAnesthesia Nurses (ASPAN) is 20°C to 24°C (68°F-75°F).14 The ambient temperature can have a significant effect on peripheral/limb hypothermia when operating on a limb with a tourniquet inflated, as the limb has no blood supply to distribute heat from the core to the periphery. However, the direct effect of ambient room temperature on the patient’s core body temperature is unlikely to be clinically significant if standard active warming interventions are implemented.15
COMPLICATIONS
The increased incidence of mortality due to inadvertent hypothermia in the perioperative period has already been discussed. Several other complications of inadvertent perioperative hypothermia include increased incidence of coagulopathy, acidosis, stroke, sepsis, pneumonia, myocardial infarction, surgical site infections, altered drug metabolism, and longer hospital stays.8,9,16-18 Hypothermia, coagulopathy, and acidosis have long been recognized as a lethal triad more commonly seen in polytrauma patients than in elective orthopedic surgery, as this occurs at extremes of temperature, usually <32°C. When compared with patients who did not develop perioperative hypothermia, patients who developed hypothermia during elective operations were shown to experience an overall doubled complication rate (13.9% vs 26.3%; P < .001) of which the incidence of stroke (1.0% vs 6.5%; P < .001), pneumonia (1.3% vs 5.1%; P < .001), and sepsis (2.6% vs 7.5%; P < .001) were much more likely than myocardial infarction (1.1% vs 3.3%; P = .01) and wound infection (3.3% vs 5.0%; P = .14).9
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