d. Induction of anesthesia inhibits shivering, produces vasodilation and redistributes blood flow from the warm core to the periphery. Normally body heat is unevenly distributed, the temperature of core tissues being 2 ? to 4 ?C greater than skin temperature. Redistribution leads to a 0.5 ? to 1.5 ?C decrease in core temperature. Radiant heat loss accounts for about 60% of total heat loss in the perioperative period.

b. Dry anesthetic gases, large volumes of cold IV fluids and the usually cold ambient OR temperatures will all contribute to increased heat loss in the perioperative period. Anesthetic medications, however, will reduce, not increase, the temperature at which the hypothalamus will initiate normal thermoregulatory mechanisms. Under normal conditions the hypothalamus will initiate thermogenic responses when core temperatures drop as little as 0.1 ? below 37 ?C. Most anesthetics can lower this normal threshold by as much as 4 ?C (20 times lower than the normal threshold trigger).

a. Most anesthetics are potent inhibitors of the compensatory responses to even mild hypothermia: shivering and vasoconstriction, in addition to their ability to lower the hypothalamic threshold for cold response by as much as 3 to 5 ?C. Versed, tested at doses exceeding those used for premedication, only lowers the point at which thermogenic controls respond to cold by 0.2 ? to 0.7 ?C. It also has less of an effect on the ability to respond to cold. Nitrous oxide similarly has less effect on thermoregulation when compared to other more potent volatile anesthetics.

d. None of the above. Propofol, fentanyl and remifentanyl, frequently used in TIVA techniques, produce significant changes in thermoregulation and place the patient at risk of developing hypothermia. Regional anesthesia impairs both central and peripheral thermoregulation. Regional techniques produce vasodilation and impair thermoregulation by blocking autonomic signaling. This results in core hypothermia similar to that seen with general anesthetics. Because peripheral skin sensation is altered by regional anesthesia, this hypothermia is often not perceived by patients and can cause a potentially dangerous clinical paradox: a hypothermic patient who denies feeling cold. Patients under regional anesthesia can shiver in non-blocked areas even though they do not feel cold. All thermoregulatory responses are neurally mediated; anything that blocks accurate neural input and output will affect thermoregulation and place the patient at risk for hypothermia. Despite the fact that locally injected anesthetics have no direct action on the hypothalamus, the thermoregulatory center still appears to become impaired. Through still unknown mechanisms, the thermoregulatory center incorrectly senses skin temperature in anesthetized regions to be abnormally elevated. This misinterpretation of skin temperature in the blocked region fools the regulatory system into tolerating lower core temperatures. The net result is that the threshold necessary to trigger a response decreases 3 to 4 times (from 36.8 ?C to between 36.4 and 36.2 ?C) and the patient is still at risk for developing mild hypothermia. Any IV sedation added to local procedures raises the risk of hypothermia further.

c. Numerous factors can contribute to increased risk of developing perioperative hypothermia. Patient factors include extremes of age, overall health and physiologic reserve and muscle mass. Pediatric patients are at increased risk of becoming hypothermic because increased body-surfac-area-to-mass ratio accelerates radiant heat loss. Young infants are at significant risk of hypothermia as they are unable to shiver and are dependent on non-shivering thermogenesis to generate heat. Elderly and cachectic patients have both a decreased muscle mass and physiologic reserve. Regulatory thresholds in patients 60 to 80 years of age are 1 ?C less than those in 30- to 50-year-olds. The increased incidences of chronic diseases and the use of multiple medications such as anti-hypertensives and anti-psychotics, which can impair compensatory responses, increase their risk further. Obese patients have actually been found to have higher perioperative core temperatures compared to non-obese patients and thus do not represent a population at increased risk for developing hypothermia. It should be noted, however, that obese patients can still experience perioperative hypothermia and should receive the same preventive and active treatment methods that other patients receive.

c. Sustained shivering can increase metabolic heat production by 50 to 100% in adults. By comparison, however, exercise can increase metabolism by as much as 500% and is far more effective in increasing metabolic heat production. As noted earlier, infants depend on non-shivering thermogenesis and the pediatric patient cannot compensate for hypothermia by shivering until they are several years old. The rapid movement seen with shivering consists of asynchronous muscular activity that suggests there is no central modulator to this activity. There is, however, a slower synchronous pattern of activity that is superimposed on the fast muscular contraction. This is presumed to be centrally mediated.

b. Coagulation is impaired by even mild hypothermia leading to potentially increased blood loss. A cold-induced defect in platelet function is the primary problem, although hypothermia directly impairs the enzymes of the coagulation cascade as well. Initial respiratory response to mild hypothermia is a centrally mediated hyperventilation often producing an initial respiratory alkalosis. Hypoventilation and abnormal breathing patterns are seen with more severe drops in body temperature and/or residual effects of anesthetics. Humeral stress responses to the development of mild hypothermia lead to increases in plasma glucose and drops in plasma insulin levels. The resultant hyperglycemia further increases the risk of surgical site infection, which is directly increased by the effects of hypothermia on immune cell function. Drug metabolism is markedly decreased by perioperative hypothermia. Prolonged duration of action of neuromuscular paralytics, IV sedatives and inhalational agents are all observed in hypothermic patients. Hypothermic patients have prolonged stays in the PACU and delayed discharge from ambulatory facilities, all of which can increase cost and decrease patient satisfaction.

c. The body reacts to even mild hypothermia by producing vasoconstriction. As a result, increased vascular resistance leads to increased blood pressure and myocardial afterload. This limits the ability of the myocardium to contract and decreases cardiac output. Post-operatively, ?if core temperature is more than 1 ?C below normal, the body activates the shivering mechanism and places a large demand on the cardiovascular system. Upon emergence, adrenergic and metabolic responses can create an imbalance between myocardial oxygen supply and demand, potentially leading to ischemia and myocardial infarction. Maintenance of normothermia throughout surgery has the potential of decreasing cardiac morbidity by 55%. Ventricular fibrillation and arrhythmias tend to occur with more significant hypothermia in most patients. It is important to remain vigilant with patients who have pre-existing myocardial disease or cardiac dysrhythmias, however, as smaller degrees of hypothermia may induce serious changes in rhythm.

a. Wound problems are one of the most common serious complications of anesthesia and surgery. Hypothermia contributes in multiple ways to wound complications. Mild hypothermia directly suppresses the function of the immune system, making the patient more vulnerable to wound infection. Additionally, hypothermia will cause cutaneous vasoconstriction, which can regionally decrease blood flow to the surgical site, decreasing oxygen tissue delivery and increasing tissue acidosis. This can cause delayed healing or wound dehiscence even in the absence of infection.

d. The risk of post-operative shivering in adults is largely determined by younger age and drop in core temperature. The incidence of post-anesthetic shivering was once as high as 40% of patients being admitted to PACU. It has decreased, however, with aggressive efforts to keep all surgical patients normothermic and the increased use of opioids, which directly inhibit shivering. Shivering can increase O2 consumption about 100% in proportion to heat loss; however, shivering is actually poorly correlated to the development of cardiac ischemia. Post-operative shivering in the elderly is actually rare and usually of low intensity when it does occur. This suggests that increased metabolic rate is not the primary mechanism by which myocardial ischemia occurs. Shivering can place stress on the surgical wound, augmenting pain and stretch on surgical sutures and increasing the risk of wound dehiscence.

b. Post-anesthetic shivering can be treated with a variety of drugs. Specifically how many of these agents prevent shivering remains unknown. Both clonidine and dexmedetomidine reduce the vasoconstriction and shivering thresholds, making modulation of the central thermoregulatory center likely. Studies on meperidine suggest its effect on shivering is modulated by non-opioid receptors as well as central adrenoreceptors. Compared to other opioids, meperidine appears to have the best efficacy for treating shivering.

a. Most heat is lost in the first 30 minutes following induction when warm core blood is redistributed to the colder periphery due to vasodilation. Once core temperature drops, compensatory vasoconstriction of AV shunts will begin. This constriction is limited, however, by potent vasodilating anesthetics. Post-operatively, in hypothermic patients, the ability to vasoconstrict in response to cold returns. The sequence of the physiologic changes has a striking influence on the efficacy of hot-air warmers. Forced hot-air warming devices are most effective at preventing hypothermia when applied in the pre-operative period. Normally dilated skin vessels enable peripheral blood to gain heat in as little as 30 minutes during the pre-op period. This can significantly attenuate the drop in core temperature during the initial redistribution of blood. Forced hot-air devices are also somewhat helpful during the intra-operative period when anesthetically induced vasodilation allows for better transfer of heat to the skin. This situation, however, is rapidly reversed in the postoperative period when anesthetically induced vasodilation dissipates. The peripheral vasoconstriction that results makes forced-air warming least effective in the post-operative period. Post-op patients under regional anesthesia will warm faster than patients who have had general anesthesia due to the residual vasodilation produced by regional anesthetics.

d. Less than 10% of metabolic heat production is lost via the respiratory tract. Because so little heat is lost via this modality, both heating and humidifying gases have little effect on raising core temperature. This is especially true during long open surgeries, when far more heat is lost via evaporation from the surgical wound. Airway heating and humidification is more effective in decreasing heat loss in infants and children, compared with adults, but cutaneous warming still transfers 10 times as much heat.

b. While it is not possible to actively warm patients by administering heated IV fluids, because the fluids cannot exceed body temperature without causing damage, it is possible to prevent further heat loss by using fluid warmers. This is especially important when large volumes of fluids or blood are needed. For routine outpatient cases, no clinically significant difference is noted using the various types of fluid warmers on adults. At low flows the fluid will cool in the tubing but the amount delivered is small. At higher flows, little cooling will occur and less heat will be lost.

c. The OR temperature is what determines the gradient for heat loss by radiation and convection from the skin and evaporation from the surgical site and wound. Warming the OR can minimize heat loss. However, to maintain normothermia, a room temperature greater than 23 ?C (74.3 ?F) is required. Most scrubbed OR staff would find this uncomfortably warm.