What are the modalities to prevent and treat perioperative hypothermia?
· Redistribution is best prevented by initiating skin warm-ing before induction.
· Radiation heat loss is proportional to the fourth power of the difference between the absolute temperatures of the surfaces and can be limited by increasing the room tem-perature. Radiant warmers are only used for infants, facilitate keeping the patient warm while still visible, and preclude the need to increase the room temperature.
· However, radiant warmers become less effective as the distance to the patient increases. In addition, they do not decrease convection heat loss.
· Gas and fluid heating have limited efficacy. Less than 10% of the heat loss is via respiration. Fluid warming should be used if a large volume of fluid is administered. One liter of crystalloid at room temperature or one unit (250 mL) of refrigerated blood will decrease the temper-ature by about 0.25°C.
· Insulation will decrease heat loss by only 30% with one layer of fabric (sheet or blanket), with little additional benefit from additional layers.
· Overall, forced-air warming (e.g., Bair Hugger blankets) is the most effective warming and rewarming method. It is more effective when patients are vasodilated. It is, therefore, better to maintain normothermia from the start of the procedure rather than to rewarm postopera-tively. Rewarming a vasoconstricted patient can lead to hypotension secondary to vasodilation if the volume status is not maintained.
· Circulating-water mattresses placed on the operating room table have little efficacy, since 90% of the heat is lost from the surface of the body that is exposed and is not in contact with the table.
· Never use hot-water bottles! They are the leading cause of perioperative thermal injury according to the ASA Closed-Claims database.
· Shivering can be treated by skin-surface warming (core hypothermia is better tolerated before response mecha-nisms are activated when cutaneous warm inputs are increased) and/or pharmacologically, with either meperi-dine 12.5–25 mg intravenously or clonidine 75–150 μg intravenously. The mechanism of action of meperidine is unclear and may be due to kappa receptor activation. The mechanism of action of clonidine is a decrease in vasoconstriction and shivering thresholds.