Chapter Outline
Passive Strategies 264
Ambient Temperature 264
Thermal Insulation 264
Mass Insulators 264
Radiant Insulators 264
Other Considerations 264
Active Warming Devices 264
Forced Air Warming Devices 265
Complications 265
Liquid Circulating Devices 266
Complications 266
Radiant Heaters and Heat Lamps 266
Complications 267
Resistive Heaters 267
Other Heat Loss Prevention Strategies 268
Additional Methods 269
Complications 269
Unintended perioperative hypothermia has been associated with multiple adverse effects. Numerous studies have found hypothermia to be associated with increased blood loss and transfusion requirements, longer surgical wound healing times and increased wound infections, prolonged action of neuromuscular blocking agents and other medications, prolonged recovery in the postanesthesia care unit (PACU), postoperative shivering, and cardiac morbidity. Hypothermia has been found to increase risk of a number of morbid cardiac events postoperatively, including ischemia, unstable angina, cardiac arrest, and myocardial infarction. Patients who experience hypothermia during the perioperative period may have longer hospital stays than patients who were normothermic during the perioperative period.
The ACC/AHA 2007 Guidelines on Perioperative Cardiovascular Evaluation and Care for Noncardiac Surgery introduced a level I recommendation that body temperature should be maintained in a normothermic range for most procedures other than during periods in which mild hypothermia is intended to provide organ protection. The maintenance of normothermia during the perioperative period has become a performance measure and is rapidly emerging as a standard of care.
Causes of Hypothermia
The surgical patient is at risk for hypothermia for several reasons. Patient factors that increase the risk of hypothermia include preexisting hypothermia, extremes of age (e.g., neonates, infants, and older adults), trauma, extensive burns, low body weight, hypothyroidism, dysautonomia (e.g., diabetic neuropathy), and chronic antipsychotic or antidepressant use. Perioperative factors that increase the risk of hypothermia include a cold operating room environment, large open cavity surgery, intravenous infusion of larger volumes of unwarmed fluid or blood products, use of unwarmed irrigant solutions in body cavities, and use of pneumatic tourniquets.
Both general and neuraxial anesthesia reduce the body’s heat production and impair thermoregulation. During general anesthesia, perioperative hypothermia develops in three stages. The most rapid decrease in core temperature occurs during induction and the first 30 to 60 minutes of anesthesia. This is due predominantly to redistribution of heat from the core to the peripheral body. Subsequently, a more gradual, linear decrease of core temperature occurs over several hours as heat loss from the body exceeds heat production by metabolic processes. Finally, core temperature plateaus and does not change for the remaining duration of anesthesia.
Heat energy is transferred from the patient to the ambient operating room environment by radiation, conduction, convection, and evaporation. Surgical exposure and evaporation of water from the patient also contribute to heat loss; for every gram of water evaporated, 0.58 kcal of heat is removed from the patient.
Preventing Hypothermia
Common methods to retard heat loss and warm patients in the perioperative setting include passive strategies and active strategies. Passive strategies include warming the ambient temperature of the operating room and covering the patient. Passive strategies slow heat loss but do not actively transfer heat to the patient or increase core temperature. In addition to slowing heat loss, active warming strategies may transfer heat to the patient and increase core temperature. Technologies that actively warm patients include convective warming devices and conductive warming devices. Convective warming devices transfer heat to the patient by circulating warmed air rapidly across the body surface. Conductive warming devices transfer heat to the patient through areas of direct contact. Radiant warming devices transfer heat to the patient by emitting infrared waves. Additional strategies for preventing perioperative hypothermia include administering warm intravenous fluids and using warm humidified inspired gases.
Passive Strategies
Passive strategies for keeping the patient warm perioperatively include reducing the temperature gradient between the patient and surroundings by raising the operating room temperature and insulating the patient with a single layer or multiple layers of coverings. These measures will not raise core temperature by transferring heat energy to the patient, but may help to decrease the risk of hypothermia by slowing heat loss.
Ambient Temperature
Patients are often exposed to cold operating room environments during the perioperative period. Lower ambient temperatures have been thought to create a more hostile environment for bacterial growth, and thus to reduce the risk of bacterial contamination or infection. This may be offset, however, by the increased risk of surgical wound infection associated with patient hypothermia. In addition, ambient temperature is often kept low for operating room personnel, who may be uncomfortable working in warmer temperatures, particularly when wearing sterile gowns or lead aprons for x-ray, CT, or fluoroscopy.
Certain situations may warrant increasing the ambient temperature to keep the patient warm or prevent hypothermia. One study found that increasing the operating room temperature to 26° C reduced the incidence of core hypothermia in at-risk younger and older patient populations. A warmer room temperature may improve patient satisfaction if the patient is awake in the operating room for any extended period of time, such as during surgery under local or regional anesthesia or during long periods of preparation before induction of general anesthesia. Patient populations at risk for hypothermia include pediatric patients, elderly patients, and patients with large total body surface area (TBSA) burns. Neonatal and pediatric patients have a large body surface area to volume ratio and lose heat faster than adult patients. Large TBSA burn patients lose both heat and water quickly because of epithelial barrier loss and are at significant risk for hypothermia and its sequelae.
Thermal Insulation
The most basic method of preventing heat loss is to avoid exposure and keep the patient covered. Sheets of various materials may be applied to the patient to cover the body. The head may be wrapped or covered with cloth or plastic sheets or with fitted reflective caps. Passive coverings are inexpensive compared with electrically powered active warming systems. In most hospitals thermal insulators are often used in combination with active warming devices. While some studies have demonstrated coverings of certain materials to be more effective than others at preventing heat loss, one study found that a single layer of passive insulation reduced heat loss by about 30% regardless of the material. There are two types of thermal insulators, mass insulators and radiant insulators.
Mass Insulators
Mass insulators entrap air within a fiber matrix. Cotton blankets, cloth and paper surgical drapes, and plastic surgical drapes are all examples of mass insulators. Most of the insulation provided by mass insulators comes not from the fiber matrix itself, but from the still air entrapped between the insulator and the patient’s skin surface. Still air is a very effective insulator, and the type of material used to trap the air is of less importance than the amount of air enclosed beneath the material.
Radiant Insulators
Radiant insulators reflect radiant heat back to the patient and emit little radiant heat to the exterior. Metallized “space blankets” and sheets are radiant insulators. Radiant insulators have been found in experiments to be more efficient insulators than cloth surgical drapes and cotton blankets. The difference, however, is slight; one study found a “space blanket” to be only 13% more efficient than a cloth surgical drape. The minimal difference in effectiveness between a radiant insulator and ordinary mass insulators appears to have little clinical significance. Still air beneath the reflective blanket is still a large contributor to insulation, as it is with mass insulators. The effectiveness of radiant insulators appears to be reduced by the placement of other materials between the metallic surface and the patient’s skin; the addition of a radiant insulator between layers of mass insulators does not reduce heat loss more than the addition of another mass insulator.
Other Considerations
Multiple layers are slightly more effective than a single layer of passive insulation in helping to prevent heat loss. One study found that a single cotton blanket reduced heat loss by about 33%, and that additional blankets to a total of three layers conferred an additional 18% reduction in heat loss. This was regardless of whether the blankets were warmed or kept at room temperature.
Cotton blankets are frequently stored in an oven in the operating room area to keep them warm. Prewarmed blankets often provide immediate comfort to the awake patient and may increase patient satisfaction. Prewarmed blankets do not, however, raise core temperature or transfer heat to the patient. Prewarmed cotton blankets also do not prevent heat loss more effectively than unwarmed cotton blankets stored at room temperature. One study found that although warmed blankets did reduce heat loss to some degree, this benefit dissipated after approximately 10 minutes, after which there was no significant difference between warmed and unwarmed blankets.
Active Warming Devices
Active warming devices transfer heat to the patient and may raise core temperature. Heat may be transferred to the patient by convection, conduction, or radiation. Convective warming devices transfer heat by circulating warmed currents of air over the patient. Conductive warming devices transfer heat by direct contact with a warm surface. Radiant heating transfers heat by application of energy in the form of rays or waves.
Forced Air Warming Devices
Forced air warmers transfer heat to the patient by convection. They are effective at preventing hypothermia when used before induction of anesthesia, during anesthesia and surgery, and after emergence in the postanesthesia care unit. Forced air warmers are the most frequently used and studied active warming devices in the perioperative setting. Studies have shown that preinduction warming with forced air warmers also significantly reduces postoperative hypothermia and results in higher postoperative core temperatures.
The forced air warmer consists of an electrically powered control unit, hose, and inflatable “blanket” ( Figure 19–1 ). The control unit has an air filter and heater, which warms air entrained from the environment. The hose connects to a blanket. Small holes located throughout the blanket allow currents of warmed air to pass through and blow across the patient. Disposable forced air warming blankets have been manufactured in various shapes for placement under the patient as a mattress, over the upper body and arms, over the lower body and legs, or surrounding the head. Placement of a passive covering such as a cotton blanket, surgical drape, or sheet over the forced air blanket may help to warm the patient more rapidly than the forced air blanket alone.
Complications
Concern has been raised that forced air may seed the surgical site with bacteria from the environment, control unit, or hose. This has not been proven in any large studies, however, and some small studies have demonstrated no increase in wound or prosthesis infections associated with intraoperative use of forced air warming. One small study found bacterial colonization in the hoses of forced air convection warmers and detected bacteria in the airstream flowing directly from unattached hoses, but no positive cultures from air which passed through the perforated forced air warming blankets when the hose was connected according to the manufacturer’s intended design. Forced air warmers have in-built filters to reduce microbial risk. Nonetheless, it remains a common practice to avoid turning on the forced air warmer until after surgical prepping and draping to avoid blowing air over the surgical site.
Burns from forced air warmers have been reported in certain settings. Forced air warming should not be applied to poorly perfused or ischemic areas, such as over the lower extremities during aortic or iliac cross-clamping in vascular surgery procedures. Second- and third-degree burns have been reported in neonatal and pediatric patients at risk for cyanosis or global ischemia, such as during cardiopulmonary bypass for surgical correction of cyanotic congenital heart disease.
Burns may also occur when air flow is turned on at the control unit without attaching the hose to the appropriate forced air warming blanket ( Figure 19–2 ). In 2002, the FDA cited several complaints in which burn injuries occurred from concentrated hot air blowing directly at the patient’s skin from the unattached hose. These included first-, second-, and third-degree burns, with one case of severe muscle necrosis that resulted ultimately in above-the-knee amputation. The American Society for Testing and Materials specifies that forced air warming devices should not exceed an average contact surface temperature of 46° C, and maximum contact surface temperature should not exceed 48° C under normal operating conditions. When the hose is not attached to the intended forced air warming blanket, concentrated air at the hose nozzle may exceed 48° C. In addition, the hose surface may be hot and may cause burns if it comes into direct contact with the patient’s skin. It is recommended that a forced air warmer should always be used with the device’s appropriately fitting blanket.
