Conductive layer warming units are most commonly used to help maintain normal body temperature (normothermia) during and after surgery; they are also used in postanesthesia care, labor and delivery, chemotherapy and dialysis treatments, orthopedic procedures, and emergency situations.
When the body loses too much heat and cannot maintain its normothermia of 36.6° to 37.5° C (97.9° to 99.5° F), it is considered to be in a state of hypothermia. Even mild hypothermia (a core temperature of 34° to 36° C [93.2 to 96.8° F]) can have severe repercussions for the patient, such as delayed wound healing, reduced resistance to infection, impairment of platelet function, increased intraoperative blood loss, and prolonged postoperative recovery time. Inadvertent perioperative heat loss typically results in average temperature drops of 0.5° to 1.5° C, but in severe cases the body temperature may drop 2° to 3° C. Surgical patients are at risk of becoming hypothermic due to significant heat loss sustained during surgery for a number of reasons, including:
The body attempts to regain heat lost in the OR by shivering during the postoperative period. Shivering, which can intensify to tremors or violent shaking, poses extreme danger to the patient due to the increased metabolic demand. One study revealed a 92 percent increase in oxygen (O2) consumption in response to a drop in temperature of only 0.2° to 1.3° C; a 500 percent increase in O2 consumption was observed in response to violent shaking. The adverse consequences of the metabolic stress imposed by this increased energy demand include the following:
In addition, tremors and shaking from shivering may adversely affect patient monitoring equipment.
Principles of Operation
Thermal conduction refers to the transfer of energy (heat) between adjacent molecules of a conducting medium in response to a temperature gradient. With conductive layer warming methods, effective warming of the patient is dependent upon direct contact between the patient and heated surfaces, in contrast with forced-air warming which involves moving heated air across the patient’s skin to raise the core body temperature. Conductive layer patient warming units are comprised of a heat-conducting (e.g., carbon polymer) layer in a blanket and/or a compressible underbody mattress. They are powered by a low voltage power source that is regulated by a temperature controller.
The blanket(s) and/or mattress pad may contain water or gel to allow the warming layer to conform to the shape of the patient’s body for optimal heating. They should be made of a latex-free material that can withstand repeated cleaning and disinfection (e.g., urethane); they should also be nonflammable. Blankets are available in various sizes, including pediatric, adult, lower body, upper body, and full body. Specialized sizes designed for pre- and postoperative care are available from some manufacturers. Warming blankets used in the OR are designed to cover only the upper or lower body; a full blanket cannot be used because of the need to establish the operating field and to maintain sterility. Blankets for the post-anesthesia care unit (PACU) or emergency department are full-body blankets. Both reusable and disposable blankets are available. Mattresses have straps or clips to secure them to an operating room table and may also be radiolucent. Some manufacturers offer mattress pads of pressure-reducing foam to reduce instances of vascular occlusion and heat-accelerated necrosis. The foam pad may be enclosed in a shell made of antimicrobial or disinfectable material.
Conductive layer patient warming units have a controller for setting and regulating temperature and activating alarms, and some means (e.g., blankets) to deliver heat to the patient. The controller converts the mains input to a low voltage to achieve and maintain the required heating of the conductive blanket(s) and/or mattress layer(s). The controller may be mounted on a stand or IV pole, or attach to the bedrail via a hook or clip. At least one manufacturer offers a controller capable of regulating multiple blankets and a mattress simultaneously. Blanket temperature is monitored by one or more thermistors, and controllers typically have primary and secondary alarms to ensure that clinicians are alerted if maximum temperature is exceeded.
Conductive layer warming units, like other heating devices, have the potential to cause burn injuries. Patient burns can result from use of the device for extended periods of time on high temperature settings.
A general guideline to follow is that the maximum temperature setting should not be used on patients in the presence of the following conditions, which could increase the risk of thermal injury:
The operator should frequently check patient temperature and vital signs during extended usage. In all cases, operators should reduce the temperature or end treatment when normothermia is achieved.
ECRI Institute Recommendations
As with any device or technology used in health care, organizations planning to purchase conductive layer patient warming units need to consider a wide variety of factors, including performance, safety, and maintenance.
ECRI Institute recommends that conductive patient warming unit controllers have audible and visual overtemperature and malfunction alarms because they increase the likelihood that a caregiver will quickly respond to a device-related problem. The highest temperature setting should be limited to 43°C (109.4°F) and blankets and mattresses should not be capable of reaching temperatures greater than 46°C (114.8°F); higher temperatures increase the risk of thermal skin injury. The cable/hose connecting the blanket(s) and/or mattress to the control unit should be at least 3 m (9.8 ft) in length, and the unit should be equipped with adequate storage for the device components. For a warming unit that will be used in a post-anesthesia care unit, it would be advantageous for the controller to have outputs for more than one blanket and/or mattress.
This article is adapted from ECRI Institute’s Healthcare Product Comparison System (HPCS), a searchable database of technology overviews and product specifications for capital medical equipment. The source article is available online to members of ECRI Institute’s HPCS; learn more at www.ecri.org /HPCS.
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