3 hot trends driving patient warming practice

Nearly all clinicians know that maintenance of normal core temperature during surgery is associated with improved perioperative outcomes; however, inadequate or deferred warming and temperature monitoring can leave patients dangerously hypothermic.1

Fortunately, new warming and monitoring technologies, as well as an improved appreciation for intraoperative thermoregulation, are making it easier to properly maintain patient normothermia throughout the surgical journey.

Patient warming's role in successful surgical outcomes
Both general and neuraxial anesthesia interfere with thermoregulation and produce substantial hypothermia in unwarmed patients. Perioperative hypothermia is associated with several important complications, including: 2

• Bleeding and increased need for blood products
• Surgical site infections
• Prolonged drug effects
• Longer PACU times
• Shivering and thermal discomfort

Although the evidence is not quite as strong, there are indications that intraoperative hypothermia is associated with myocardial injury.3

Despite awareness of the consequences and the existence of several patient-warming guidelines, perioperative hypothermia remains high in many facilities, even in those in which patients are warmed.4 There are several reasons for the persistently elevated rates of intraoperative hypothermia, but the most important ones include an under-appreciation for the dominance of redistribution and a failure to monitor core temperature, especially during shorter surgical procedures.

Increase in active prewarming
While it may seem counterintuitive, the major cause of intraoperative hypothermia is not heat loss from the skin, but redistribution of heat from the central (brain and central organs) to the peripheral thermal compartment (arms and legs). Much of the heat within the body isn't lost after anesthesia; it simply moves from one area of the body to another.5

Hypothermia caused by redistribution is almost impossible to reverse quickly because heat transfer from external sources is slowed due to an increased skin temperature. However, temperature management can be improved when warming is started prior to anesthesia induction.6 Recent work has shown that as few as 10 minutes of aggressive active warming (prewarming) before the commencement of anesthesia can virtually eliminate postinduction hypothermia.7

Many guidelines and institutional protocols still do not recommend active prewarming or intraoperative warming for shorter surgeries of one hour or less. However, in adult patients, hypothermia caused by redistribution can occur at very rapid rates of approximately 2°C per hour. This means that patients having shorter surgeries are actually more likely to experience postoperative hypothermia because there is simply no way to quickly overcome the effect of redistribution with intraoperative warming.

An appreciation of redistribution as the cause of intraoperative hypothermia has motivated an increased use of aggressive prewarming for all types of surgical patients, especially for those with shorter procedures. It is important to remember that thermal comfort depends upon the loss of heat from the body exactly equaling the amount produced within the body, or what is known as heat balance.8 Because prewarming upsets the conditions for heat balance, patients may experience a brief period of thermal discomfort (feeling too warm) during preoperative warming. However, improved postoperative outcomes, including enhanced thermal comfort in the PACU, more than compensate for this fleeting, mild discomfort. Moreover, it is now known that as few as ten minutes of aggressive warming with a forced-air system is enough to eliminate intraoperative hypothermia in many surgical procedures.7

Temperature monitoring
Remarkable as it may seem, many surgical patients do not receive any form of core-temperature monitoring. Although temperature is a vital sign, it is frequently thought of as being less important than the other signs clinicians must monitor during anesthesia.

When patients are monitored, multiple methods are often used throughout the perioperative journey. Each method introduces variation due to accuracy and technique.

While it is mostly true that small changes in temperature do not generally have important immediate consequences, it is well known that even mild intraoperative hypothermia is associated with several important perioperative complications, especially bleeding and hospital-acquired infections.

One significant challenge with temperature monitoring is that reliable estimates of core temperature usually require the invasive placement of a thermometer within the patient's body. Fortunately, a newly developed, non-invasive technique known as zero-heat-flux thermometry can provide accurate measurements of deep tissue temperature that have been shown to be equivalent to invasive methods.9

Zero-heat-flux thermometers gauge deep tissue temperature in a similar way as blood pressure cuffs gauge blood pressure. The blood pressure cuff applies a pressure that is equivalent to the arterial pressure. Once the clinician determines the pressures are exactly the same from the skin surface to the artery below, the arterial pressure is easily read from a gauge. Similarly, the zero-heat-flux thermometer applies just enough heat to equal the heat leaving the body. Once those two temperatures are equivalent, the body's deep tissue temperature can be monitored.

Improved skin surface recruitment
Regardless of the method of heat transfer, the recruitment of the largest amount of skin surface area is vital to maximizing the amount of energy transferred to the patient. Forced-air warming systems have an advantage in this regard because the heated air can reach areas of the skin that are not directly in front of the blanket. A new forced-air upper body blanket, known as the multi-position upper body blanket, makes it possible to recruit more skin surface area because the blanket can be bent in positions that were previously impossible without causing the blanket to kink or deflate. The new blanket is useful in warming patients when more complex positioning is required.

Summary
Maintenance of perioperative normothermia is an important component of the global standard of surgical care.10 The growing recognition of postinduction redistribution as the dominant cause of intraoperative hypothermia, especially for shorter duration surgeries, has motivated the increased use of prewarming as a successful warming strategy.

New temperature monitoring technologies have made it easier to measure perioperative core temperature continuously, without the use of invasive instruments. Forced-air warming blankets and gowns continue to improve as well, recruiting more skin surface area while being optimized to conform to several specialized surgical positions.

About the Author
Al Van Duren is the global director of scientific affairs and education in 3M's Infection Prevention Division. He has a master's degree in physiology from the University of Minnesota and holds 26 patents for warming, pulmonary and thermometry devices.

References
1. Torossian A. Survey on intraoperative temperature management in Europe. European Journal of Anaesthesiology (EJA). 2007;24(8):668-675.
2. Sessler DI. Perioperative thermoregulation and heat balance. The Lancet. 2016;387(10038):2655-2664.
3. Scott AV, Stonemetz JL, Wasey JO, Johnson DJ, Rivers RJ, Koch CG, Frank SM. Compliance with Surgical Care Improvement Project for Body Temperature Management (SCIP Inf-10) Is Associated with Improved Clinical Outcomes. Anesthesiology. 2015;123(1):116-125.
4. Sun Z, Honar H, Sessler DI, Dalton JE, Yang D, Panjasawatwong K, Deroee AF, Salmasi V, Saager L, Kurz A. Intraoperative Core Temperature Patterns, Transfusion Requirement, and Hospital Duration in Patients Warmed with Forced Air. Anesthesiology. 2015;122(2):276-285.
5. Matsukawa T, Sessler DI, Sessler AM, Schroeder M, Ozaki M, Kurz A, Cheng C. Heat flow and distribution during induction of general anesthesia. Anesthesiology. Mar 1995;82(3):662-673.
6. Camus Y, Delva E, Sessler DI, Lienhart A. Pre-induction skin-surface warming minimizes intraoperative core hypothermia. J. Clin. Anesth. Aug 1995;7(5):384-388.
7. Horn EP, Bein B, Böhm R, Steinfath M, Sahili N, Höcker J. The effect of short time periods of pre-operative warming in the prevention of peri-operative hypothermia. Anaesthesia. 2012;67(6):612-617.
8. Fanger PO. Conditions for thermal comfort - Introduction of a general comfort equation. In: Hardy JD, Gagge AP, Stolwijk JAJ, eds. Physiological and behavioral temperature regulation. Springfield, Ill.,: Thomas; 1970:152-176.
9. Eshraghi Y, Nasr V, Parra-Sanchez I, Van Duren A, Botham M, Santoscoy T, Sessler DI. An Evaluation of a Zero-Heat-Flux Cutaneous Thermometer in Cardiac Surgical Patients. Anesth Analg. 2014;119(3):543-549.
10. Berríos-Torres SI, Umscheid CA, Bratzler DW, et al. Centers for disease control and prevention guideline for the prevention of surgical site infection, 2017. JAMA Surgery. May 3 2017.

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