Anesthesiologists spend much of their waking hours in one of the world’s most complex infrastructural and technological work environments, the modern operating room.1 There, they are surrounded by advanced equipment continuously monitoring patients’ physiological functions; precision anesthetic delivery systems; computerized anesthetic, hospital, and radiologic information systems; surgical instrumentation including lasers, robots, optical fiberscopes, and advanced imaging modalities; and operating room ventilation systems designed to facilitate sterile operating fields. The operating room’s complexity is said to be exceeded only by the “clean rooms” used in manufacturing microelectronic microprocessors and other such chips, where the ventilation systems are designed to prevent environmental particles and substances from interfering with the chemicals and materials used in the manufacturing process, as well as supporting the creation of silicon chips containing billions of transistors. It is said that the average surgical operating room is 3 times dirtier than the “dirtiest” clean room.2 Because of the operating room’s extremely complicated infrastructure, made even more complex by the equipment and clinical activities within, the interaction of its various structural features, mobile equipment, and human components can become problematic. Examples abound and include the combination of electrosurgical cauterization by the surgeon and oxygen administration by the anesthesiologist, which can lead to ignition of paper or cloth drapes and resulting in serious patient injury from the operating room fire.3 The design and heat generated by operating room lights and the arrangement of equipment on the floor can hamper the effectiveness of laminar flow ventilation systems designed to reduce wound contamination.4,5 Operating room lights (especially the older non–light emitting diode models) also generate much heat, making gloved, gowned, and masked surgeons uncomfortable, leading to requests to lower the ambient temperature, which in turn contributes to patient hypothermia. The connection between forced-air warmers and surgical site infections (SSIs) is this paradoxical situation, wherein the patient’s body temperature needs to be preserved but the surgeon requires personal cooling, that is among the reasons why patient-warming devices have evolved. It is in this context of the increasing awareness of the many man–machine–infrastructural interactions that occur in operating rooms that the simulation study by Belani et al.,6 on how a forced-air patient-warming system might contribute to the contamination of a sterile field, is timely.
Intraoperative hypothermia is both the friend and foe of the anesthesiologist. The former occurs when hypothermia is intentionally induced for organ preservation during cardiopulmonary bypass, total circulatory arrest, post–cardiac arrest, and during neurosurgical procedures. However, most often anesthesiologists are engaged in combating its development. Since the early 1980s, the mechanisms causing hypothermia,7 its clinical consequences8,9 and methods to prevent its development,10 have been well studied. Many reports have shown the association of intraoperative hypothermia with greater blood loss and transfusion requirements; more SSIs, and along with the stress of the subsequent rewarming, increased cardiovascular complications.10,11 These studies have provided the impetus to recommendations and guidelines that routine measures be implemented to prevent unplanned intraoperative hypothermia. Furthermore, there are ongoing initiatives that propose to make maintenance of perioperative normothermia a performance-based payment condition.11–13 This emphasis on preventing unintended intraoperative hypothermia has led to the routine use of warming measures before, during, and after surgery.
Historically, warm water “blankets” (even though they were mostly used underneath the patients as “mattresses”) preceded forced-air devices. Several review articles discuss the value of “warm blankets,” but do not specify the placement of the heat-generating source. Generally, circulating warm water blankets placed underneath patients are less effective than warming garments that cover more surface area and use various technologies such as warm water, resistive heat, and a slight vacuum with electric heat. For example, circulating water garments and a water garment warmer were found to be more effective than forced-air systems.10,14 On the contrary, forced (convective)-air devices were found to be more effective than circulating water blankets,15 water coil warming blankets,16 12-V resistive-heating blankets,17 and circulating water mattresses.18 Newer devices (e.g., a 3-extremity carbon-fiber resistive-heating blanket) have been found to be equal to forced-air devices and better than circulating water mattresses.19 Maintaining intraoperative and postoperative normothermia is especially important in decreasing postoperative infections. Many publications have shown that preventing hypothermia (irrespective of the type of warming device) decreases postoperative infection rates. For instance, perioperative normothermia was found to decrease the incidence of surgical wound infections,20 while a randomized controlled study of clean surgery reached similar conclusions.21
In contrast to intraoperative warming, which decreases the incidence of surgical wound infections, some of the contemporary literature has pointed to the potential increased SSI risk posed by forced-air devices. Several publications, including the article by Belani et al.6 in this edition of Anesthesia & Analgesia, have produced indirect evidence for the mechanism by which these forced-air devices potentially increase the risk of infection. For example, forced-air devices were postulated to present a potential risk for infection in prolonged vascular and hip surgery.22,23 The proposed mechanism was the generation of an increased number of particles above and in the surgical field.24 Given the ubiquitous use of forced-air devices, assumptions can be made that forced-air devices were used on a significant percentage of patients in the studies showing decreased infection when normothermia was maintained. The lower infection rates would presumably not have been present if forced-air devices caused a major increase in infection rates. Dr. Daniel Sessler summarizeda the controversy very eloquently: “Recently, some investigators have proposed that forced-air warming might disperse bacteria within operating rooms. This is a curious assertion since six studies demonstrate that properly used forced-air systems do not increase bacterial count.”22,23 Therefore, there remains the controversy of whether on the one hand, keeping a patient warm reduces the SSIs, while on the other hand, using forced-air warming might mitigate some of this advantage due to an increase in airborne particles.25 Unfortunately, simulation studies in a single operating room and collecting data over a limited time period, as performed by Belani et al.,6 will not help resolve this issue, since there are many variables that need to be considered. Therefore, the challenge is to design and perform in vivo studies that specifically examine the bacterial count over and within the surgical field. The bacterial count, and not just particles, is a key variable since it is not only anesthesiologists who produce operating room airflow issues; surgeons also do by producing tissue aerosols, blood mists, and laser smoke.26–28 Furthermore, comparison studies of SSIs with non–forced-air warmers must be performed to specifically determine the effects of forced-air warmers. At this stage, given the controversies over the best method for warming patients and the possibility of increased airborne contaminants with forced-air warming, the prudent course for clinicians might be to continue with the presently proven successful warming therapies, but keep an open mind about the possible future need to change practice.
Name: Charles Weissman, MD.
Contribution: This author suggested, and helped design, the study, conduct the study, analyze the data, and write the manuscript.
Attestation: Charles Weissman approved the final manuscript.
Name: W. Bosseau Murray, MD.
Contribution: This author helped design the study, conduct the study, analyze the data, and write the manuscript.
Attestation: W. Bosseau Murray approved the final manuscript.
This manuscript was handled by: Sorin J. Brull, MD, FCARCSI (Hon).
a Daniel Sessler. APSF 25th Anniversary Edition of the ASA Newsletter, May 2011, p. 34–40. Available at: http://viewer.zmags.com/publication/7359936c#/7359936c/36. Accessed May 23, 2013.
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