Forced-Air Warming and Resistive Heating Devices. Updated Perspectives on Safety and Surgical Site Infections

Introduction: Perioperative hypothermia is one of the most common phenomena seen among surgical patients, leading to numerous adverse outcomes such as intraoperative blood loss, cardiac events, coagulopathy, increased hospital stay and associated costs. Forced air warming (FAW) and resistive heating (RH) are the two most commonly used and widely studied devices to prevent perioperative hypothermia. The effect of FAW on operating room laminar flow and surgical site infection is unclear and we initiated an extensive literature search in order to get a scientific insight of this aspect. Material and Methods: The literature search was conducted using the Medline search engine, PubMed, Cochrane review, google scholar, and OSU library. Results: Out of 92 Articles considered initially for review we selected a total of 73 relevant references. Currently there is no robust evidence to support that FAW can increase SSIs. In addition, both of the two warming devices present safety problems. Conclusion: As unbiased independent reviewers, we advise clinicians to weigh the risks and benefits when using either one of these devices; no change in the current practice is necessary until further data emerges.


INTRODUCTION
Perioperative hypothermia (PH), defined as a core temperature below 36 • C, is one of the most common phenomena seen among surgical patients (1). Hypothermia during surgery can be generated by various factors such as: exposure to surgical environment, thermoregulatory dysfunction during general or regional anesthesia and medications. PH leads to intraoperative blood loss, cardiac events, coagulopathy, an increase in hospital stay and associated costs (1,2). Even mild hypothermia (approximately 2 • C below normal temperature) can triple the incidence of wound infection and prolong hospitalization by about 20% (2). Passive and active warming methods are used in the operating room (OR) to prevent the incidence of PH. Blanket use to provide insulation and radiant heat loss prevention (passive method), warming intravenous fluids, and patient warming devices (active methods) are well known (3).
Currently, a plethora of patient warming devices are utilized to reduce the incidence of intraoperative hypothermia among surgical patients (2, 3) ( Table 1). We focus our review on two most commonly used and widely studied methods-forced air warming (FAW) and resistive heating (RH)-based on recent publications associated with their use (3).
FAW system employs air convection to transfer heat, utilizing a heat generator with a temperature management unit, a blower to circulate heated air and a temperature control system equipped with various sensors (4-6). The air enters through a 0.2 µm rated intake filter (4-6). The blower unit is connected, via rubber hose, to a disposable perforated air blanket (6). The warm air passes through this blanket and helps maintain patient's surface temperature within a physiological range (6).
RH system converts electrical energy into heat and warms the patient through conduction (2). Effective warming is delivered by non-disposable carbon polymer fiber fabric strips (7,8).
Recently, the debate between FAW and RH has intensified. There is a concern that FAW device can increase the incidence of surgical site infections by disrupting the OR laminar air flow, (9)(10)(11)(12) and by transporting contaminants from the floor to the sterile surgical field (13). Laminar flow ventilation is described as an entire body of highly filtered air (>99.997%) within a designated space (operating room) moving with uniform velocity (0.3 to 0.5 m/s) in unidirectional downward parallel flow (2,9). Laminar flow ventilation system reduces the number of contaminants in the OR air by generating a continuous flow of low bacteria containing air and sweeping the contaminants away from the surgical area (9,14). Some authors stressed the importance to check the compatibility of the devices used in the operating room (OR) with the laminar airflow system (13). The heat from FAW devices creates hot air convection current and disrupts the OR laminar air flow with a potential compromise Exothermic pads Releases warmth through exothermic heat released when pads are exposed to air Heated humidifiers Warms and moistens air that passes over a heated water reservoir of surgical site sterility and increased incidence of infections (2,9,15,16). McGovern argued that FAW devices do not disrupt the laminar flow and the effect of OR laminar flow on the incidence of surgical site infections is questionable (9). Recently, several cohort studies (2,17,18) reported no significant benefit of laminar air flow in reducing the incidence of surgical site infections. Furthermore, the difference in warming efficacy of these two patient warming devices is debatable. Some studies (19,20) conclude that FAW is more effective than RH. Others (21) consider the RH being more effective or report (8,13,15,22) the same efficacy (Tables 2-4).
Recently, the concerned manufacturers have commissioned scientists to conduct independent studies and refute publications that criticized their sponsored devices. Hence we decided to perform a review of all the available published literature in order to get a scientific insight of this present controversy.

METHODS
The literature search was conducted through the United Stated National Library of Medicine using the MEDLINE search engine, Pubmed, Cochrane review, Google Scholar and OSU library. The following keywords were used alone and in combination: Forced air warming, Bair hugger, Resistive heating, Hotdog patient warming, Surgical site infections, and Perioperative hypothermia. Out of 92 articles considered initially for review, we selected a total of 73 relevant references.

The Efficacy of FAW and RH Considering Temperature Regulation
Several clinical studies analyzed the efficacy of FAW and RH devices without unanimous results. Some studies favored the use of FAW, while others recommended RH for warming patients during surgeries. Recent data indicated that core and mean skin temperatures were comparable among patients undergoing surgeries using the RH and FAW devices, both being equally effective in preventing hypothermia (7,8,15,17,22,27). A study published by Perl et al., considering a limited sample size, concluded that heat transfer in RH group was significantly greater than the FAW group (21). Leijtens et al. reported an incidence of hypothermia of 26-28% during intra-operative FAW utilization (30). In contrast, Leung et al. and John et al considered that FAW is more effective than RH in maintaining intraoperative patient temperature (1,19). Roder et al. indicated that FAW increased the patient's core temperature at twice the rate compared to RH when tested on 28 patients (20).

The Effect of FAW on or Laminar Flow
Several studies have focused on the effect of FAW on OR laminar flow using multiple techniques (smoke, temperature probes, and neutral-buoyancy detergent bubbles) to access and visualize the airflow patterns altered by patient warming devices. Recently, new experiments (some of them conducted in a simulated OR environment) evaluated the effect of FAW devices on disruption  Evaluating the efficiency of FAW intake filter. The efficiency of intake filters is subpar and leads to internal buildup of contaminants which can be emitted into sterile OR. (15) An experiment assessing OR with laminar flow.
Comparing the effect of FAW and RH on OR laminar flow.
FAW disrupts laminar flow and creates a temperature gradient directly above the patient.
(9) An experiment studying FAW and a clinical trial with 1437 patients.
Compares the effect of FAW and RH on disrupting the laminar flow and incidence of SSI FAW disrupts laminar flow ventilation and significantly increases SSI in patients undergoing arthroplasty surgeries.
A simulated study to test FAW intake filters.
Evaluating FAW intake filter and its role in increasing contaminant count.
The intake filters are not very efficient in filtering air. It emits build up contaminants from inside the blower to sterile OR. The contaminants did contain disease causing organisms.
An case report of an epidemic Isolating the source of epidemic outbreak of Acinetobacter baumannii The source of an MDRO outbreak was isolated from colonization of organism inside FAW and the ventilation system.   (28). Several studies used particle count in air samples to assess the air quality in OR and concluded that FAW does not worsen air quality in laminar flow operating room (2,14,25,26,31). Cristina et al. found no statistically significant correlation between microbial loads and particle counts when assessing OR air quality (33). Furthermore, Avidan et al. reported that although 40% of the swabbed FAW devices grew S. aureus, S. epidermidis, Corynebacterium sp., Cryptococcus albidus, and Aspergillus fumigatus, the resultant air does not grow any of these aforementioned organisms (29).

The Effect of FAW on the Rate of SSI
A limited number of clinical studies referred to the effect of FAW on SSI rates. An extensive retrospective/ simulated prospective study published by McGovern et al. in 2011, and involving 1,437 total joint arthroplasty patients, showed a significant increase in deep joint infection at the rate of 3.1% when FAW was used, compared to 0.8% when conductive heating was used (9). Nonetheless, the retrospective nature of data collection poses limitations when considering antibiotic regimens and some other non-standardized covariates. The authors isolated microbes from cultures, and found the ones responsible for septic jointspredominantly skin commensals-suggesting that the pathogens may be transported and deposited through air (9). Morreti et al. found a significantly increased bacterial load in the air samples when FAW was used, yet none of the patients developed nosocomial infections (6).

Safety Use of Forced Air Warming and Resistive Heating System
Although these devices are widely used, they pose a significant burn risk to the patients. Several articles have reported FAW related burn injuries and pressure ulcers when placed improperly on patient's skin (2,(34)(35)(36)(37). Mayo stands, trays and surgical equipment restrict the warming blanket from expanding and the non-circulating warm air increases the risk of burns (2). FAW misuse is one of the main causes of burn injuries (38). Similarly, RH was associated with thermal injuries, a case report published by Dewar et al.describing three pediatric patients who were diagnosed with thermal injuries, two of them requiring skin grafts and long term scar management (39).

DISCUSSION
There is a lot of discrepancy when comparing the efficacies of FAW and RH. This can be attributed to variation in study designs, type of surgeries, the anatomical location of the warming blankets (either anterior, posterior, upper body, or lower body), the difference temperature settings, and temperature measurement methods (1,8).
Whether FAW increases the amount of particles in the air is controversial (12,24). Recent studies have reported that the filter used in FAW devices is not very efficient (4). Reed et al. and Brown et al. suggested that replacing the FAW intake filter with a high efficiency particulate air (HEPA) filter and an additional filter at the outlet hose end will reduce contaminant counts (5,40). Some authors advised to use FAW devices according to manufacturer's instructions in order to avoid bacterial contamination of the surgical field (29,41,42).
The efficacy of UCV laminar flow in the OR was questioned by different sources (2,23,(43)(44)(45) Furthermore, studies have discussed the influence of drapes hanging between the surgical site and anesthesia on laminar flow. Vertical drapes can lead to production of accessory convection currents that disrupts laminar flow, and eliminating the vertical drapes will facilitate the release of exhaust heat without interrupting the laminar flow near the surgical field (12). Additional sources of heat in the OR, including surgical lights and personnel, play an important role in ventilation disruption (15).
Very few studies have directly researched the link between FAW and SSI with limitations. Mcgovern et al. conducted a study on 1,437 hip and knee arthroplasty surgeries and showed a significant increase in incidence of SSI using FAW (9). During the course of the study several changes in the prophylactic antibiotic and anti-thrombolytic regimen influenced the results. Moreover, multiple covariates associated with SSI such as high BMI, clinical factors (incontinence), and blood transfusions were not assessed/ documented.
Most of the studies did not mention whether the FAW devices were maintained and used according to manufacturer's instructions, whether they changed the filter every 6 months or 500 h, or whether they attached the hose to a new, manufacturerapproved warming gown for each patient. There are many methodological concerns regarding these studies related to sample size, randomization, blinding, and selected patient population in order to obtain unbiased results. Certain studies concluding that FAW warming devices do have a potential to increase particular or bacterial counts, were supported by manufacturing companies with competing interest.
A significant number of studies suggest that both FAW and RH are equally effective in maintaining core body temperatures and reducing hypothermia (7,8,15,17,22).

CONCLUSION
The benefits of maintaining normothermia during the perioperative period is well documented. Many studies suggest that FAW can disrupt laminar flow system, and transport floor contaminants directly to the sterile surgical field, but the link between disrupted laminar flow, increased contaminant count, and SSI is unclear. More prospective, controlled, well designed, large scale clinical trials are warranted to compare these two different warming systems. As unbiased independent reviewers, we advise clinicians to weigh the risks and benefits when deciding to use either one of these devices. We advise to keep the current practice unchanged until further data emerges. Conflict of Interest Statement: The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.
Copyright © 2018 Ackermann, Fan, Parekh, Stoicea, Ryan and Bergese. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.