A closer look at the scientific evidence for and against 3M's Bair Hugger

Independent experts say there is no definitive study proving or disproving that 3M's Bair Hugger patient-warming system increases risks of deep-joint infections after surgery.

But it's not for lack of trying. Dozens of peer-reviewed papers on the Bair Hugger's safety profile have been published, some critical, some supportive. None has convinced medical experts or judges that using the Bair Hugger leads to surgical infections, which is why juries are now being asked to decide whether they think it's happening.

Here's a look at a few of the leading papers on both sides.

Reports critical of the Bair Hugger

1. McGovern : "Forced-air warming and ultra-clean ventilation do not mix," published Nov. 2011 in the Journal of Bone & Joint Surgery, by P.D. McGovern, et al.

This study reported on an experiment that compared movement of neutral-buoyancy soap bubbles around a mannequin in a partial-walled operating room while 3M's Bair Hugger forced-air warming system or Augustine Surgical's HotDog conductive-fabric system were running. The researchers found that the Bair Hugger "established convection currents that mobilized resident air from non-sterile areas such as the floor and under the anesthesia/surgery drape into the surgical site." The same study also retrospectively analyzed 2½ years of infection data for 1,437 patients treated at a single hospital in the U.K. and found that the infection rate in hip and knee replacements dropped from 3 percent to less than 1 percent after the hospital switched from Bair Hugger to the HotDog. However, the study notes that the hospital's standard antibiotic regimen changed at the same time that the patient-warming systems were changed, and patients' records failed to account for important infection factors like blood transfusions, incontinence and obesity. "This study does not establish a causal basis for this association" it says of forced-air warming and joint infections. One of the study co-authors, statistician Mark Albrecht, was employed as the lead statistician/researcher for research at Augustine Biomedical + Design at the time the study was published.

2. Legg 2013 : "Forced-air patient warming blankets disrupt unidirectional airflow," published March 2013 in the Bone and Joint Journal, by A.J. Legg and A.J. Hamer.

This study documented an experiment in which neutral-buoyancy helium bubbles in a simulated operating room rose to an area above a mannequin because of "waste heat" radiating from an active Bair Hugger device. It found the Bair Hugger created convection currents that could theoretically bring germ-laden particles toward the patient. Compared to the HotDog and a "control" set-up with no warming device, the Bair Hugger "significantly" increased the number of particles in the air above the patient. However, "this study does not show that forced-air warming increases the risk of infection — only that in certain types of theater set-up it can significantly disrupt unidirectional airflow and draw particles from the potentially contaminated area below the sterile field. This is a concern." 3M discovered through litigation that Augustine provided equipment and participated in the experiment, without that being disclosed in the paper.

3. Elghobashi : "Effect of heated-air blanket on the dispersion of squames in an operating room," published March 2017 in the International Journal for Numerical Methods in Biomedical Engineering, by Said Elghobashi, et al.

This report examined the results of a three-dimensional computational fluid dynamics (CFD) simulation for how a Bair Hugger would affect the movement of 3 million particles on the floor of an operating room. An animation from the simulation, which was shown to a jury in May, shows plumes of 10-micron skin flakes moving from the floor to the area above the operating room table: "The hot air from the blower and the resultant thermal plumes are capable of lifting the particles and transporting them to the side tables, above the OT (operating table), and the surgical site." Without the Bair Hugger on, these skin flakes are removed from the room via normal airflow. The CFD simulation did not account for confounding factors like opening and closing of doors of the operating room, other heat sources in the room, movement of hospital staff, and the fact that some exhaust grilles actively pull air from the room, rather than passively accepting air blown from above.

Reports supportive of the Bair Hugger

1. Moretti : "Active warming systems to maintain perioperative normothermia in hip replacement surgery: a therapeutic aid or a vector of infection?" published in July 2009 in the Journal of Hospital Infection, by B. Moretti, et al.

This study used a machine that contains a bacterial growth medium to sample the air in 20 hip-replacement surgeries using the Bair Hugger, and 10 procedures without the device, at the Bari University Hospital in Italy. The study concluded that having people in the operating room significantly increased the number of colony-forming bacteria per cubic meter of air, whether or not the Bair Hugger was on. The study also found that these bacterial loads were lower with the Bair Hugger on in a populated room, as compared to an operating room filled with people without the Bair Hugger on, but these differences were not statistically significant for two of the three sampling locations around the table. No surgical infections happened during the study. "The main potential contamination factor in the operating theatre is the presence of the theatre medical staff themselves," the study concluded. The study had a small sample size, and its results appeared to be influenced by the presence of hospital personnel — the effects of which are "not easily quantifiable."

2. Oguz : "Airborne bacterial contamination during orthopedic surgery: A randomized controlled pilot trial," published in 2017 in the Journal of Clinical Anesthesia, by Ruken Oguz, et al.

This study randomized 80 orthopedic surgery patients at the University of Vienna in Austria to getting either 3M's Bair Hugger or Augustine Surgical's HotDog device, and then placed petri dishes filled with growing media ("agar plates") in the operating room near the patient. No infections were reported in either group. The study found that the type of warming device used didn't make any difference in the amount of bacterial sedimentation on the agar plates. "The study may obviously not be generalized for an overall safety statement on forced air warming," the study says. "Only a large, randomized, controlled trial of forced air warming vs. non-forced air warming will help to decide if patient outcome is influenced by the use of forced-air devices." Study authors said it would take a much larger study to detect relatively rare surgical infections, and they acknowledged that the upper-body position of the Bair Hugger in relation to the lower-body surgical field may have reduced the effect of air turbulence on airborne contamination.

3. ICM 2018 : "Second International Consensus Meeting (ICM) on Musculoskeletal Infection," published in full in November 2018 at icmphilly.com. (An excerpt was also published in the Journal of Arthroplasty in 2018)

This report documents the consensus meeting of 800 orthopedic infection experts who considered, among other questions, whether forced-air warming devices like the Bair Hugger increase the risk of post-surgical joint infections. A supermajority of 93 percent of voting members agreed that, after reviewing papers including McGovern, Legg 2013, Elghobashi, Oguz, and Moretti, there is no evidence to definitively link forced air warming to an increased risk of surgical joint infections. "The literature is conflicting, and there is still a lack of strong evidence linking FAW (forced-air warming) to increased risks of SSIs (surgical-site infections)," the consensus statement said. "While we recognize the theoretical risk posed by FAW, we cannot recommend discontinuing the use of these devices at this time. … We recommend a randomized prospective trial." The meeting was co-sponsored by 3M, and meeting co-chair Dr. Javad Parvizi is a former member of a 3M scientific advisory committee on infections. One of the co-authors of the forced-air warming section of the paper was Mike Reed, who was the corresponding author for McGovern.

Independent reviews

1. FDA : "Information about the Use of Forced Air Thermal Regulating Systems — Letter to Health Care Providers," published Aug. 2017 by the U.S. Food and Drug Administration.

The FDA reported: "After a thorough review of available data, the FDA has been unable to identify a consistently reported association between the use of forced air thermal regulating systems and surgical site infection. Therefore, the FDA continues to recommend the use of thermoregulating devices (including forced air thermal regulating systems) for surgical procedures when clinically warranted. Surgical procedures performed without the use of a thermoregulation system may cause adverse health consequences for patients during the postoperative and recovery process."

2. ECRI : "Forced-Air Warming and Surgical Site Infections," published in April 2013 in Heath Devices by the ECRI Institute.

The not-for-profit ECRI Institute reported: "Five studies [including McGovern and Legg 2013] demonstrate that the exhaust from FAW (forced-air warming) units results in thermal currents that rise into the downward ventilation airflow of the laminar airflow systems studied. The disruption of airflow patterns is particularly worrisome in laminar-flow and ultraclean ORs, in which a wide variety of implant surgeries are performed. … These studies, however, only raise questions about airflow disruptions. Demonstrating that airflow patterns change when FAW is used does not establish that it results in increased bacterial contamination or increased rates of SSI (surgical-site infection) and PJI (prosthetic joint infection) as compared to use of other methods of patient warming. … Based on our focused systematic review of the published literature, we believe that there is insufficient evidence to establish that the use of FAW systems leads to an increase in SSIs compared to other warming methods. Although one study (McGovern) presents data that suggests higher PJI rates with use of FAW compared to an alternative warming method, this study has serious limitations such that its findings on PJI rates cannot be considered conclusive."

3. Cochrane Reviews : "Active body surface warming systems for preventing complications caused by inadvertent perioperative hypothermia in adults (Review), published online in 2016 by the Cochrane Collaboration.

The not-for-profit Cochrane Reviews reported: "Forced-air warming seems to have a beneficial effect in terms of a lower rate of surgical site infection and complications, at least in those undergoing abdominal surgery, compared to not applying any active warming system. It also has a beneficial effect on major cardiovascular complications in people with substantial cardiovascular disease, although the evidence is limited to one study. It also improves patients' comfort, although we found high heterogeneity among trials. … Nothing suggests that ABSW (active body-surface warming) systems pose a significant risk to patients." (The analysis did not review McGovern, Legg 2013, or Moretti.)