Author: Michael Vlessides
Anesthesiology News
More than 5,000 federal lawsuits against the 3M Bair Hugger forced-air warming system have been dismissed by a federal court, a decision that comes some six months after a Minnesota Court of Appeals upheld a state court order dismissing 61 similar cases filed on behalf of Minnesota plaintiffs.
As part of the lawsuits—which were consolidated in a multidistrict litigation in federal court in Minnesota—plaintiffs’ attorneys claimed the warming system was responsible for surgical site infections suffered by patients during surgery, particularly joint replacement surgery. The federal court did not see it that way.
As part of her 49-page decision, Ericksen noted that the Court found the plaintiffs’ medical expert opinions to be “unreliable,” and excluded them because:
- there was “too great an analytical gap between the literature and the experts’ general causation opinions”;
- the experts “failed to consider obvious alternative explanations”; and
- “the causal inferences made by the experts have not been generally accepted by the scientific community.”
The federal court decision does not dismiss two similar cases that are filed in state courts in Missouri and Texas.
Controversial History
The decision is the latest development in the stormy history of the Bair Hugger. Scott D. Augustine, MD, invented the device in the 1980s, and he helped pioneer the idea of forced-air warming to prevent intraoperative hypothermia. Bair Huggers soon were a mainstay in operating rooms around the world, and was a significant financial success for Dr. Augustine and his company, Augustine Medical.
Yet, despite Bair Hugger’s success, the relationship between Dr. Augustine and the company he founded was not always smooth. In the early 2000s, Augustine Medical underwent a corporate reorganization and became Arizant Inc. In 2002, after a dispute with other board members, Dr. Augustine resigned as the chairman and CEO of the company. In 2010, Arizant was acquired by 3M, in a deal worth more than $800 million. It was around that time that Dr. Augustine began speaking out against the Bair Hugger.
Dr. Augustine claims the Bair Hugger’s forced air can spread bacteria and cause surgical site infections, particularly in patients undergoing total joint arthroplasty. Meanwhile, he invented what he says is a safer patient-warming alternative: the air-free HotDog system.
As expected, 3M lauded the court’s decision. “There is no legitimate scientific support for the plaintiffs’ theory,” Todd Fruchterman, MD, PhD, the president and general manager of 3M Medical Solutions, said in a statement. “We are pleased that the court has dismissed all of the cases in the multidistrict litigation.”
Dr. Augustine did not see the outcome in the same way. “Our hearts go out to the horribly injured patients who may not get their day in court,” he said in a statement. “Of course, the ruling will likely be appealed, so this may just be a pause in the litigation.”
Multiple Parties: Forced-Air Warming Poses No Risk
In the years since 3M acquired Arizant and Bair Hugger, Dr. Augustine has become a vocal critic of the technology he invented. The problem, he insists, is that forced-air warming interferes with laminar flow and therefore is responsible for infections. Research on both sides of the issue has produced conflicting results.
The only trial related to Dr. Augustine’s theory that included actual patients was conducted by Oguz et al (J Clin Anesth 2017;38:160-164). The prospective, randomized pilot study included 80 patients (mean age, 43±15 years; 44 males) undergoing minor orthopedic surgery at the Medical University of Vienna.
Participants underwent surgery with or without a unidirectional, turbulent free laminar airflow system, and were randomly assigned to warming by a forced-air or an electrical warming system. The researchers measured the number of airborne bacteria, using sedimentation agar plates and nitrocellulose membranes placed at six standardized locations in the OR.
Multivariate analysis revealed that longer duration of surgery increased the bacterial count on plates 1 to 4, while the absence of laminar airflow significantly increased it on plates 1 to 6. The study also found a trend of longer duration of surgery increasing the bacterial count on plates 5 to 6 (P=0.07).
Nevertheless, the study found no difference between forced-air warming and resistive warming for bacterial count at any sampling site. Perhaps most tellingly, all 80 patients were followed to hospital discharge (up to four days), and no surgical site infections were reported.
The authors concluded that it was not possible to detect higher bacterial counts on any plate for the forced-air warming group than for the resistive-warming group. “The study may obviously not be generalized for an overall safety statement on forced air warming, and is primarily applicable in the particular surgical setup,” they wrote in their conclusion. “Only a large randomized controlled trial of forced air warming versus non–forced air warming will help to decide if patient outcome is influenced by the use of forced-air devices. Until this study has been performed, the hypothesized risks of forced air warming remain unclear.”
The study’s senior author, Oliver Kimberger, MD, PhD, saw little reason to think forced-air warming poses a risk for increased surgical site infections, even during joint replacement surgery.
“In my opinion, the available evidence supporting this hypothesis is rather weak,” Dr. Kimberger, the chair of anaesthesia, general intensive care medicine, and pain therapy at the Medical University of Vienna, told Anesthesiology News. “In my department at the general hospital of Vienna/Medical University of Vienna, forced-air warming is used as a standard warming method perioperatively.”
A 2018 study by Shirozu et al (Anesthesiology 2018;128[1]:79-84) examined the effects of forced-air warming on airflow around the operating table. The researchers used a 3D ultrasonic anemometer to examine changes in airflow speed and direction, as well as effects of laminar airflow, from a lower-body forced-air warming device with sterile drapes mimicking abdominal surgery or total knee arthroplasty. As part of the investigation, both the airflow from forced-air warming and the effects of laminar airflow were visualized using special smoke and laser light.
The study revealed that forced-air warming caused upward airflow of 39 cm per second in the patient head area, and a unidirectional convection flow of 9 to 14 cm per second along the ceiling from head to foot. Despite this air movement, no convection flows were observed around the sides of the operating table.
Furthermore, the investigators found that the downward laminar airflow (approximately 40 cm per second) counteracted the upward flow caused by forced-air warming, resulting in downward airflow ranging from 36 to 45 cm per second. Downward airflows (34-56 cm per second) moving diagonally away from the operating table were also detected at the height of the operating table, on both sides.
The researchers concluded that airflow caused by forced-air warming is counteracted by downward laminar airflow from the ceiling. “Thus, it would be less likely to cause surgical field contamination in the presence of sufficient laminar airflow,” they wrote.
FDA Weighed In
The FDA appears to agree that forced-air warming poses no appreciable risk. In fact, the agency went so far as to issue a letter to health care providers, dated Aug. 30, 2017, concerning the subject. In the letter, William Maisel, MD, MPH, who is now the director of the Office of Device Evaluation at the FDA’s Center for Devices and Radiological Health, noted that some health care providers might be avoiding forced-air warming systems during surgical procedures because of concerns about potentially increased risk for surgical site infections.
The FDA conducted its own review, collecting and analyzing data from various sources, including medical device reports it received, information from manufacturers and hospitals, publicly available medical literature, OR guidelines, and ventilation requirements.
“After a thorough review of available data,” Dr. Maisel wrote, “the FDA has been unable to identify a consistently reported association between the use of forced-air thermal regulating systems and surgical site infection.” With this finding, the FDA continued to recommend the use of patient-warming devices—including forced-air systems—for surgical procedures when clinically warranted.
The FDA was not alone in reaching such a conclusion. In fact, four years previously, the independent ECRI Institute was contracted by Dr. Augustine to evaluate the safety of forced-air warming during implant surgery. As part of its analysis, ECRI reviewed more than 180 studies.
ECRI concluded that the disruption of airflow patterns caused by forced-air warming systems is “particularly worrisome in laminar flow and ultraclean ORs,” but nevertheless declined to advise hospitals to discontinue using forced-air warming during surgery, stating that the evidence at that time was insufficient to support such a recommendation.
This position seemed to be bolstered by a late 2018 meta-analysis conducted by Ackermann et al (Front Surg 2018;5:64), that examined the effect of forced-air warming on OR laminar flow and surgical site infection. The investigators searched a variety of databases, finally including 73 references deemed to be relevant.
“Currently there is no robust evidence to support that forced-air warming can increase surgical site infections,” the authors wrote. They also noted that both forced-air warming and resistive heating present safety problems.
“As unbiased independent reviewers,” the authors concluded, “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.”
Conflicting Results
Given the conclusions of various independent bodies, the consensus among clinicians—a position now backed by the courts—is that the benefits of forced-air warming outweigh theoretical safety concerns. But the literature is not entirely clear.
Indeed, HotDogWarming.com lists some studies that cast doubt on forced-air warming, including:
McGovern et al. J Bone Joint Surg Br. 2011;93B(11):1537-1544:
During simulated lumbar surgery, forced-air warming generated convection currents that mobilized floor air into the surgical site area, whereas conductive-fabric warming had no such effect. A retrospective review of data also found an elevated odds ratio for infection (hazard ratio, 3.8; P=0.024) during the period when forced-air warming was used, compared with when conductive-fabric warming was used.
Legg et al. Bone Joint J. 2013;95-B(3):407-410:
The authors attempted to visualize airflow over a simulated total knee replacement surgery using neutral-buoyancy helium bubbles. The trial demonstrated that waste heat from the forced-air warming blanket increased air temperature on the surgical side of the drape by more than 5° C, creating convection currents that rose against the downward unidirectional airflow. The researchers found that the convection currents increased particle concentration by 1,000 times for forced-air warming (2.174 million particles/m3 vs. 1,000 particles/m3 for radiant warming and 2,000 particles/m3 for control condition).
Legg et al. J Bone Joint Surg Br. 2012;94(2):254-256:
The authors compared the effects of forced-air warming with both radiant warming and no warming on the room temperature and number of particles over the surgical site. Forced-air warming was found to significantly increase the mean temperature (1.1° C vs. 0.4° C; P<0.0001) and number of particles (1,038.2 vs. 274.8/m3; P=0.0087) over the surgical site compared with radiant warming.
Belani et al. Anesth Analg. 2013;117(2):406-411:
In this mannequin study, the investigators compared the effects of forced-air and conductive-fabric warming with control conditions on ventilation performance in an orthopedic OR for simulated total knee replacement surgery. It was found that the direct mass-flow exhaust from forced-air warming generated convection currents that mobilized detergent bubbles over the anesthesia drape and into the surgical site. The forced-air system resulted in an average bubble count of 132.5 versus 0.48 for the conductive-fabric system (P=0.003) and 0.01 for control conditions (P=0.008).
Dasari et al. Anaesthesia. 2012;67(3):244-249:
This study also used a mannequin to investigate floor-to-ceiling temperatures in a laminar-flow OR with a forced-air warming blanket, a conductive warming blanket and an under-body resistive mattress. Mean air temperatures were found to be significantly elevated over the surgical site with forced-air warming compared with the conductive blanket (2.73° C increase; P<0.001) and resistive mattress (3.63° C increase; P<0.001).
Given the often acrimonious relationship between Dr. Augustine and 3M, it’s not surprising that each party publicly promotes its position. For its part, 3M is the driving force behind both the “truthaboutbairhugger.com” and “bairhuggerfacts.com” websites.
In the end, clinicians will need to make up their own minds about the utility of forced-air warming systems and their alternatives. For now, the court has made its opinion clear.