Bair Hugger: A potential enemy within the operating room

To the Editor—The American Society of Anesthesiologists practice guidelines recommend normothermia as a goal during anesthetic emergence and recovery ¹ in part to reduce adverse cardiac events. ² The Bair Hugger (3M, St Paul, MN) is a type of forced-air warming (FAW) device commonly used to maintain intraoperative normothermia; however, its use is not without the potential for harm. ³ Surgical site infection (SSI) is a major risk of any surgical procedure. Rates of SSI have been reported to be as high as 6.8% in some types of orthopedic surgery. ⁴ Contamination of operating room (OR) air with human pathogens has been directly attributed to FAW ⁵ ; however, contamination of OR air is not sufficient for demonstrating SSI causation. ⁶

Contamination of the OR air with human pathogens may be an intraoperative safety hazard, and adherence to recommended FAW-device filter-replacement guidelines may improve patient safety. Bair Hugger devices draw in ambient air through a HEPA filter before heating and traveling through a distal hose to a disposable patient blanket. ⁷ The manufacturer recommends that the HEPA filter should be changed “every 12 months or 500 hours of use,” whichever comes first. ⁷ Our institution changes these filters on an annual basis presuming that the usage of each device is below the 500 hours per year limit. We examined mean annual usage among 36 Bair Hugger devices at our institution and compared the results to manufacturer recommendations.

Methods

Bair Hugger usage was determined by utilizing the Bair Hugger alternative modes feature (activated by a button concealed within the logo), which displays accumulated run time. The serial number of each Bair Hugger unit along with its total run time was logged. Each device’s purchase date and last filter change date was obtained from our institution’s technology registry system. The average annual run time in hours was calculated by dividing the total number of run hours for each Bair Hugger unit by time elapsed since its purchase date in years. A mean annual run time in hours was calculated for all devices. A 1-sample t test was used to compare the sample mean to the recommended maximum run time of 500 hours.

In addition, we surveyed 60 regional hospitals and surgical centers in California regarding their Bair Hugger filter-change practice.

Results

The mean annual run time for our 36 Bair Hugger units was 785.09 hours (SD, 239.72 hours) (Fig. 1). Furthermore, 32 units (89%) had >500 hours average annual run time. The sample mean (785.09 hours) was significantly different from the recommended maximum of 500 hours (P < .0001).

Fig. 1. Average annual run hours per Bair Hugger. Scatter plot with each data point representing 1 Bair Hugger device (x-axis) and its annual run time (y-axis). The overlaid red square with error bars represents mean annual run hours (785 ± 240) among all 36 Bair Hugger devices studied.

The results of this survey revealed that none of the 60 hospitals or surgical centers in California followed manufacturer recommendations for filter change.

Discussion

The Bair Hugger device may increase the risk for an SSI in several ways. First, a Bair Hugger unit draws air in from a potentially contaminated floor, warms it, then blows it into the disposable warming blanket. This warmed air is released through small pores on the underside of the blanket, often in the vicinity of the surgical field. Contamination of the Bair Hugger, the outlet hose, and/or the warmed air passing through them could occur as the efficiency of the inlet HEPA filter decreases over time.

Our study revealed that only 4 of the 36 Bair Hugger devices examined were compliant with the manufacturer’s filter-change recommendations. The fact that Bair Hugger devices have been shown to potentially spread human pathogens across open surgical wounds ⁵ should alert healthcare institutions to strictly follow the recommended filter replacement after 500 hours of run time rather than simply changing the filter on an annual basis. To see how our institution compared to others regarding compliance with manufacture’s recommendations, we informally surveyed 60 regional hospitals and surgical centers in California that used Bair Hugger units. The survey revealed that none of the institutions followed the manufacturer’s 500-hour maximum use-time recommendation. Instead, most of them changed filters on an annual basis without regard to actual usage hours.

Regarding potential modifications to Bair Hugger devices, simply incorporating an alarm to indicate when 500 hours of run time had elapsed may lead to improved patient safety. Recognizing that 500 hours of use in an environment with a high particulate load may not be equivalent to 500 hours in a “cleaner” environment; the best alarm system would indicate when filter efficacy had dropped below an acceptable minimum. Even more simply, placing a disposable HEPA filter at the end of the Bair Hugger hose where it attaches to the blanket may prove most advantageous. ⁵

Although it remains unproven that the Bair Hugger or other FAW devices may cause surgical site or implant-associated infections, ⁶ we recommend that alternative patient-warming methods ⁸ be used, especially in immunosuppressed patients and in procedures involving surgical implants. We believe that FAW devices may represent an unnecessary risk in these cases. Unfortunately, no randomized controlled clinical trials have been conducted to directly answer this question. Future studies should investigate a possible link between higher Bair Hugger run hours and increased SSI.

In conclusion, we recommend that institutions track Bair Hugge run time and change filters at least every 500 hours or at 1 year, whichever comes first. 3M should also consider implementing a 500-hour filter use alarm or installing a disposable HEPA filter at the end of the hose as it enters the warming blanket.