Jim Kling

September 18, 2013

DENVER — Air flow in operating rooms could be a risk factor for surgical-site infections, a new study has found.

The problem appears to be in the differential pressure between air supply and exhaust. There is a strong association between risk for surgical-site infection and a variation in differential pressure of more than 100 cubic feet per minute during a procedure, warn researchers.

The unusual report, presented here at the 53rd Interscience Conference on Antimicrobial Agents and Chemotherapy, began nearly a decade ago when infection control practitioner and lead author Kim Horn, MPH, arrived at the Flagstaff Medical Center in Arizona. At the time, its infection rate for knee surgeries was about 3 times the national average.

"One of the surgeons said he felt that the air wasn't right in his room — that it was blowing in instead of blowing out — or the pressure wasn't right. I investigated and found that the pressure was actually lower than that required for a negative-pressure room," Horn told Medscape Medical News.

 
This is a breakthrough study.
 

Conversations with building engineers led her to consider variations in the air supply and exhaust. "We don't pay enough attention to the exhaust. If we have high-velocity supply and don't have enough exhaust, the particles are turbulent in the room," she explained.

The researchers conducted a retrospective case–control study of patients undergoing spinal fusion, total hip or knee arthroplasty, or coronary artery bypass graft (CABG) procedures. The researchers used ductwork sensors to measure differential pressure between air supply and total exhaust. They took measurements each hour a patient was in the operating room.

"This is a breakthrough study," said session moderator Didier Pittet, MD, director of the infection control program at University of Geneva Hospitals and director of the World Health Organization patient safety program.

The researchers matched each patient who developed a surgical-site infection with 3 patients who underwent the same procedure during the study period but did not develop an infection.

Table 1. Study Cohort

Surgery Case Subjects (n = 95) Control Subjects (n = 353)
Spinal fusion 39 168
Total hip or knee arthroplasty 42 127
CABG 14 58

 

They found no association between rate of infection and age, sex, race, procedure, operation duration, or operation year.

As expected, a differential pressure variation of more than 100 cubic feet per minute more than 4 times during surgery had the highest degree of statistical significance of any factor on multivariable analysis.

Table 2. Risk Factors for Surgical-Site Infections

Risk Factor Odds Ratio 95% Confidence Interval P value
BMI >35 kg/m² 2.10 1.06–4.14 .03
American Society of Anesthesiologists score >2 1.95 1.15–3.32 .01
Differential pressure variation of >100 CFM >4 times during surgery 1.81 1.02–3.20 .04
Povidone/iodine skin preparation 0.64 0.32–0.90 .02

 

Bivariable analyses revealed associations between surgical-site infection and a National Nosocomial Infection Surveillance System risk index score of 2 or higher, 2 specific operating rooms, and 1 specific surgeon.

In addition to povidone/iodine solution, cefazolin prophylaxis was found to be protective.

A contributing factor could be laminar air flow, which "has a high velocity. Some work done by engineers indicates that high velocity interrupts the surgical heat plume. They hypothesize that there's a heat plume over the wound. If the velocity is too high, it interrupts the plume and epithelial cells from the people standing around the table will shed into the wound," Horn explained.

This research suggests that air flow in the operating room is underappreciated. "I think it's something infection control professionals need to be a part of. We've studied laminar flow, but there are a lot of pieces to laminar flow that we haven't addressed, such as the arrangement of the diffusers in the ceiling and flow and exhaust rates. It's a new field that needs to be addressed by engineers and infection control," said Horn.

"Situations in the operating theater such as these happen every day in every hospital around the world," Dr. Pittet told Medscape Medical News. "If the maintenance regulation of the flux of air in the operating room is not optimal, you can increase the risk for infection."

Ms. Horn and Dr. Pittet have disclosed no relevant financial relationships.

53rd Interscience Conference on Antimicrobial Agents and Chemotherapy (ICAAC): Abstract K-446a. Presented September 11, 2013.

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