©2014 This excerpt taken from the article of the same name which appeared in ASHRAE Journal, vol. 56, no. 2, February 2014.
By Jennifer A. Wagner, Ph.D., Kevin J. Schreiber, Member ASHRAE, Ralph Cohen, P.E., Member ASHRAE
About the Authors
Jennifer A. Wagner, Ph.D., is a principal infection control consultant at Prism Environmental Health & Safety Solutions. Kevin J. Schreiber is director of healthcare at Huntair. Ralph Cohen, P.E., is principal at Ralph M Cohen Consultancy.
Annually, the CDC reports nearly 99,000 deaths per year resulting from health-care associated infections (HAIs). According to the U.S. Department of Health and Human Services (HHS), it is estimated that of the more than 290,000 incidences of surgical site infection (SSI) annually, more than 13,000 people die each year due to infections acquired during surgical procedures.
In addition to quality of care implications, according to the HHS Action Plan to Prevent Healthcare Associated Infections, the added health-care costs of treating SSIs are no longer reimbursed for Medicare patients, placing the financial burden on hospitals. On average, the cost to hospitals per SSI is $25,546. In aggregate, this amounts to $7.4 billion in additional health-care costs every year.
It has been estimated that airborne transmission accounts for 10% to 20% of HAIs, although more recent studies have concluded that the role of airborne transmission may be underestimated due to the difficulty of culturing many airborne organisms and the complexities of assessing the role such pathogens play in the contamination of environmental surfaces and subsequent contact transmission. Landmark studies performed by Lidwell and his colleagues along with many other studies have indicated a strong connection between contamination in the air during surgeries and SSI rates. Clinical trials carried out in Britain, Europe, and the United States have confirmed that between 80% and 90% of bacterial contaminants found in the wound after surgery come from colony forming units (cfu) present in the air of the operating theatre. With respect to bacteria transmitted to the surgical site through the air, squames (or skin scales) are the primary source of transmission. Approximately 1.15 x 106 to 0.9 x 108 squames are generated in a typical two- to four-hour surgical procedure. Viral and fungal contamination also can be present in these skin scales.
An interesting parallel can be drawn between the control of these airborne contaminants in operating rooms (ORs) and semiconductor manufacturing cleanroom environments. In each case, the design principle of using unidirectional, controlled (often referred to as laminar) airflow to contain and remove airborne particles, particularly in the area targeted as the sterile field, is generally accepted as the most effective method of preventing airborne contaminants from landing in that sterile field. However, cleanroom technology has proven effective in virtually eliminating human particle contamination, vastly improving product yields in the semiconductor manufacturing industry.
This article identifies the differences in air delivery between semi-conductor manufacturing cleanroom and operating room designs, and discusses how employing the single-large diffuser (SLD) configuration used in semi-conductor manufacturing cleanrooms can significantly reduce the incidence of airborne contaminants reaching the patient in an operating room (OR).
There are three basic design concepts evaluated for ceiling-mounted air delivery in a surgical suite: air curtain (AC), multi-diffuser array (MDA) and single-large diffuser (SLD).
The AC design uses a laminar flow diffuser array over the surgical table with a high velocity slot diffuser at the perimeter of the sterile field. The concept is to create a barrier of air from the slot diffusers, protecting the sterile field from contaminants outside of that space. The MDA uses multiple laminar diffuser panels that are set into an array in the ceiling. According to ASHRAE Standard 170-2013, Ventilation of Health Care Facilities, minimum guidelines, the supply air array must extend a minimum of 12 in. (305 mm) beyond the footprint of the operating table, but up to 30% of the area encompassed can be used for non-air delivery devices (booms, light troffers, etc.). Light booms in the air field create blockages in the air, with turbulent zones beneath the lights. Similarly, gaps between diffusers allowed by Standard 170-2013 create turbulent zones beneath the areas where airflow is not being delivered. The SLD design supplies air from a single, large diffuser that concentrates the air delivery in a controlled air field over the surgical table and reduces these turbulent zones.
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