By Farhad Memarzadeh, PhD, PE and Andy Manning, PhD
The National Institutes of Health (NIH) Office of Research Services, Divisionof Engineering Services, conducted an extensive study on the issue of operatingroom (OR) ventilation systems and their effect on the protection of the surgicalsite.
The risk of nosocomial infection is present in all surgical procedures, butcan be particularly serious in certain operations, such as joint replacement.The many factors that influence infection are shown in Figure 1.
The standards suggested for air conditioning systems for ORs in differentcountries vary widely in content and recommendations and are often based on oldresearch. For example, the ASHRAE Handbook1 is based on workoriginally carried out in the 1950s. As a point of reference, the ASHRAEHandbook suggests that "the delivery of air from the ceiling, with adownward movement to several exhaust inlets located on opposite walls, isprobably the most effective air movement pattern for maintaining theconcentration at an acceptable level."1
The handbook indicates that the temperature range should be between 62degrees Fahrenheit (16.67 Celsius) and 80 degrees F (26.67 C), and that positivepressurization should be maintained. It also suggests that the air should beexhausted or returned from at least two locations near the floor. It suggeststhat supply diffusers should be of the unidirectional type and thathigh-induction ceiling or sidewall diffusers should be avoided. The recommendedACH is 15 ACH for systems that use all outdoor air and 25 ACH for re-circulatingair systems.
Some studies have been published which consider the relative merits ofdifferent systems, such as Lidwell2 and Schmidt.3 However,since they do not include specific system design data, it is difficult toestablish definitive recommendations. Further, while laminar flow systemsprovided lower general concentration levels in the room, they are sometimesblamed for higher infection rates than more conventional systems, for example,Salvati et al.4 The theory put forward by Lewis5 is thatlaminar flow systems cause impingement on the wound site. However, with Schmidtdefining a laminar system as having velocities of at least 90 fpm (0.45m/s),this probably is due to high supply velocities.
The aforementioned studies were experiment based; however, an alternativetechnique, Computational Fluid Dynamics (CFD), sometimes known as airflowmodeling, has been proved to be very powerful and efficient in research projectsinvolving parametric study on room airflow and contaminant dispersion (Ziang, etal.6 and Haghighat et al.7) In the study documented here,airflow modeling is used to consider the dispersion of squame-sized particles(which are recognized as the main transport mechanism of bacteria in operatingrooms (Woods, et al.)8 under various ventilation system-designconditions. To establish the relative ranking of the different systems, twotarget areas of concern are considered: the surgical site and the top surface ofthe back table where instruments are located.
TheCFD and particle tracking routine methodology are described in detail inMemarzadeh and Manning.9 (They are not included here because of spacelimitations. A copy of the paper is available at: http://des.od.nih.gov/eWeb/research/farhad/index.htm.)
Outline of Base Model
A typical OR layout was considered for the baseline model. The room measured20 feet (6.1m) by 20 feet (6.1m) by 12 feet (3.66m) high; general features aregiven in Figure 2 and Table 1. The room's layout was agreed upon by a panel ofphysicians and engineers. The total heat dissipated in the room was 2166W.
Several ventilation systems were considered in this study and are shown inTable 2. These systems approximately replicate those outlined in Schmidt.3Models of the various diffuser types considered in this project were allvalidated prior to the room parametric study against manufacturers data.
Squames are cells which are released from exposed regions of the surgerystaff and patient, for example, neck, face, etc., and are the primary transportmechanism for bacteria in the OR. They are approximately 25 microns (mm) by 3 to5 microns thick. Approximately 1.15 by 106 to 0.9 to 108 squames are generatedduring a typical (two-hour to four-hour) procedure.10 In this study,the squame particles would be tracked to see how many hit the back table, shownin Figure 3, or the surgical site. The surgical site was taken as a 1-foot(0.3m) by 1-foot (0.3m) square with the surface temperature at 100 degrees F(37.78C), and is shown in Figure 3.
A representative number of particles was introduced from three arrays ofsources in the CFD models. The locations and sizes of the sources, designated asMain, Nurse and Surgery, are shown in Table 3. As the particles could readilypass to the instruments at this point, the surgery source/top surface of backtable target analysis was not performed in this study.
There are three potential particle outcomes:
Table 4 shows there is a wide range of particle removal effectiveness. Casesthat have the same ACH show marked differences in terms of the percentage ofparticles removed via ventilation. For example, Case 10 demonstrates a moreeffective removal of particles than Case 1.The reason is that the ventilationsystem in Case 1 results in the formation of two large re-circulations in theroom where particles can become trapped; this is illustrated in Figure 4. InCase 10 the ventilation system works with the thermal plume in the center of theroom in driving the particles up to the high level exhausts. Taking Cases 3, 4,5, 6 and 9 as a group that adopts the same general approach to ventilation, thepercentage vented from all locations becomes more uniform on increasing ACH andsupply array size. The reason is, for the smaller laminar arrays, the areasoutside the direct influence of the supply have very low velocity flow fields,and particles are more likely to drop to the floor via gravity.
Percentage of Particles, Which Hit Surgical Site or Top Surface of BackTable
Table 5 shows that the percentages of particles which hit the surgical sitefrom the main or nurse sites are low (less than 1 percent) because of therelative dominance of the thermal plume caused by the surgical site. Forexample, Figure 5 shows such a plume for Case 2. It is only when the particlesare released close to the site, in particular, the surgery source, that thepercentage becomes significant. It is also clear that ACH is not as significantin the surgery source/surgical site analysis as design of the ventilationsystem. In particular, a lower percentage of particles hit the site in Case 4,which has an ACH of 20, than Case 2, which has an ACH of 150.
The results also show, with the exception of Case 11, the percentage ofparticles that hit the back table from the main or nurse sites is relativelylow. In Case 11, the particles are blown onto the table. The results for Cases4, 5 and 6 indicate that a mixture of exhaust location levels is better than lowor high only. Finally, the cases which can be placed together in a laminar flowtype group, namely, Cases 2, 3, 4, 5, 6 and 9, show lower strike rates than theother systems. In fact, Cases 4 and 9 represent the lowest strike percentages ofall the cases considered.
Conclusions and Discussion
Conclusions of the study are:
To expand on the issue of diffuser array size, it appears that the mainfactor in the design of the ventilation system is the control of the centralregion of the OR. In particular, the operating lights and surgical staffrepresent a large heat density in the middle of the room. Particulates could getcaught in buoyant plumes created by these heat-dissipating objects, at whichpoint control of them is lost; however, if a laminar flow type system isemployed, the particles are instead driven by the flow to the exhaust. Ideallythen, the array size should be large enough to cover the main heat dissipatingobjects.
This is illustrated in Figure 6, which shows the flow field for Case 9.
Further, another factor is the thermal plume created by the surgical site,shown for Case 2 in Figure 5.A laminar flow regime which provides air at 30-35fpm (0.15 to 0.18 m/s) ensures that particles are not impinged on the surgicalsite, a danger highlighted by Lewis5 as the thermal plume should besufficient to protect the surgical site.
Farhad Memarzadeh, PhD, PE, is chief of technical resources in the Officeof Research Services, Division of Engineering Services at the NationalInstitutes of Health in Bethesda, Md. Andy Manning, PhD, is director ofengineering for Flomerics, Inc. of Southboro, Mass.