Operating theatre ventilation system design

This is an excerpt from the published technical article entitled Design Considerations for Operating Theatre Ventilation Systems written by Kristian Kirwin (B.ENG Mechanical) and Shannon Roger (B.Ed) for Airepure Australia (a Camfil Group company), and published in Healthcare Facilities Volume 40, No 3, September 2017.

For detailed information regarding design objectives and risks, minimum acceptable requirements within Australia, individual HEPA modules versus laminar flow (UCV) systems and lower velocity systems designed to meet the European DIN 1946-4 standard, please refer to https://www.airepure.com.au/operating-theatre-ventilation-system-design/.

The effect of blanking sections of a laminar flow system

Figures 8 and 9. CFD modelling of laminar flow system with blanked center section: a circulating flow region is created, suspending particles. For a larger image, click here.

Figures 8 and 9 represent how spaces between individual diffusers — or how the blanking of a section of a laminar flow system — can introduce an area of non-uniformity and turbulent air. This results in non-controlled air or air contaminated with particulates (such as squames or skin cells from operating staff) possibly entering the wound site and increasing the risk of surgical-site infections (SSIs).

Airborne particles and SSIs

A number of technical papers and reports have been written relating to the significance of airborne particles contributing to SSIs.

“The majority of SSIs are a result of hygiene-related factors associated with surgical personnel. With respect to bacteria transmitted to the surgical site through the air, squames or skin scales are the primary source of transmission.”[i]

Airborne particles are found to be responsible for about 80–90% of microbial contamination (CDC 2005).

It is generally understood that indoor air in an operating theatre may contain particulates from a number of sources (including people and processes or activities in the operating theatre), and that micro-organisms on these air particles can settle on the wound, dressings and surgical instruments and cause infections.

Reductions in hospital-acquired infections can have a significant impact on improved patient outcomes and minimising the cost to the healthcare facility. While hygiene-related prevention is the most practiced and proven method, airborne-related contamination control offers one area that could play a much larger role. One area of ongoing discussion is the role of operating theatre ventilations systems and system design in airborne containment control to assist in the reduction of hospital-acquired SSIs.

Operating theatre air quality

In 2010, Airepure undertook a review of the air quality within two operating theatre systems utilising independent industry resources: one with a traditional design for the ventilation system incorporating four terminal HEPA filters and 20 air changes per hour; and one with laminar air flow theatre ventilation with 40 air changes per hour (2.4 x 2.4 m² laminar flow system with a face velocity of approx 0.4 m/s).[ii]

Figure 10: Traditional four-terminal HEPA module arrangement

Figure 11: Laminar Flow / UCV Theatre

The traditional theatre (Figure 10) showed a high level of particle contamination, both at the operating theatre table level and throughout the theatre. The tests were carried out for three traditional theatres in the same surgical department with similar results for each theatre.

The results of the laminar flow theatre (Figure 11) showed a dramatic reduction in the airborne particle contamination both at the operating theatre table level and throughout the theatre.

The assessment was carried out using a calibrated particle counter with particle counts measured and recorded in the 0.3 micron, 0.5 micron and 5 micron particle size ranges.

For both theatre ventilation systems, the results were zero particle counts for all three particle sizes when the air quality was measured at the discharge from the diffusers below the HEPA filters, however the results showed significant improvement in the air-quality readings at the table height in the laminar flow theatre compared to the traditional turbulent flow theatre.

A summary of the results of the particle counts recorded can be found in Table 3.

Location	Particle Count/m3	Particle Count/m3	Particle Count/m3
	Size 0.3 micron	Size 0.5 micron	Size 5.0 micron
Traditional theatre at 1 m below the terminal HEPA diffuser	34,500	8000	824
Traditional theatre at operating theatre table	304,000	119,000	6950
Traditional theatre at the wall	563,000	677,000	4360
Laminar flow theatre at operating theatre table	0	0	0
Laminar flow theatre outside perimeter of laminar flow diffuser	6000	2130	706
Laminar flow theatre at the wall	15,900	5300	1680

Table 3: Particle count results for Traditional 4 x Terminal HEPA module arrangement and Laminar Flow / UCV Theatre

The most interesting observation is the rapid decline in air quality below the HEPA filters in a traditional theatre with the individual HEPA filters arrangement.

This is due to the entrainment of particles from the adjacent space. On comparison with cleanroom design principals, the turbulent flow arrangement would not be acceptable. High turbulence leads to pollution or contamination as well as surface areas.[iii]

During the theatre observations there were numerous staff entries from the sterile corridor to set up for the next series of procedures — this had no discernible effect on the observations at the table location (zero values returned).

A well-designed laminar flow/UCV system provides two protective effects: positive pressurisation, with no contaminated external entering the theatre by inflow from open doors reaches or perimeter areas can migrate to the protection zone and any air contaminated with in the protected zone is rapidly displaced by clean air from the laminar flow /UCV system.[iv]

References

[i] Woods; 1996
[ii] Sutherland, A: Operating Theatre Ventilation System Review, Part 1: AHE Journal Issue 37, Dec 2014, Part 2: AHE Journal Issue 38, Mar 2015
[iii] Baumgarth Set. Al; Compendium of Air Conditioning Technology; Vol 1: Basics. 4th Ed. Karlsruhe (Germany): 2000
[iv] CEN, Ventilation for Buildings – test procedures and measuring methods for handing over installed ventilation and air conditioning systems. German Version EN 125999; 2000
AS 1668.2: The use of ventilation and air conditioning in buildings. Part 2: Mechanical ventilation in buildings: SAI Global Limited 2012