New Standard for Operating Theatres

A new Australian Standard contains a section, which prescribes the operation of air systems for operating theatres and associated rooms. The requirements of the standard differ somewhat from the guidelines generally applied in New Zealand and there are implications for the way such systems should be designed, operated and tested.

There are few standards for operating theatre air systems and a lack of uniformity among the few that exist. In 1989, the Technical Services section of the Ministry of Works released Design Guidelines for Environmental Conditions in Operating Theatres. (1) According to this document, theatres for major surgery were to be class 350 cleanrooms with at least 20 air-changes per hour of HEPA-filtered supply air and 12.5 Pa positive pressurisation. In 1995 the Ministry of Health prepared Draft Ventilation Guidelines for Operating Theatres which were revised in 1997. (2) These guidelines were similar to the Works guidelines except that the pressure differential was relaxed to 2 Pa and the airflow direction over the operating table was required to be downwards. The 1997 draft guidelines have become the de-facto standard against which the majority of operating theatres in New Zealand have been tested.

AS 1668.2-2002 is the Australian standard for the use of ventilation and airconditioning in buildings, part 2: ventilation for indoor air contaminant control. (3) It is a rewrite of an earlier 1991 standard and now includes a section on “Mechanical ventilation of enclosures used for particular health care functions.” This section covers operating theatres, sterile store and set-up rooms, isolation rooms, recovery rooms, autopsy rooms and dirty utility rooms. The standard is not a New Zealand standard but is likely to influence operating theatre design in this country. While the standard is similar to the Ministry of Health’s draft guidelines, there are some important differences as shown in table 1.

AS 1668.2 reinforces the need for HEPA filtration of air supplies to operating theatres and sterile set-up and sterile store rooms. Most theatres in New Zealand are supplied with HEPA-filtered air, but there are a few that are fitted with lower efficiency filters. Our experience of testing such theatres is that they often comply with the class 350 air quality requirement - but only marginally. We tested one theatre which failed in the evening due to increased external particle concentration and passed the following morning! We recommend terminal HEPA filtration in line with AS 1668.2.

The allowed recirculation rate (or the required fresh air percentage) has been a matter for debate over the years. The requirement of 10% per the MOH Draft Guidelines is the lowest we have seen. NZS 4303:1990 requires at least 15 L/s per person or 5m2 and this requirement will usually require more than 10% (typically 18%) for a conventional theatre. Although lower fresh air rates reduce operating costs, higher rates dissipate anaesthetic gases, which inevitably escape from the anaesthetic equipment and patient’s breathing circuit during connection and disconnection and from the interface with the patient. We agree with the AS 1668.2 requirement for 20 L/s per person or 5m2. However, we are not aware of any problems with systems that have fresh air rates of only 25 – 30% in conventional theatres. So we suspect that the AS 1668.2 requirement for 50% fresh air may be too conservative.

The requirement for pressurisation of the operating theatre to an adjacent sterile set-up room or sterile store in AS 1668.2 is in direct contradiction to that in the MOH Draft Guidelines. AS 1668.2 requires the sterile set-up and sterile store to be at positive pressure to the theatre so that air would flow from the sterile set-up to the theatre. The MOH Draft Guidelines require the theatre to be at positive pressure to the sterile set-up or sterile store so that air would flow from the theatre to the sterile set-up. The issue is which is the most critical area? Which room needs to be protected against contamination from the other? In which room is the contamination risk the greater? AS 1668.2 implies that the sterile set-up area is of greater concern and this is in line with the British document, Health Technical Memorandum 2025, Design considerations, Ventilation in healthcare premises. (4) We agree with AS 1668.2 and recommend that air systems be re-balanced as required to comply with this standard.

In the past we have tested operating theatre air systems to the MOH Draft Guidelines and “passed” theatres which were at positive pressure to an adjacent sterile set-up room. From November 2002, we will measure the pressure differential and report the result, but we will not fail a theatre for having a lower pressure than the sterile set-up room

John Eccles is laboratory manager at Total Air Care, an IANZ accredited test laboratory which tests the majority of operating theatres in New Zealand.

1. O R A Burgess, Technical services, ‘WORKS’ Design Guidelines, Environmental Conditions in Operating Theatres January 1989.
2. National Licencing Office, Ministry of Health, Wellington. Draft ventilation guidelines for operating theatres, January 1997.
3. AS 1668.2-2002 Australian Standard, The use of ventilation and airconditioning in buildings part 2: Ventilation design for indoor air contaminant control, Standards Australia.
4. NHS Estates (UK Health Department). Health Technical Memorandum 2025 Design considerations, Ventilation in healthcare premises, London: HMSO.