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Why the rate of air changes in rooms matter

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As planned surgery has restarted in countries like Australia and New Zealand, transmission-based precautions remain crucial to minimising risk and ensuring procedures go ahead. But the rate of air changes possible in operating rooms is also a key factor.

As planned surgery has restarted in countries like Australia and New Zealand, transmission-based precautions remain crucial to minimising risk and ensuring procedures go ahead. But the rate of air changes possible in operating rooms is also a key factor.

Aside from reducing the risk of infection, the time that is required for aerosols to be cleared from clinical areas by ventilation systems can have a significant impact on the time needed between surgical and diagnostic procedures, and therefore efficiency levels in operating rooms and procedure rooms.

Minimising Risk

Risk factors with the potential to influence transmission of infection in rooms include the likelihood of patients having Covid-19 in the first place, which in turn depends on the adequacy and effectiveness of isolation and pre-screening of patients, as well as general prevalence of the virus and community transmission rates.

The level of risk when undertaking elective surgery and diagnostic procedures is also dependent on local adherence to relevant infection control precautions, such as patient preparation, hand washing and the use of appropriate PPE.

But even where a range of measures are taken for safe entry into the non-Covid pathway, such as self-isolation, screening for absence of symptoms, raised body temperature and negative antigen tests before admission, some level of risk still remains. To further reduce this risk, additional precautions are needed to ensure infection is not transmitted through the air.

Air filtration systems in operating rooms reduce the risk of infection from airborne particles. Minimum standards are similar in many countries as they are based on international recommendations, but not necessarily the same.

The minimum Australian requirement for an operating room is outlined the standard AS1668 Part 2, as outlined by the Building Code of Australia. The key requirements are that the air coming into the room needs to pass through a HEPA grade filter, and a minimum air change rate (ACH) of 20 air changes per hour is required. The air should also be at a positive pressure to the surrounding areas.

The HEPA filter ensures the air supplied into the room is clean, the air changes ensure fresh air is provided in a quantity sufficient to dilute particles in the room to a suitable level, while the positive pressure ensures the air entering the room does so in a controlled way.

Aerosol-generating procedures (AGPs)

Enhanced airborne precautions are needed when an aerosol-generating procedure (AGP) is performed on a patient known to be infected with Covid-19 or for whom the Covid-19 status is unknown, or a procedure is undertaken in a ‘hotspot’ location. As a minimum, PPE for staff in rooms where aerosol-generating procedures are undertaken should include a fluid-resistant surgical mask to minimise the risk of droplet dispersal.

Aerosol-generating procedures can be divided into respiratory and surgical AGPs, with surgical AGPs generally likely to be of lower intrinsic risk, although may generate aerosols for longer periods. Examples of common AGPs are intubation, extubating, tracheostomy-related procedures, non-invasive ventilation, bronchoscopy, upper gastro-intestinal endoscopy and certain dental procedures.

How many air changes that are required for an operating room depends on the type of surgery undertaken in it, and the guidelines it needs to comply with. Different states in Australia have slightly different guidelines, although on the whole they conform to national and international standards.

Aerosol Clearance Times (ACTs)

While a standard operating room needs to have 20 air changes per hour, this rate is regularly exceeded and laminar flow rooms tend to have a much higher rate than that. Laminar flow systems allow for airflow to be more tightly controlled across the operating area.

Widely accepted international guidelines refer to data indicating that each air change removes approximately 63% of airborne contaminants, with >99% being removed after five air changes. Although there is limited evidence to support this method of calculation, it is widely used. Based on this assumption, only 0.004% of the circulating aerosols remain after 10 exchanges.

How long a particular number of air changes take to perform depends on the ventilation system in the room, room or building used. The calculation for the ‘aerosol clearance time’ (ACT) - the time taken in minutes for an air exchange in a room - is 60 divided by the number of air changes per hour measured or estimated for that room. The resulting time in minutes is then multiplied by the number of air changes required.

However, generic guidelines and recommendations should be used with caution. In reality, aerosol clearance times depend on room characteristics, and are location-specific. In order to work out the aerosol clearance time for an individual room or room, the health service must know the air exchange rates for each room location. While most operating rooms have a high ventilation rate per hour, this can vary and ventilation rates are measured at regular intervals over time. 

Clearance time between procedures

A letter published by the Association of Anaesthetists in June suggested that an aerosol clearance time (ACT) equal to the time it takes for five air changes to cycle through - as measured for a particular room - should be left between procedures to allow for sufficient aerosol clearance. This recommendation is commonly referred to, however, estimates from other sources range from two ACTs up to 10-15 ACTs.

Based on the assumption that five air changes are required between procedures, a standard operating room with 20 air changes per hour would need a 15-minute clearance time. Laminar flow room ventilation systems can have a much higher number of air changes per hour, meaning a shorter time is required between procedures.

It is up to each health provider to decide on its own policy in terms of what air change frequency is acceptable for the safe clearance of aerosols, based on their known air exchange rates in clinical areas. While typical ranges for different types of facilities are available, using estimates rather than hard data can be connected with risk.

In addition, for Covid-positive patients or patients whose Covid status is unknown, negative pressure may be required to reduce the risk of infection outside the room environment.

Improving room turnaround times

Regardless of the number of air changes a hospital has between procedures, there is no doubt that the policy each hospital decides on can have substantial implications on room productivity levels. A reduction in the time taken for each air change, and therefore room waiting times, can lead to effectiveness gains and directly impact how many patients can be treated in one day.

The higher the airflows and the number of air changes an hour that are possible in a room or room, the faster aerosols can be cleared, and the quicker the patient turnaround times can be.  Although the difference and the time saved in between each procedure may seem small, it can make a substantial difference over the number of procedures undertaken in a day or a week, ultimately reducing patient waiting times for planned surgical and diagnostic procedures.

Mobile and modular operating rooms represent highly controlled environments, which facilitates the calculation of the number of air changes required in each case. As well as adding capacity to help reduce waiting lists, these units can in some cases offer a higher number of air changes per hour than a hospital’s internal facilities, increasing throughput even further.

Q-bital’s mobile operating rooms and endoscopy suites offer HEPA-filtered environmental air with up to 30 fresh air changes per hour passing over the patient, while a mobile laminar flow room can perform up to 60 air changes per hour in the room room.

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