This case study shows how operating theatre CFD simulation can be used to assess whether a ventilation system meets wound-site air velocity requirements. Using a hybrid operating theatre model, we look at how room layout, equipment, and staff positions affect airflow in the critical area around the patient.
Ever since I had an operation due to a sports accident, I have had a keen interest in knowing more about the principles on which operating theatre designs are based and what criteria their ventilation system has to meet. From a ventilation viewpoint, these spaces function as cleanrooms with a specific purpose.
When I was in one as a patient, I was too scared about the oncoming procedure and had no time to look at supply and extract grille types and locations. So I started to search for more details about how fluid dynamics is involved in their design. Since then we have completed a few operating theatre CFD simulations and learned not only about room types and surgical procedures, but also about how large machinery and medical staff positions influence airflow patterns.
One of the purposes of an OT’s ventilation system is to provide 0.2 – 0.3 m/s air velocity near the patient. Achieving this requires careful design and special ventilation equipment. And it needs CFD simulations with fine geometric details to predict compliance.
How surgical procedure drives room design
We have worked with OT room designs having only a few pendant lights and a patient table. We also have simulated complex architectural concepts too. This case study uses a hybrid operating theatre, which was inspired by one of the recent trends. We created a ventilation system and interior design to highlight some features a real-life system would have, but it is just an example.
A hybrid operating theatre is a fairly new concept though. It involves the installation of large medical imaging systems like an MRI (Magnetic Resonance Imaging, the big doughnut on the right) in the same room where the surgical procedure is carried out. These machines come with screens the size of a family dinner table. These screens are hung from the ceiling and can be moved around, when not in use. Apart from pendant lights and ceiling-mounted supply units there is also an anaesthesia machine, usually near the patient.
Medical staff positions and airflow obstructions
The case study model also highlights what we have learnt about the medical staff positions in the room and its interaction with air pathways around the operating table. In this model there are nine medical team members arranged to perform laparotomy. During this procedure doctors work on the lower half of the patient’s torso.
Medical staff positions can be very much different for another procedure. But either way, the patient’s surroundings are always busy. Hence the ventilation system has to use a special supply and extract arrangement to overcome obstructions to airflow in the critical area at the wound site.
Key components of the ultra clean ventilation (UCV) system
The UCV hood alone would not be enough to build up the necessary overpressure and air changes in the OT. So there are three 600 x 600 ceiling supply grilles on the left.
The OT also receives airflow from the preparation room (where staff scrub in) via an air pressure stabiliser (APS). This is the yellow block in the bottom left corner.
The other three yellow blocks are also air pressure stabilisers. Each allows air to leave the OT towards a different space with lower designed pressure.
APS’ are all at high level, in this example above doors. But there are three low level extraction points in the system too. Two are in the lower corners of the right-hand side wall near cabinets. The third is in the scrub-up room (in red in the top left corner). These extract grilles are connected to the mains mechanical ventilation network, each removing air at a pre-set flow rate.
For a real-life operating theatre ventilation system these components work together to maintain overpressure and provide high air changes. But again, another important design criterion is to achieve high flow velocities at wound site.
How airflow paths achieve ventilation system compliance
Air velocity and also flow direction are critical to keep the wound site clean of airborne contaminants. This helps minimise the possibility of surgical site infections. As far as our experience shows, in real-life OT ventilation systems the required 0.2 – 0.3 m/s wound site air velocity can be achieved with air supply positioned right above the operating table. The supply can be a purpose built hood or some other arrangement. But it must use such a high flow rate that would, in living rooms, be felt as quite a draught.
Flow rates I chose for the UCV hood can produce more than adequate air velocities at wound site. Maybe the 0.46 m/s, which we can measure very accurately, is even a bit too much. Air could be better distributed among ceiling ports, but it shows what I meant to show.
In any case, the point is that well-arranged fresh air supply above the patient can make the operating theatre ventilation system comply with wound site velocity requirements.
When an ultra clean ventilation system is used, the patient table is surrounded by downward flowing high-speed air. It acts like an air curtain. Though the pendant light above the patient deflects air, the flow is still focused enough to build up the necessary velocity at surgical site.
This helps carry away any airborne contaminants. It forces them to follow flow paths that will leave the OT through one of the high level UCV hood extract ports, or low level extraction vents, or via air pressure stabilisers.
Why deatiled CFD simulation matters in operating theatre design
We believe details matter. Our simulation method relies on capturing geometric features like shape and size of machinery and accurate representation of ventilation system elements. Experience tells that medical staff positions also have key importance, as the area around the operating table is always busy during a procedure.
Providing the right level of wound site air velocity is getting more attention recently. It is becoming an additional design goal for operating theatre refurbishment and new OT design projects.
There are operating theatre types that only need regular, ceiling-mounted supply grilles and air pressure regulators to comply with the standard overpressure and high air changes per hour requirements. But when ventilation systems have to meet stricter criteria like wound site air velocity must be at a certain level, a detailed operating theatre CFD simulation can predict compliance.
Dr Robert Dul