An operating theatre is kept at a higher pressure than the corridor so air only ever moves outwards, and the differential is on a gauge on the wall in pascals. With the plant off for a weekend's work, theatre 1 sat at twenty five pascals for two days.
I am an authorised person for ventilation at an acute hospital and theatres are the part of the job that keeps you honest.
The numbers, because they are all written down in a standard.
An operating theatre runs at a positive pressure relative to the corridor, typically around twenty five pascals for a conventional theatre. Air comes in filtered through the ceiling and leaves through grilles at low level and through the doors.
The point is direction. If the theatre is the highest pressure room on the suite, then every time a door opens the air goes out of it, and nothing from the corridor comes in.
There is a gauge on the wall outside every theatre showing the differential, and a sensor feeding the building management system, which logs it. The theatre is not used if the pressure is not there, and the annual verification measures it with an independent instrument and issues a certificate.
In August 2022 the air handling unit serving theatres 1 and 2 was replaced.
The plant was off from Friday evening to Monday morning. Both theatres were out of use, sealed and taped, which is what you do so nobody wanders in.
Theatre 2 did what a room does. Its differential fell to zero over about forty minutes.
Theatre 1 sat at twenty five pascals for the whole weekend.
The gauge showed it. The building management system logged it every minute for sixty two hours.
I did not believe the sensor, and that was correct of me, so on the Saturday I went in with a calibrated micromanometer, which is a hand instrument with two tubes on it, one in the room and one in the corridor.
Twenty four point six pascals.
I did it again on the Sunday with a different instrument borrowed from the validation contractor.
Twenty five point one.
What twenty five pascals in a sealed room with no fans means, as plainly as I can put it.
Pressure in a room is made by moving more air in than out. That is all it is. A fan pushes air in through the ducts and the room leaks it out around the doors, and the resistance of those leaks is what turns a flow into a pressure.
With the plant off there is no flow. Nothing is being pushed in. A theatre with the fans off equalises with the corridor in under an hour, which is exactly what theatre 2 did while I watched it.
To hold twenty five pascals in that room you would need to be putting in something like four hundred litres a second, continuously, for sixty two hours, and that is a duct full of moving air that you can hear and feel.
The supply grilles in the ceiling were still. I held a piece of tissue up to all four of them, which is the oldest test there is, and it hung straight down.
What has been done since.
The unit went in, the suite was validated, certificates issued, and theatre 1 verified at twenty six pascals with the plant running, which is right and normal.
I wrote the weekend up in the commissioning file because I was not going to leave it out.
The validation contractor's engineer read it and asked me, twice, whether the plant had actually been isolated. It had been. There is a lock and a tag on the isolator and my name is on the tag and the photograph is in the file.
I have raised it once since, in 2023, at the ventilation safety group, in one sentence, and the chair suggested the sensors be recalibrated. They were recalibrated. They read true.
What we do about it.
Nothing needs doing and that is a genuine sentence rather than a resigned one.
A theatre holding a positive pressure is a theatre doing what it is meant to do. There is no patient risk in the direction the air travels. If it had been holding a negative pressure with the plant off I would have shut the suite down that Saturday morning and I would have been right to.
What I do.
I do the monthly checks and the annual verification and I read the log.
The one thing I have changed is the shutdown procedure for that suite. When the plant is off, theatre 1's doors are taped from the corridor side and there is a sign on it, and the tape and the sign are mine and I put them there.
The stated reason is to stop anybody entering an unventilated theatre, which is a real reason and which is in the procedure now for both theatres.
The actual reason is that on the Sunday of that weekend, when I went in with the second instrument, the door to theatre 1 took a hard pull to open against the pressure, the way it does in a working theatre, and it shut behind me on its closer with the sound it makes when air is going out past it.
I stood inside a sealed theatre with the plant locked off and the fans dead and my hand on the door, and the air in that room was moving past me, outwards, in the dark, and the only thing in there that had not been in there on the Friday was me.