Thanks for all your enthusiasm and willingness to help in these difficult times.
I’m an anaesthetist in Australia. I use ventilators every day. I’ve seen many ventilator projects around the internet but I haven’t seen a lot of detail on the control systems for these ventilators, and there’s a few things I think need pointing out. I hope this post can be useful for those working on these projects.
The first thing to understand is that COVID-19 patients that require invasive ventilation inevitably do so because they develop Acute Respiratory Distress Syndrome - ARDS. Ventilation in ARDS is notoriously difficult, but the right ventilation strategy is associated with a decrease in mortality and complications.
Being able to ventilate someone appropriately could mean the difference between survival and death. It’s not as simple as squeezing a bag, and grossly oversimplified approaches to ventilation may be worse than nothing at all.
I’d encourage anyone considering working on these projects to do some independent reading on ventilation in ARDS. Much of it will assume a level of base knowledge and be written in medicalese but post here if you want me to explain anything specific.
Down to the details:
Pressure sensing is mandatory - excess ventilatory pressures cause all kinds of problems and it’s a key parameter that can be used to safely ventilate someone with ARDS.
Flow sensing, and by integration with time, volume determination, is also critical. This allows the clinician to determine a few things - firstly, if expiratory flow hasn’t reached zero before the next breath start, a phenomenon call gas trapping can occur. This can lead to volutrauma, pneumothorax etc. Secondly, decreasing tidal volumes over time can alert the clinician to changing lung compliance which needs actioning.
A method of setting and maintaining positive end expiratory pressure (PEEP) is required.
- There must be an over pressure alarm and failsafe mode
- I would advocate for a pressure control mode only
- Adjustable respiratory rate
- There should be a display of flow/time and pressure/time
- Ideally a method of adjusting inspiratory time or I:E ratio
As for the overall architecture, I think the self inflating bag concept makes a lot of sense.
Again, thanks everyone for your enthusiasm but please temper it with some thought of actual clinical utility.
An introduction to the ventilator waveform
Basic components of a mechanical ventilator
Methods of generating positive ventilator circuit pressure