A condensed recent Twitter rant of mine regarding this influx of DIY ventilators based on bag-valve masks (BVMs):

There are quite a few BVM-based ventilators being designed right now. However, there are a lot of caveats I have become aware of.
In short: ventilators for use in intensive care need to be safe and adjustable. Oh, and it is not clear whether ventilators actually change the outcome.

Lungs of COVID-19 patients are often damaged, and ventilation causes additional damage. As the patients' alveoli no longer capture enough O2, you need to increase air pressure to force more O2 into them. More pressure = more damage to alveoli, so you need to be very careful. So you need to be able to adjust the pressure curve very finely. None of the BVM solutions I have seen so far has such a feature. COVID-19 already causes severe lung damage in some people - you don't want to damage them even more due to wrong ventilation.

Unlike emergency care (where BVMs are usually used), intensive care of COVID19 patients needs to play the long game. AFAIK, patients need constant care, supervision and medication over a period of 10+ days. Any damage you cause to the lungs early on will make it harder later on.

But won't it be better to keep people alive with a simple ventilator even if they will suffer lung damage? Maybe not. I am aware of the gut-wrenching situation doctors face in Italy and elsewhere - deciding whom to connect to the few available ventilators. It seems (I'm not really sure about it yet) that ventilators just delay inevitable death by a few days in most cases. A study from Wuhan notes: “32 patients required invasive mechanical ventilation, of whom 31 (97%) died.” [1]

"Not everyone is a good fit for ICU. Even outside of COVID, if you are of a certain frailty and you have certain underlying conditions, your chances of making it off a ventilator when you are deathly sick is close to 0%." [2]

So the question is indeed whether one should really extend a patient's suffering for a few days or whether one should instead focus on making their last few days as comfortable as possible, and give them the chance to stay conscious and say goodbye to family.

Even if there is a subset of patients who might benefit from a BVM-based ventilator (and I am not sure about this), I would be very hesitant to rely on them. If a motor burns out, a tube gets loose, or a microcontroller goes crazy, the patient might choke, panic, even die. None of the DIY ventilators has gone through rigorous testing yet. None of the ones I have seen has backup mechanisms (spare motor, continuous surveillance of all parameters, fail-safe power supplies, etc.) I would not want to inflict them on dying patients - or caregivers.

This leads me to the most important point: the most severe bottleneck are qualified professionals. Should we really hand these people some DIY Arduino-based ventilators that might fail any moment, that lack important features, and that need additional training? A ventilator that malfunctions at the wrong time (or does not reliably function at all) will take away time and attention from exactly those people whom we really don't want to get distracted or burnt out.

There is a UK government guidance which gives a good overview of essential requirements for ventilators [3]. It states:

"A ventilator with lower specifications than this is likely to provide no clinical benefit and might lead to increased harm, which would be unacceptable for clinicians and would, therefore, not gain regulatory approval."

Disclaimer: I'm no expert on medical tech or intensive care. I have just followed a few of those projects over the past weeks and read reports of experts. It could be that I am wrong and those projects will save many lives. There could also be use cases where DIY ventilators may help (e.g. in home care). But overall, I am rather skeptical and would rather suggest focusing on stuff that might be more useful.

[1] https://www.sciencedirect.com/science/article/pii/S014067362...

[2] https://www.reddit.com/r/Coronavirus/comments/fnl0n6/im_a_cr...

[3] https://www.gov.uk/government/publications/coronavirus-covid...