I am not a doctor but I have read quite a bit on how COVID-19 affects the body. The ability of a critically afflicted patient to efficiently exchange oxygen drops off a cliff as the lungs fill with a slimy fluid that clogs the alveoli, as the body tries to combat the viral cell damage.
Even with ventilator assisted breathing, their blood-oxygen saturation can go down to 50% or less, which then causes permanent injury or complete failure of the kidneys and other organs due to prolonged oxygen starvation.
A ventilator does not do much to help when the lungs are full of fluid, and seems to be the wrong technology when the lungs are not working properly anyway.
Apparently we should really be using artificial lung technology, which Wikipedia tells me is called Extra-Corporeal Membrane Oxygenation (ECMO), and is less complex than a heart-lung machine as it is assistive to the working heart of the patient.
But I never hear anything in the news about this being used for COVID-19. All the focus has been on ventilators. So why isn't it being widely used and why isn't it being widely discussed?
I expect that the main problem is that it is an even more rare, expensive, and hard to obtain technology than a ventilator. The ratio of artificial lung machines to ventilators is probably 1:100 or even 1:1000.