Covid-19: The machine that breathes for Covid patients
For people who are most sick with Covid-19 - those whose lungs have not been helped by a ventilator - there is another alternative: ECMO.
These are specialist intensive care life support machines which pump oxygen into the patients blood, allowing the lungs to rest.
There are only 6 NHS centres in the UK that offer the treatment, to a few dozen patients but now they're under pressure to take on more.
Health correspondent Sophie Hutchinson was given rare access to an ECMO unit at the Royal Papworth Hospital in Cambridge.