eli5: Why do Heart-Lungs bypass machine can’t keep someone alive for more than days?

Question

eli5: Why do Heart-Lungs bypass machine can’t keep someone alive for more than days?

In: 155

Answer 1

While standard ECMO is 5 days, they have gone as long as 25-30 days.
The big issue is that the heparin they use to prevent clotting can also cause brain bleeds, etc.