Thanks for the query
From the history you have given me I think your mom is suffering from pulmonary edema
( fluid in the lungs), since she has generalised body swelling also I suppose she is suffering from hypo albuminemia which causes this situation. However since she has multiple issues there could be other causes for the fluid in the lungs like pneumonia
which is quite common is ventilator cases too.
In this case she might need to be replenished with albumin and diuretics to reduce the edema.
Fluid in the lungs would have reduced her capacity to oxygenate hence the oxygen is required
I hope I was of help
I wish her good health and a speedy recovery