1.   For much of the time he has been on a respirator and has undergone dialysis, in which a machine cleans the blood the way that kidneys should.

2.   He also undergoes hemodialysis daily, during which a machine cleans the blood the way the kidneys should.

3.   He has also been undergoing hemodialysis daily since Monday, during which a machine cleans his blood the way his kidneys should.

4.   He has had to undergo hemodialysis daily, a process in which a machine cleans the blood the way the kidneys should.