Dialysis and Water RemovalDialysis can only remove water that is in your blood. Only a small part of the water weight you gain is in your blood. If your feet are swollen (edema), fluid is in your tissues—not your blood.During a treatment, pressure forces fluid out of your blood, into the dialysate, and down the drain. Some water from other compartments can then move into your bloodstream (this is called vascular refilling), so it can be removed. In the 3 or 4 hours of a treatment, only so much water is in your blood or has time to move there. The machine is set to a fluid goal— your “dry weight,” or weight without excess water. The dialysis machine keeps pushing, even if the water is not in your blood. And this is why you may feel awful. If your blood becomes too “dry,” your blood pressure drops. This happens if you go below your dry weight, or even if you are above your dry weight, but the extra fluid is not in your bloodstream.
When I was on hemo, it happened a few times to me as well. I could usually feel it coming on before the big crash though. Never to the point of passing out. My cases were always because I was pulling too dry. We'd adjust my blood pressure medicines (would start taking my daily dose AFTER I left hemo) and a few times we'd cut the session short. Not as much of an issue now that I'm on PD.