In medicine, hemodialysis, also haemodialysis, is a method for removing waste products such as potassium and urea, as well as free water from the blood when the kidneys are incapable of this (i.e. in renal failure). It is a form of renal dialysis and is therefore a renal replacement therapy.
Hemodialysis is typically conducted in a dedicated facility, either a special room in a hospital or a clinic (with specialized nurses and technicians) that specializes in hemodialysis. Although less typical, dialysis can also be done in a patient's home as home hemodialysis.
The principle of hemodialysis
The principle of hemodialysis is the same as other methods of dialysis; it involves diffusion of solutes across a semipermeable membrane. In contrast to peritoneal dialysis, in which transport is between fairly static fluid compartments, hemodialysis relies on convective tranport and utilizes counter current flow, where the diasylate is flowing in the opposite direction to blood flow in the extracorporeal circuit. Counter-current exchanges maintain the concentration gradient across the membrane at a maximum and increase the efficiency of the dialysis.
The efficiency of waste clearance during hemodialysis is much higher than in natural kidneys. Therefore, dialysis treatments do not have to be continuous and can be performed intermittently, typically three times a week.
Fluid removal (ultrafiltration) is achieved by altering the hydrostatic pressure of the dialysate compartment, causing free water to move across the membrane along a pressure gradient.
The dialysis solution that is used is a sterilized solution of mineral ions. Urea and other waste products, such as potassium and phosphate, diffuse into the dialysis solution. However, concentrations of most mineral ions (e.g. sodium) are similar to those of normal plasma to prevent loss.
Note that this is a different process to the related technique of hemofiltration.
Hemodialysis access
There are three primary modes of access to the blood in hemodialysis: an intravenous catheter, an arteriovenous (AV) Cimino fistula, or synthetic graft. The type of access is influenced by factors such as the expected time course of a patient's renal failure and the condition of his or her vasculature. Patients may have multiple accesses, usually because an AV fistula or graft is maturing, and a catheter is still being used.
