Article: Dialysis

This article is about renal dialysis; for the laboratory technique, see dialysis (biochemistry); for the treatment for liver failure, see liver dialysis.

In medicine, dialysis is a type of renal replacement therapy which is used to provide an artificial replacement for lost kidney function due to renal failure. It is a life support treatment and does not treat any kidney diseases. Dialysis may be used for very sick patients who have suddenly lost their kidney function (acute renal failure) or for quite stable patients who have permanently lost their kidney function (end stage renal failure). When healthy, the kidneys remove waste products (for example potassium, acid and urea) from the blood and also remove excess fluid in the form of urine. Dialysis treatments have to duplicate both of these functions as dialysis (waste removal) and ultrafiltration (fluid removal).

The Principle of Dialysis

Dialysis works on the principle of the diffusion of solutes along a concentration gradient across a semipermeable membrane. In all types of dialysis, blood passes on one side of a semipermeable membrane, and a dialysis fluid is passed on the other side. By altering the composition of the dialysis fluid, the concentrations of undesired solutes (chiefly potassium and urea) in the fluid are low, but the desired solutes (for example sodium) are at their natural concentration found in healthy blood. The undesired solutes then diffuse across the membrane into the dialysis fluid and are removed.

Types of dialysis

There are two main types of dialysis, hemodialysis and peritoneal dialysis. Hemofiltration is not strictly speaking a dialysis treatment, but is extremely similar.

Main article: hemodialysis

• In hemodialysis, the patient's blood is passed through a system of tubing (a dialysis circuit) via a machine to a semipermeable membrane (dialyser) which has dialysis fluid running on the other side. The cleansed blood is then returned via the circuit back to the body. Ultrafiltration occurs by increasing the hydrostatic pressure of the blood in the dialysis circuit to cause water to cross the membrane down a pressure gradient. The dialysis process is very efficient, allowing the treatment to be undertaken intermittently, usually three times a week, but often fairly large volumes of fluid must be removed in a session which can sometimes be demanding on the patient.

Main article: peritoneal dialysis

• In peritoneal dialysis, a special solution is run through a tube into the peritoneal cavity, the abdominal body cavity around the intestine, where the peritoneal membrane acts as a semipermeable membrane. The fluid is left there for a period of time to absorb waste products, and then is removed through the tube. This is usually repeated a number of times during the day. Ultrafiltration occurs via osmosis in this case, as the dialysis solution is supplied in varying osmotic strengths to allow for some control over the amount of fluid to be removed. The dialysis process in this case is less efficient than hemodialysis and is carried out daily, but the ultrafiltration process is slower and gentler.

Main article: hemofiltration

• Hemofiltration is a similar treatment to hemodialysis, but in this case, the membrane is far more porous and allows the passage of a much larger quantity of water and solutes to pass across it. The fluid which passes across the membrane (the filtrate) is discarded and the remaining blood in the circuit has its desired solutes and fluid volume replaced by the addition of a special hemofiltration fluid. It is a slow continuous therapy with sessions typically lasting 12-24 hours, usually daily. This, and the fact that ultrafiltration is very slow and thus gentle, makes it ideal for patients in intensive care units, where acute renal failure is common. A combination of hemofiltration and hemodialysis, called hemodiafiltration (incorporating a hemofilter to a standard hemodialysis circuit), is being used in some centres for chronic maintenance therapy.