Dialysis Support Group

Dialysis works on the principle of the diffusion of solutes across a semipermeable membrane. Blood flows by one side of a semipermeable membrane, and a dialysis solution or fluid flows by the opposite side. Smaller solutes pass through the membrane. The concentrations of undesired solutes (for example potassium, urea, and phosphorus) are high in the blood, but low or absent in the dialysis solution and constant replacement of the dialysate ensures that the concentration of undesired solutes is kept low on this side of the membrane. The dialysis solution has levels of minerals like sodium and chloride that are similar to their natural concentration in healthy blood. For another solute, bicarbonate, dialysis solution level is set at a slightly higher level than in normal blood, to encourage diffusion of bicarbonate into the blood, to neutralise the acidosis that is often present in these patients.

In hemodialysis, the patient's blood is pumped through the blood compartment of a dialyzer, exposing it to a semipermeable membrane. Dialysis solution is pumped through the dialysate compartment of the dialyzer, which is configured so that the blood and dialysis solutions flow on opposite sides of the semipermeable membrane. The cleansed blood is then returned via the circuit back to the body. Ultrafiltration occurs by increasing the hydrostatic pressure across the dialyzer membrane. This usually is done by applying a negative pressure to the dialysate compartment of the dialyzer. This pressure gradient causes water and dissolved solutes to move from blood to dialysate, and allows removal of several liters of excess salt and water during a typical 3-4 hour treatment. Hemodialysis treatments are typically given three times per week, but more frequent sessions, which are usually 2-3 hours in duration given 5-6 times per week can be sometimes perscribed. Hemodialysis treatments can be given either in outpatient dialysis centers or can be done by a patient at home, providing he or she has suitable help and accommodations.

In peritoneal dialysis, a sterile solution containing minerals and glucose 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 dialysate is left there for a period of time to absorb waste products, and then it is drained out through the tube and discarded. This cycle or "exchange" is normally repeated 4-5 times during the day, (sometimes more often overnight with an automated system). Ultrafiltration occurs via osmosis; the dialysis solution used contains a high concentration of glucose, and the resulting osmotic pressure causes fluid to move from the blood into the dialysate. As a result, more fluid is drained than was instilled. Peritoneal dialysis is less efficient than hemodialysis, but because it is carried out for a longer period of time the net effect in terms of removal of waste products and of salt and water are similar to hemodialysis. Peritoneal dialysis is carried out at home by the patient and it requires a substantial degree of motivation and support to perform. It does free patients from the routine of having to go to a dialysis clinic on a fixed schedule multiple times per week, and it can be done while traveling with a minimum of specialized equipment.

Hemofiltration is a similar treatment to hemodialysis, but it makes use of a different principle. The blood is across a very permeable membrane and a pressure gradient is applied; as a result, water moves across the membrane rapidly, facilitating the transport of dissolved substances, importantly ones with large molecular weights, which are cleared less well by hemodialysis. Because the rate of water and solute removal from the blood is rapid, and no dialysate is used, the salts and water are replaced, using "substitution fluid" that is infused into the extracorporeal circuit during the treatment.Hemodiafiltration is a term used to describe several methods of combining hemodialysis and hemofiltration in one process.