How Dialysis Works

by Food and Drug Administration (FDA)

(Page 2 of 3)

Dialysis acts as an artificial kidney. There are two types of treatment: hemodialysis and peritoneal dialysis. About 90 percent of dialysis patients receive hemodialysis, in which the blood is circulated outside the body and cleaned inside a machine before returning to the patient.

Before hemodialysis can be done, a doctor must make an entrance, called an access, into the patient's blood vessels. This is done by minor surgery in the leg, arm or sometimes neck. The best access for most patients is called a fistula. Minor surgery is performed to join an artery to a vein under the skin to make a larger vessel.

If no vessels are suitable for a fistula, the doctor might use a soft plastic tube called a vascular graft to join an artery and vein under the skin. For temporary dialysis in the hospital, a patient might need a catheter implanted into a large vein in the neck. Once the access is made and healed, two needles are inserted in the fistula or graft, one on the artery side and one on the vein side.

Blood drains into the dialysis machine to be cleaned. The machine has two parts, one side for blood and one for a fluid called dialysate. A thin, semipermeable membrane separates the two parts. As dialysate passes on one side of the membrane, and blood on the other, particles of waste from the blood pass through microscopic holes in the membrane and are washed away in the dialysate. Blood cells are too large to go through the membrane and are returned to the body.

The benefits of hemodialysis are that the patient requires no special training, and he or she is monitored regularly by someone trained in providing dialysis.

The other type of treatment, peritoneal dialysis, uses the patient's own peritoneal membrane as a filter. The peritoneal membrane is a sac around the abdominal organs. This membrane (like the dialysis machine membrane) is semipermeable. Waste particles can get through it, but larger blood cells cannot.

The patient has a plastic tube called a peritoneal catheter surgically implanted into the belly. He or she slowly empties about two quarts of dialysate fluid through the catheter into the abdomen. As the patient's blood gets exposed to the dialysate through the peritoneal membrane, impurities in the blood are drawn through the membrane walls and into the dialysate. The patient drains out the dialysate after three or four hours and pours in fresh fluid. The draining takes about half an hour and must be repeated about five times a day. This is called Continuous Ambulatory Peritoneal Dialysis (CAPD).

The main benefit of CAPD is freedom — the patient doesn't have to stay at a dialysis clinic several hours a day, three times a week. The dialysate can be exchanged in any well-lit, clean place, and the process is not painful. The drawback to this treatment is that some people get an infection of their peritoneal lining, and the process may not work well enough on very large people.

Children often do a similar type of dialysis called Continuous Cycling Peritoneal Dialysis (CCPD). Their treatments can be done at night while they sleep. A machine warms and meters dialysate in and out of their abdomens for 10 hours continuously. Then they are free from treatments during the day.

As a college student in the spring of 1985, Kris Robinson chose CAPD when her kidney (she was born with only one) began to fail.

Doctors quickly determined Robinson would need dialysis until a kidney transplant could be done. Robinson's father was willing and able to give her one of his kidneys, and for several months before the operation was arranged, she drained dialysate in and out of her abdomen five times a day. She became adept at draining it out in the shower, putting fresh fluid in during breakfast, and so on throughout the day.

"I'm extremely independent," Robinson says. "This let me be in charge of my own dialysis. I knew I could do it, and I wanted to be responsible for my own care. I didn't like to have to sit for four hours, three times a week, and I didn't like the idea of dealing with my own blood in such an open way as in hemodialysis."

The transplant from her father was successful and today Robinson, now 32, still has her kidney transplant and is the executive director of the American Association of Kidney Patients in Tampa, Fla., a nonprofit organization dedicated to patient education about dialysis and kidney disease.

One thing all dialysis patients must know a great deal about is diet. They need a good amount of protein and lower amounts of potassium and phosphate, which tend to accumulate in the blood and cannot be removed very well with treatment. French fries, for example, are off-limits, and ice cream and cheese must be eaten with caution. Dialysis patients also must limit fluids because the treatment removes only a certain amount of water. Excess fluids make body tissues swell.

About the Author

www.fda.gov
FDA is A United States government body that oversees medical devices, including contact lenses, intraocular lenses, excimer lasers and eyedrops. In the US, these products must be approved by the FDA before they can be marketed.