Treating Kidney Failure With Peritoneal Dialysis
III. How Peritoneal Dialysis Works
Peritoneal dialysis works by means of two processes: diffusion and osmosis. Diffusion is what happens when particles pass from one side of a membrane to the other--from an area of stronger solution, to an area of weaker solution. Think of a tea bag: It holds the tea leaves in place, but allows tea to float out, and water to flow in. If you place a tea bag into boiling-hot water, the tea moves from the area of stronger solution (within the tea bag), to the area of weaker solution (the water). The longer you have the tea bag in the water, the more color and flavor you get.
Osmosis occurs when fluid moves through a membrane from an area of weaker solution to an area of stronger solution. Think again of water flowing through a tea bag, and how both sides are affected: The tea gets wet, and the water turns into tea.
Now let’s look at peritoneal dialysis and you’ll see that it works on the same principle. Think of the tea bag as your peritoneal lining, the tea leaves as your red blood cells, the particles of tea as the waste products carried in your blood, and the water as the dialysate fluid that goes into your abdomen. The sterile dialysate fluid has less of the waste products (fluid, creatinine, potassium, etc.). As the fluid comes in contact with your peritoneal lining, the waste products cross the lining into the fluid. However, as the tea bag keeps the tea leaves from going into the water, your peritoneal lining keeps your blood cells from going into the sterile solution.
A. The Peritoneal Membrane
The inner wall of the abdominal cavity, and the organs inside are contained in a sac of smooth, thin, blood-rich tissue. This tissue is known as the peritoneal membrane, or peritoneum. Like other membranes, the peritoneum allows very small particles to pass from one side to the other. This makes it a perfect internal filter since blood cells are too large to pass through, but some waste products and fluids do pass through and are drained away with the dialysate.
| It is important to remember that strict sterile technique must be used at all times to prevent serious infections. |
B. The Peritoneal Dialysis Cycle: Exchanges
Three basic stages, called Drain, Fill and Dwell, are involved in peritoneal dialysis. They combine to form a complete peritoneal dialysis cycle, which is then repeated over and over again. In summary, after the dialysate has been in your abdominal cavity for several hours, an exchange for fresh solution is made. This involves draining the used dialysate from your abdomen and immediately replacing it with a fresh solution. This begins the filtering process over again. The number of exchanges done in a 24-hour period varies, depending on factors like body size, amount of kidney function you have left, the type of peritoneal dialysis you use and how efficient your peritoneum is in removing wastes and fluid. Your doctor will decide the number of exchanges that are best for you. Here's how the stages in the PD cycle work:
Before starting a new cycle, the used dialysate must be drained out of your abdominal cavity, so you can fill with fresh dialysate. During the drain stage, gravity pulls used dialysate out of your abdomen, through the PD catheter and into an empty, sterile, plastic container. The used dialysate contains the waste products and excess fluids that have filtered out of the blood. This "drain" or "outflow" stage usually takes about 15 to 20 minutes.
Approximately 2 to 3 liters (about 2 to 3 quarts) of sterile dialysate are put into your abdomen through a soft plastic tube, or catheter, which is permanently implanted in your abdomen. (Click to "Gaining Access: Your Peritoneal Dialysis Catheter" for more information.) This sterile cleansing solution is similar in makeup to normal body fluid. It usually takes about 5 to 10 minutes to complete the "fill stage" or "inflow" stage.
Once the fluid has filled the abdominal cavity, the dialysate is left to "dwell" inside your body for four to six hours. During the dwell stage, waste products and excess fluid filter out of the blood vessels in the abdominal wall, through the peritoneal lining, and into the dialysate solution. The dialysate acts like a magnet, helping to draw the excess fluids and wastes from the blood and into the dialysate. Red blood cells and important nutrients that the body needs are too large and cannot pass through the peritoneal membrane. These processes of osmosis and diffusion continue until wastes, fluids and other particles are more equal on both sides of the peritoneal membrane. When this occurs, the used dialysate is drained and fresh dialysate is flushed in to begin the process of diffusion and osmosis again.
The process is very slow and gentle. The length of time depends on many factors, including body size, amount of kidney function left and the type of peritoneal dialysis system being used. Most patients notice nothing more than a slight feeling of fullness. In addition, you are free to do whatever you'd like--work, travel, shop, walk or sleep--during this dwell stage of PD.
