Peritoneal Dialysis Dose and Adequacy
Peritoneal Dialysis Dose and Adequacy
When kidneys fail, waste products such as urea and creatinine build up in the blood. One way to remove these wastes is a process called peritoneal dialysis (PD). The walls of the abdominal cavity are lined with a membrane called the peritoneum. During PD, a salt and dextrose (sugar) solution, called dialysate, is placed into a person’s abdominal cavity through a catheter. The peritoneal membrane enclosing the digestive organs allows waste products and extra fluid to pass from the blood into the dialysate without letting the dialysate seep out of the abdominal area into the body. These wastes then leave the body when the dialysate is drained from the abdomen. Each cycle of filling and draining is called an exchange.
Many factors affect how much waste and extra fluid are removed from the blood. Some factors cannot be controlled, such as the patient’s capacity for dialysate and the permeability, or speed of diffusion, of the peritoneum. Dialysate comes in 1.5-, 2-, 2.5-, or 3-liter bags. The dialysis dose can be increased by using a larger bag, but only within the limits of the person’s abdominal capacity. Everyone’s peritoneum filters wastes at a different rate. In some people, the peritoneum does not allow wastes to enter the dialysate efficiently enough to make PD feasible.
Other factors that determine how efficiently a person’s blood is filtered can be controlled. Controllable factors include the number of daily exchanges and the dwell time, which is the length of time the dialysate stays in the abdomen. When fresh dialysate is first placed in the abdomen, it draws in wastes rapidly. As the dialysate becomes more nearly saturated with wastes, it cleans the blood less efficiently. For example, a patient may perform one exchange over the course of 6 hours, until the dialysate is saturated with wastes. In the second half of that exchange, wastes are being removed from the blood very slowly. If the patient performed two 3-hour exchanges instead, the amount of waste removed would be substantially greater than that removed in one 6-hour exchange.
Types of Peritoneal Dialysis
The three types of peritoneal dialysis differ mainly in the schedule of exchanges. In continuous ambulatory peritoneal dialysis (CAPD), the patient empties a fresh bag of dialysate into the abdomen. After 4 to 6 hours of dwell time, the patient returns the dialysate containing wastes to the bag. The patient then repeats the cycle with a fresh bag of dialysate. CAPD does not require a machine; the process uses gravity to fill and empty the abdomen. A typical prescription for a CAPD patient requires three or four exchanges during the day and one long overnight exchange as the patient sleeps. The dialysate used for the long overnight exchange may have a higher concentration of dextrose so that it absorbs wastes for a longer time.
For added clearance, a mini-cycler machine can be used to exchange the dialysate once or several times overnight during sleep. Such additional exchanges may also help prevent excessive absorption of fluid from the overnight exchange.
Continuous cycler-assisted peritoneal dialysis (CCPD) uses a machine to fill and empty the abdomen three to five times during the night while the person sleeps. In the morning, the CCPD patient performs one exchange with a dwell time that lasts the entire day. Sometimes one additional exchange is done in the mid-afternoon to increase the amount of waste removed and to prevent excessive absorption of fluid. The dialysate used for the long daytime exchange may have a higher concentration of dextrose.
Nocturnal intermittent peritoneal dialysis (NIPD) is like CCPD, only the number of exchanges overnight is greater (six or more), and the patient does not perform an exchange during the day.
NIPD is usually reserved for patients with a peritoneum that is able to transport waste products very rapidly, or for patients who still have substantial residual renal function.
Testing for Efficiency
To see if the exchanges are removing enough urea, the patient’s health care team must perform several tests. These tests are especially important during the first weeks of dialysis to determine whether the patient is receiving an adequate amount, or dose, of dialysis.
The peritoneal equilibration test (often called PET) measures how much glucose has been absorbed from a bag of infused dialysis solution, and how much urea and creatinine have entered into the solution, during a 4-hour exchange. The peritoneal transport rate varies from person to person. People who have a high rate of transport absorb glucose from the dialysis solution quickly, and they should be given a dialysis schedule that avoids exchanges with a very long dwell time because they tend to absorb too much fluid from such exchanges.
In the clearance test, samples of dialysate drained over a 24-hour period are collected, and a blood sample is obtained during the day when the dialysate is collected. The amount of urea in the dialysate is compared with the amount in the blood, to see how effective the PD schedule being used is in clearing the blood of urea. If the patient has more than several hundred milliliters per day of urine output, the urine should also be collected during this period for measurement of its urea concentration.
From the dialysate, urine, and blood measurements, one can compute a urea clearance, called Kt/V, and a creatinine clearance rate (normalized to body surface area). The residual clearance of the kidneys is also considered. Based on these measurements, one can determine if the amount of PD prescribed is adequate.
If the laboratory results show that the dialysis schedule is not removing a sufficient amount of urea and creatinine, the doctor may change the prescription byIncreasing the number of exchanges per day for patients treated with CAPD or per night for patients treated with CCPD or NIPD. Increasing the volume (amount of fluid in the bag) of each exchange in CAPD. Adding an extra, automated middle-of-the-night exchange to the CAPD schedule. Adding an extra middle-of-the-day exchange to the CCPD schedule.
One of the big problems with PD is that patients sometimes do not perform all of the exchanges recommended by their medical team. They either skip exchanges, or sometimes skip entire treatment days when using CCPD or NIPD. Skipping PD treatments has been shown to increase the risk to the patient of hospitalizations and death.
Residual Renal Function
Normally the PD prescription factors in the amount of residual, or remaining, renal function. Residual renal function typically falls, although slowly, over the months or even years of treatment with PD. This means that, more often than not, the number of PD exchanges prescribed, or the volume of exchanges, needs to be increased as residual renal function falls.
The doctor should determine the patient’s dose of PD on the basis of practice standards established by the Health Care Financing Administration (see the contact information below). Health care providers should work closely with their PD patients to ensure that the proper dose is administered. To maximize health and prolong life, patients should follow instructions carefully to make sure they get the most out of their dialysis exchanges.
For more information, contact the following organizations:
Health Care Financing Administration
7500 Security Boulevard
Baltimore, MD 21244
Home page: http://www.hcfa.gov/
National Kidney Foundation
30 East 33rd Street
New York, NY 10016
Home page: http://www.kidney.org/
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NIH Publication No. 99-4578