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How peritoneal dialysis works.

Successful peritoneal dialysis is based on a patent access, the instillation into the peritoneal cavity of a dialysate prescription solution, and an intact, functioning peritoneal membrane with adequate blood flow. This article describes the basic concepts of peritioneal dialysis.


To explain the basic concepts of peritoneal dialysis


1. Identify and describe the components needed to perform peritoneal dialysis.

2. Summarize the physiologic processes, which occur during the three phases of a peritoneal dialysis exchange.

3. Discuss ultrafiltration and clearance in peritoneal dialysis.

Peritoneal dialysis (PD) occurs inside the body. To perform peritoneal dialysis, four things are necessary: an indwelling catheter, a peritoneal membrane, dialysis solution, and blood flow to the membrane (Boen, 1989).

Basic Concepts of PD

Access. First, a soft tube, known as a catheter, is surgically placed through the abdominal wall. This catheter is a permanent access to allow sterile dialysis solution flow in and waste products and excess water from the blood to drain out. The ideal catheter provides reliable, rapid dialysate flow rates without leaks or infections. Variations of the peritoneal catheter include the number of cuffs (one vs. two), the design of the subcutaneous pathway (permanently bent versus straight), and the intra abdominal portion (straight vs. coiled) (Gokal et al., 1998).

Peritoneal membrane and cavity. The peritoneal membrane, is a thin, translucent, porous layer of tissue with numerous blood vessels. It consists of two layers, the parietal layer, which lines the inner surface of the abdominal wall and the visceral layer, which covers the abdominal organs in the peritoneal cavity (see Figure 1). The visceral peritoneum accounts for about 80% of the total peritoneal surface area (Blake & Daugirdas, 2001). The space between the parietal and visceral peritoneum is called the peritoneal cavity. It normally contains less than 10 ml of fluid (Khanna, Nolph, & Oreopoulos, 1993), but can accommodate several liters without patient discomfort. This membrane is semipermeable and acts as a dialyzer, permitting waste to cross (filter through) the membrane from the blood into the dialysis solution in the peritoneal cavity.


Dialysis solution. Sterile dialysis solution is instilled into the peritoneal cavity through the catheter. The choice of solution volume is made by two factors (Sorkin, 1993). The first factor is the capacity of the patient's peritoneal cavity to hold the solution. Two to three liters of dialysis solution is used for the average adult. The volume of solution may be altered during the day mad night depending upon the size and activity of the patient.

The second factor is the amount of small molecular-weight solutes (urea and creatinine) that must be removed to keep the patient feeling well. The capillary blood flow to the peritoneal membrane is approximately 50-100 mL/per minute (Blake & Daugirdas, 2001) and is more than adequate for even small solutes.

The Dialysis Procedure in PD

There are three phases involved in performing a PD cycle or exchange: infusion, dwell and drain (Prowant & Gallagher, 1995) (see Figure 2). In the infusion or fill phase, warmed sterile dialysis solution flows into the peritoneal cavity through the catheter. It takes approximately 10 minutes for a 2-liter volume of dialysis solution.


During the dwell phase, the dialysis solution remains in the peritoneal cavity. The dwell time provides equilibration of solutes for diffusion and osmosis to take place. The amount of time varies depending upon which PD therapy is performed. In continuous dialysis, the dwell time is several hours.

To complete the process, the drain phase allows the used dialysis solution be emptied by gravity from the peritoneal cavity, removing the waste and excess water. The drain phase may take 20-30 minutes depending on the volume of solution in the patient.

A good functioning catheter is essential for proper drainage of the solution.

Factors that affect the drain rate include the catheter position, internal and external occlusion of the lines, intra-abdominal pressure, catheter configuration, the tubing diameter, tubing length, and the height distance from the abdomen to the drain bag (Prowant & Gallagher, 1995). This process of exchanging the fluid is repeated throughout the treatment to meet each patient's dialysis needs. New solution is replaced following each drain.

Kinetics of Peritoneal Dialysis

Dialysis occurs during the dwell time through the process of diffusion. Diffusion allows solutes to cross the peritoneal membrane from an area of higher solute concentration to an area of lower concentration (see Figure 3). Uremic solutes and potassium diffuse from the capillary blood vessels across the membrane into the dialysis solution. Glucose and lactate diffuse in the opposite direction (Blake & Daugirdas, 2001).

Peritoneal diffusion depends on the ibllowing factors:

* Concentration gradient. Urea is maximal in the blood at the start of the dwell when the concentration in the dialysis solution is zero. It gradually decreases during the course of the dwell as it is removed from the blood.

* Effective peritoneal surface area. This depends on both the total peritoneal surface area and the degree of vascularity.

* Fill volumes. Peritoneal diffusion can also be increased by using larger fill volumes.

* Molecular weight of solutes. Solutes with lower molecular weight, such as urea (MW60) are more easily transported than those with higher molecular weights, such as creatinine (MW113).

Ultrafiltration In Peritoneal Dialysis

During the dwell phase, water removal (ultrafiltration), also occurs as a consequence of the osmotic gradient between the relative hyperosmolarity of the PD solution and the peritoneal capillary blood (Blake & Daugirdas, 2001). Glucose is added to the peritoneal solution to increase the osmotic (concentration) gradient. This gradient is maximal at the start of the dwell and decreases with time due to the dilution of the glucose by the ultrafiltrate and to the diffusion of the glucose from the dialysis solution into the patient's blood.

The gradient can be maximized by using more hypertonic solutions. The glucose solutions range from 1.5% to 4.25%. Use of dialysis solution containing 1.5% dextrose, will result in little or no fluid removal, especially during long dwell exchanges (Prowant & Gallagher,1995). To increase ultrafiltration, a higher concentration of dextrose is used. For patients on continuous PD regimens, shortening the dwell time will result in removal of increased ultrafiltrate volume compared to long dwells. Persistent use of hypertonic dialysis solutions may cause excessive fluid removal resulting in an increase of the serum osmolality and/or hypovolemia, causing excessive thirst which leads to increased fluid intake necessitating more ultrafiltration. Patients are taught to take their daily weight and blood pressure and evaluate which glucose solutions they will use depending on their need to remove excess fluid.

Drug Transport

Systemic vasodilators may increase peritoneal clearances by augmenting blood flow to adjacent capillary beds (Prowant & Gallagher, 1995). Use of drugs that decrease blood flow will decrease clearances. Factors that affect solute clearances also affect clearance of systemic drugs. Drugs that have a low molecular weight, are poorly bound to protein, and are water soluble are more readily transported across the peritoneal membrane. Drugs can be administered intraperitoneally and may be transported into the systemic circulation.

Planning the Dialysis Prescription

To achieve desired dialysis clearance levels, individually tailored PD prescriptions are essential (Blake et al., 1996). Body surface area (BSA), residual renal function, and peritoneal membrane transport characteristics are fundamental to developing the PD prescription. Residual renal function makes it easier to achieve clearance guidelines, however, as residual renal function declines, the PD prescription must be adjusted. The standardized Peritoneal Equilibration Test (PET) was developed by Dr. Zbylut Twardowski and defines the peritoneal membrane's clearance and ultrafiltration rates by measuring the dialysate to plasma ratios of creatinine and glucose under specific conditions. Results from the PET classify patients into tour basic groups: High, High Average, Low Average, and Low (Twardowski, Nolph, & Khanna, et al., 1987). Once the membrane type is known, a PD regimen can be planned for the individual patient. Routine monthly blood tests show the results of the dialysis prescription and will indicate if changes are needed in the patient's prescription.

Types of Peritoneal Dialysis

Two types of PD are continuous ambulatory peritoneal dialysis (CAPD) and continuous cycling peritoneal dialysis (CCPD) (Khanna, Nolph, & Oreopoulos, 1993). CAPD was described in 1976 by Popovich, Moncrief, Decherd, Bomar, and Pyle (see Figure 4). This type of self-dialysis is done 7 days a week. Four to five exchanges of new solution are performed each day. During an exchange, which takes about 30 minutes, the solution inside the peritoneal cavity is drained and new solution is instilled. The new solution remains in the cavity for 4 6 hours. The last evening's exchange dwells overnight, to allow for an uninterrupted night's sleep. Using the CAPD dialysis method gives the patient die freedom to do dialysis at home, work, or during vacation.


CCPD is a form of automated peritoneal dialysis. During this type of dialysis, the exchange of dialysis solution is performed by a machine (cycler) while the patient sleeps (see Figure 5). Each exchange is referred to as a "cycle." Patients are taught how to set up the machine and generally connect to the cycler at bedtime, 7 days a week, for 8-10 hours each night. The machine controls the three phases of the cycle: draining used solution, re-filling with new solution, and monitoring the dwell time. In the morning, the machine does a "final fill," which remains in the patient throughout the day (Baxter, 2003).


Training and Follow-Up

Patients preparing for home dialysis receive one-on-one training to give them the skills they need to perform their dialysis treatments. This custom training usually takes between 5 days to a few weeks and is given by a qualified nurse. Once trained, patients then begin doing their own dialysis at home. They are periodically examined by the renal team to evaluate the dialysis regimen. Supplies are delivered to the patient's home each month. Patients who wish to travel may have their supplies sent directly to their destination. PD allows patients to perform their dialysis at home, according to their daily work and life schedule.


Baxter Healthcare. (2003). Choices: The Baxter patient education program. McGaw Park, IL: Author.

Blake, P.G., Burkart, J.M., Churchill, D.N., Daugirdas, J., Depner, T., Hamburger, R.J., Hull, A.R., Korbet, S.M., Moran, J., Nolph, K.D., Oreopoulos, D.G., Schreiber, M., & Soderbloom, R. (1996). Recommended clinical practices for maximizing peritoneal dialysis clearances. Peritoneal Dialysis International, 16(5), 448-456.

Blake, P.G., & Daugirdas, J.T., (2001). Physiology of peritoneal dialysis. In J.T. Dangirdas, P.G. Blake, & T.S. Ing (Eds.). Handbook of dialysis (pp. 281-296). Philadelphia: Lippincott, Williams, & Wilkins.

Boen, S.T. (1989). History of peritoneal dialysis. In K.D. Nolph (Ed.), Peritoneal dialysis (3rd edition, pp. 2-3). Norwell: Kluwer Academic Publishers.

Gokal, R., Alexander, S., Ash, S., Chen, T.W., Danialson, A., Holmes, C., Joffe, E, Moncrief, J., Nichols, K., Piraino, B., Prowant, B., Slingeneyer, A., Stegmayr, B., Twardowski, Z., & Vas, S. (1998) Peritoneal catheters and exit-site practices toward optimum peritoneal access: 1998 update. Peritoneal Dialysis International, 78, 11-33.

Khanna, R., Nolph, K.D., & Oreopoulos, D.O. (1993). The essentials of peritoneal dialysis (p. 4). Norwell: Kluwer Academic Publishers.

Popovich, R.P., Moncrief, J.W., Decherd, J.F., Bomar, J.B., & Pyle, W.K. (1976). The definition of a novel portable/wearable equilibrium peritoneal dialysis technique. Abstract. Transactions of the American Society of Artificial Internal Organs, 5, 64.

Prowant, B.F., & Gallagher, N.M. (1995). Peritoneal dialysis. In L. E. Lancaster (Ed.) ANNA core curriculum for nephrology nurses (pp. 281-322). Pitman, NJ: American Nephrology Nurses' Association.

Sorkin, M.(1993). Peritoneal dialysis solutions. In A. R. Nissenson, R. N. Fine (Ed.) Dialysis Therapy (2nd ed) (pp.157-158). Philadelphia: Hanley & Belfus, Inc.

Twardowski, Z.J. Nolph, K.D., Khanna, R., Prowant, B., Moore, H.I., & Nielson, M.P. (1987). Peritoneal equilibration test. Peritoneal Dialysis Bulletin, 7, 138-147.

How Peritoneal Dialysis Works Karen T. Kelley, BSN, RN, CNN Posttest--1.0 Contact Hour Posttest Questions (See posttest instructions on the answer form, on page 491.)

(1) Variations of the peritoneal dialysis catheter include

a. number of cuffs only.

b. number of cuffs and design of the subcutaneous pathway only.

c. number of cuffs, design of the subcutaneous pathway, and the intra-abdominal catheter design only.

d. number of cuffs, design of the subcutaneous pathway, the intra-abdominal catheter design and the material used to make the catheter.

(2) The visceral peritoneum accounts for what percent of the total peritoneal surface?

a. 25%

b. 60%

c. 80%

d. 95%

(3) The choice of solution volume may be determined by the

a. amount of large-molecular-weight solutes that must be removed.

b. capacity of the patient's peritoneal cavity.

c. dialysate glucose concentration.

d. type of PD, CAPD or CCPD.

(4.) Mr. Smith comes to the clinic complaining of long drain times. You begin to evaluate possible causes, which include

a. increased intra-abdominal pressure only.

b. increased dialysate glucose concentration.

c. increased solution volume.

d. number of cuffs on catheter.

(5.) Peritoneal diffusion depends on the

a. concentration gradient only.

b. concentration gradient and peritoneal surface area only.

c. concentration gradient, peritoneal surface area, and fill volumes only.

d. concentration gradient, peritoneal surface area, fill volumes, and molecular weight of solutes.

(6.) Mr Johnson arrives for his clinic visit. He complains of shortness of breath with exertion. His BP is 170/70. He is 4 kg above his target weight and has 3+ edema. You would advise Mr. Johnson to do the following tonight when he goes on the cycler.

a. Use lower concentrations of glucose and increase dwell time

b. Use higher concentrations of glucose and increase dwell time

c. Use lower concentraion of glucose and decrease dwell time

d. Use higher concentration of glucose and decrease dwell time

(7.) Clearance of drugs across the peritoneal membrane is improved when drugs

a. have a high molecular weight.

b. are protein bound.

c. are water soluable.

d. increase glucose absorption.

(8.) The Peritoneal Equilibration Test (PET) defines the membrane's clearance and ultrafiltration rates by:

a. determining the amount of urea and creatinine in the dialysate.

b. measuring the dialysate to plasma ratios of creatinine and glucose.

c. calculating the true serum creatinine levels after the exchange is completed.

d. subtracting the urine creatinine level from the serum creatinine level.

(9.) Mrs. Cherry comes to the clinic. She is complaining of dizziness and excessive thirst. Her blood pressure is 90/60 and she has a blood sugar reading of 200. What is your best advice to Mrs. Cherry?

a. Decrease glucose concentration

b. increase sodium intake

c. increase fluid intake

d. Decrease dwell time

(10.) Mrs. Lee comes to the clinic complaining of decreased appetite. She has noted a decrease in her urine output. You decide to do a PET test. You find that she has decreased urea clearance. How might you improve her clearance?

a. Increase fill volume only

b. Increase fill volume and dwell time only

c. Increase fill volume, increase dwell time and decrease glucose concentration only

d. Increase fill volume, increase dwell time, and decrease glucose concentration and advise patient to decrease protein intake


How Peritoneal Dialysis Works Karen T. Kelley, BSN, RN, CNN

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* Enclose a check or money order payable to ANNA. Fees listed in payment section.

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Karen T. Kelley, BSN, RN, CNN, is a Clinical Educator with The Renal Division at Baxter Healthcare Corporation, McGaw Park, Il. She is a member of the ANNA PD SIG and The Jersey North Chapter.
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Title Annotation:Continuing Education
Author:Kelley, Karen T.
Publication:Nephrology Nursing Journal
Geographic Code:1USA
Date:Sep 1, 2004
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