Technical aspects of peritoneal dialysis
1. What is peritoneal dialysis (PD), and how does it work?
PD is a means of removing waste (such as urea, creatinine, and phosphate), other solutes (i.e., sodium and chloride), and excess fluid from the body when the kidneys have failed. A sterile solution (PD fluid) containing a balanced concentration of electrolytes and an osmotically active agent is introduced into the patient’s peritoneal cavity by a PD catheter or tube. The latter is placed surgically (laparoscopic or open) or by interventional (bedside or radiologic) technique. The introduced PD fluid bathes the expansive network of capillaries covering the surface area of the peritoneum. As the PD fluid is devoid of any waste, solutes move down concentration gradients across peritoneal capillaries into the PD fluid over time until near-equilibrium of uremic solute concentrations is reached between the blood and the PD fluid. Once this occurs, the PD fluid (“dialysate effluent”) is drained, again through the PD catheter. The process is then repeated with fresh PD fluid. The osmotically active agent present in the PD fluid draws fluid, called ultrafiltrate, across the peritoneal capillaries into the PD fluid. Wastes and excess fluid are removed when “spent” PD fluid effluent is drained. The cycle of infusion of PD fluid, followed by its dwell in the peritoneum, and subsequent drainage from the patient is referred to as an “exchange.”
2. What are the indications for and clinical benefits of PD?
PD can be performed in any patient who has end-stage kidney disease (ESKD) and has intact peritoneal anatomy and function. Specific contraindications are discussed later (see Question 9).
The main clinical benefit of PD is that it allows patients the flexibility and lifestyle choices inherent in a home-suitable kidney replacement therapy. It also has an advantage in providing continuous removal of waste and fluid, similar to the continuous function provided by the kidneys. The resulting physiologically gentle means of dialysis is thought to contribute to better preservation of existing residual kidney function (RKF). Maintenance of RKF has been shown to provide survival advantage for patients with ESKD. The continuous nature of PD also affords greater hemodynamic stability and avoids rapid transcellular shifts of fluids and electrolytes. These features help maintain circulatory integrity and tissue perfusion, factors potentially compromised by intermittent renal replacement therapies.
Initiating renal replacement therapy (RRT) with PD also helps preserve vasculature for future vascular access as part of an “integrated therapy” strategy for patients anticipated to require multiple RRTs (PD, transplant, home hemodialysis [HD], in-center HD) over their lifetime. As such, a RRT strategy of “PD first” is one advocated by an increasing number of clinicians when a preemptive kidney transplant is not available. This approach could also reduce the need for central venous catheters (CVCs) in patients needing an unplanned dialysis start.
Better preservation of RRF, avoidance of compromised cardiac, brain, and gut perfusion, as well as avoidance of CVCs may help explain the apparent survival advantage reported in retrospective observational analyses of propensity matched-PD and HD-treated incident ESKD cohorts.
3. What is peritoneal membrane transport, and why is it important?
Bidirectional transport of solutes and water occurs across the capillary walls of peritoneal membrane. Solute concentration gradients between the peritoneal capillary blood and the PD fluid are the primary drivers of net transport. However, the intrinsic transport characteristics of the peritoneal membrane capillary network are variable between patients and have a significant impact on patient outcome. Understanding an individual patient’s peritoneal membrane transport type is therefore critical to appropriate tailoring of their PD prescription.
Membrane transport is classified as slow, slow average, fast average, or fast, according to the peritoneal equilibration test (PET), as described in the following sections. In general, patients with fast to fast average membrane transport characteristics should be prescribed shorter dialysis dwell times to enhance fluid and small solute removal. Patients with slow and slow average membrane transport should generally be prescribed longer dwell times.
4. What is the PET?
The PET is a standardized procedure for assessing the permeability and efficiency of a patient’s membrane to exchange small solutes and fluid. The PET uses a series of dialysate (D) and plasma (P) samples obtained over a 4-hour period to measure solute equilibration (D/P creatinine), rate of glucose absorption, and net fluid removal or “ultrafiltration” (UF; Fig. 53.1 ). After determining these values, the patient’s peritoneal membrane is categorized into one of the four membrane transport classifications. Each membrane classification (slow, slow average, fast average, fast) has specific characteristics that guide the clinician in tailoring the patient’s dialysis prescription. An example of results from a typical standard PET is included ( Fig. 53.2 ). In general, patients found to have fast to fast average membrane transport characteristics should be prescribed shorter dialysis dwell times to enhance fluid and small solute removal. Rapid equilibration of waste between dialysate and plasma, along with absorption of the osmotic agent (dextrose) by abundant peritoneal capillaries, are the reason for this.
The standard PET is usually done with a 2.5% (2.27% anhydrous) dextrose PD solution, but a 4.25% (3.86% anhydrous) dextrose solution can be used as an alternative. The benefit of using the latter is that it produces near-identical diffusive results to a 2.5% solution and provides additional information about maximal UF capacity of the peritoneal membrane being tested. Reproducible and accurate results have been demonstrated with either solution.
5. What is the “three-pore model,” and what is its relevance to peritoneal membrane transport?
Fluid and solute are transported between the blood and the peritoneal cavity across the peritoneal membrane. The capillary endothelial membrane provides the primary hindrance to this exchange. Mathematical modeling to describe, understand, and simulate this transport was captured by Bengt Rippe in his 1991 description of the “Three-Pore Model.” The accuracy of the model has been validated in clinical studies and is used extensively to predict solute and fluid removal using different PD prescriptions for given peritoneal transport characteristics. As implied, transport across the peritoneal capillary is characterized to occur across three distinct endothelial “pores”:
- 1.
Intracellular aquaporins or water channels. Aquaporins are affected by osmotic pressure and are exclusively permeable to water.
- 2.
Inter-endothelial cell or “small” pores. Small pores respond to both crystalloid and colloid osmotic forces and are permeable to both water and solutes smaller than albumin.
- 3.
Large inter-endothelial cellular pores. Large pores account for less than 0.01% of all capillary pores. While these are capable of passing larger molecules and proteins, they are effectively unresponsive to osmotic forces given their large size (precluding a transcellular gradient). Transport across these is unidirectional from plasma to peritoneal cavity and occurs by hydrostatic pressure. Large pores are responsible for leakage of protein into the peritoneal cavity.
The Three-Pore Model is a fairly accurate mathematical tool used to predict solute and water transport for specific peritoneal membrane transport characteristics in response to different PD solutions with varying osmotic contents.
6. What are the different methods of PD catheter placement?
There are currently three techniques for catheter placement:
- 1.
The dissective technique involves surgical placement of the catheter by mini-laparotomy. This is typically done under general anesthesia.
- 2.
The modified Seldinger technique involves “blind” insertion of a needle into the abdomen, placement of a guidewire, dilation of a tract, and insertion of the catheter through a sheath, all without visualization of the peritoneal cavity.
- 3.
Laparoscopic insertion using a small optical peritoneoscope for direct inspection of the peritoneal cavity. The latter can be performed as an outpatient procedure under local anesthesia with gas insufflation.
The advantage of the Seldinger approach is that it can be placed acutely at the bedside or in the interventional radiology/nephrology suite without the need for general anesthesia. Conversely, superior results have been demonstrated using the advanced laparoscopic technique. Though some of the success may be operator dependent, the technique benefits from:
• Rectus sheath tunneling, during which the transmural segment of the catheter is obliquely placed through a long musculofascial tunnel in the abdominal wall. This effectively maintains pelvic orientation of the catheter tip and reduces the risk of both exit site leak and hernia.
• Direct visualization of the PD catheter into the pelvic cavity
- •
Ability to address other abdominal peritoneal issues such as occult hernias, adhesions, redundant omentum, and epiploic appendices that may influence short- and long-term catheter success
The insertion technique used is determined by availability of expertise and economics. Success of the catheter after implantation is driven by the following of best demonstrated practices (BDPs), operator expertise, and patient comorbidities. To ensure the best patient outcomes, there needs to be cooperation among surgeons, radiologists, and nephrologists, irrespective of who places the PD catheter. These operators need to work collaboratively to develop common pathways and techniques to provide timely peritoneal access and resolve complications.
7. Can elderly, obese, diabetic, and pediatric patients receive PD?
PD can be used successfully in the majority of patients with kidney disease requiring dialysis. PD has been shown to be effective for patients with large body size or obesity, polycystic kidney disease, advanced age, diabetes, or other comorbidities (e.g., liver failure, ascites), and in patients without clinically significant kidney function (called anuria ; see Question 8).
PD is also the preferred form of dialysis for most pediatric patients with ESKD, including neonates and infants. Benefits include no need for vascular access or venous puncture, association with good blood pressure control, fewer hospital visits for dialysis and associated care, facilitation of full-time school attendance, and better psychosocial adjustment for both patients and caregivers.
Assisted PD describes support of individuals unable to perform their own PD with assistance for all or part of their dialysis procedure. Assistance is provided by a health care technician, community nurse, family member, or a trained partner. Assisted PD is an option for elderly or disabled patients, allowing them to initiate or continue to use PD despite mental or physical limitations.
8. Can patients continue receiving PD after they are anuric?
Although maintenance of even a minimal amount of kidney function has been demonstrated to have a survival benefit in patients treated with either PD or HD, patients with anuria do fine on PD. Adequate nutritional intake and UF appear to be key elements to good outcome in patients who are anuric and receiving PD.
9. What are the absolute contraindications to PD?
The definition of absolute contraindication is the presence of a clinical condition that makes a treatment either unsafe or unlikely to be effective. There are few absolute contraindications for PD. PD is contraindicated in patients with:
- •
Diaphragmatic defects (e.g., pleuroperitoneal abnormalities)
- •
Abdominal defects (e.g., unfixable hernia) or processes (e.g., acute diverticulitis) that prevent effective PD or increase the risk of infection
- •
Situations where the patient and/or caregiver are unable or unwilling to learn the therapy. As noted previously, assisted PD has provided a solution for the later problem.
10. Why would a patient want to do PD?
•
PD is generally provided in the patient’s home, precluding the need to commute to and from dialysis on a fixed schedule.
- •
PD provides patients an active role in their own care
- •
Greater independence
- •
More flexibility in dialysis prescription to accommodate school, work, travel, and recreation
- •
Unlike conventional HD, PD is a needleless form of RRT.
11. Are there requirements that would limit certain patients from being able to do PD?
PD is typically performed in the home, a setting in which either the patient or home caregiver is responsible for setup, connections, and execution of the treatment. While this may limit the ability of certain patients with visual, tactile, or motor restrictions from performing PD on their own, for most patients, “where there is a will, there is a way.” Patients are enabled by the flexibility to do either continuous ambulatory peritoneal dialysis (CAPD) or automated peritoneal dialysis (APD; see definitions that follow), family and/or caregivers (i.e., see the section on “Assisted Peritoneal Dialysis” that follows), and PD nursing teams knowledgeable about innovative care accessories that help overcome most limitations. Notably, many nursing homes offer PD, offloading any burden from the patient. Similarly, while space limitations or the presence of pets (i.e., cats) may at first seem to be a barrier to PD use, smaller, more frequent home supply deliveries and the use of CAPD, respectively, help circumvent these issues. PD should ideally be performed in a dedicated area of the living quarters that is kept clean (not sterile), and is free from blowing dust or dander.
12. Which is better: PD or HD?
PD, HD, and kidney transplant offer alternative and complementary means to treat ESKD. Most comparative analyses of observational registry studies have demonstrated similar outcomes for PD and HD in patients. A number of studies have shown better early survival (6 to 24 months) for patients treated with PD compared with those treated with HD (lower relative risk of death or higher survival probability favoring PD). While patient involvement in the decision-making process is key, clinician guidance in development of an integrated “ESKD Life Plan” can optimize clinical and lifestyle outcomes when the full spectrum of renal replacement therapies is appropriately sequenced. When possible, initial treatment with PD has the potential to improve early survival, improve transplant results, preserve vasculature for future access, and maintain more downstream renal replacement options. A “PD First” or preferred policy exists in many countries based on clinical as well as economic benefits.
13. What do CAPD, APD, CCPD, NIPD, and “high-dose” mean?
CAPD is the abbreviation for continuous ambulatory peritoneal dialysis. Typically patients manually infuse and drain 2 to 3 L of PD fluid three to four times a day. The PD fluid is allowed to dwell in the peritoneal cavity for a period of 4 to 6 hours per each of three daytime exchanges and 8 to 10 hours during the overnight exchange. Patients will usually carry PD fluid in the peritoneum continuously, 24 hours a day. Depending on the individual circumstance, a dry period may be allowed for reasons of patient comfort or convenience.
APD is the abbreviated term for automated peritoneal dialysis. This refers to use of a cycler (see Question 14) to assist in administration and drainage of PD fluid. Typically this is utilized to administer several dialysis exchanges at night while the patient is sleeping, with a final filling of the abdomen in the morning before the patient disconnects from the device. When APD is programmed to provide dialysis cycles both at night and for a “last fill” of fresh PD fluid that will remain in the peritoneal cavity during the day, it is called continuous cyclic PD, or CCPD. When a “last fill” is not programmed and the cycler only provides nocturnal dialysis exchanges, the APD is termed nocturnal intermittent PD, or NIPD. NIPD, where patients have a “dry day,” should only be considered for patients who have residual kidney function (RKF). RKF is associated with improved survival in PD patients thought perhaps to be related to removal of larger molecular weight substances (i.e., B2 Microglobulin). When kidney function is lost, removal of these larger molecular weight solutes by PD is time-dependent and therefore is enhanced with a long dwell. As such, anuric patients should be prescribed either CCPD or CAPD where there is no “dry” period (neither a “dry day” or a “dry night,” respectively).
Use of APD does not preclude additional manual exchanges from being done during the daytime hours, if needed. When a “last fill” is drained in the late morning or in the afternoon followed by infusion of an additional “midday” exchange with the intent of augmenting fluid and solute removal, the term “high-dose” is used.
14. What is acute PD?
Many patients need urgent or emergent dialysis. This occurs in patients who are not previously known to have chronic kidney disease (CKD) or when dialysis is started in situations of progressively deteriorating, but known, CKD without a permanent access (i.e., a fistula or a PD catheter). Urgent or emergent dialysis also occurs in situations of acute kidney injury (AKI). In the United States, Canada, and Europe, these patients are usually started on HD after placement of a CVC. However, there is movement to increase use of PD in these situations, with the goal of avoiding CVCs (tunneled and untunneled) and the morbidity and mortality associated with them.
PD can be used in many patients who have an unplanned start for dialysis. In a retrospective analysis comparing the outcomes of a group of patients started acutely on PD and a nonmatched group of patients with a planned start on chronic PD, there was no difference in infectious complications or technique survival rate, although mechanical complications were significantly more common in the acute group. In another small study in France, patients were nonrandomly selected for unplanned start with either PD or HD. Median time from PD catheter insertion to PD start was 4 days. The 1-year survival adjusted for comorbidity (79% survival on HD compared with 83% on PD) and the rehospitalization rate were similar. In a more recent observational cohort study from Germany, groups started on either unplanned acute PD or HD had equivalent mortality rates. HD patients had a significantly higher risk of bacteremia, presumably due to CVC use. PD was initiated within 12 hours after PD catheter implantation in this study, delivered nocturnally thrice weekly. Urgent start PD has also been proven effective in the elderly population. It should be noted that these are all single-center studies where the norm is HD. One would anticipate that increasing experience using PD for unplanned starts would improve outcomes.
Published experience with PD for AKI is limited. Ponce et al. demonstrated that high-volume PD (weekly Kt/V ~ 3.5) could achieve adequate metabolic and fluid control in AKI patients without severe fluid overload or hypercatabolism. A prospective randomized experience of 120 patients comparing high-volume PD to six times per week HD showed both similar survival (58% and 53%) and recovery of kidney function (28% and 26%). Although results here are encouraging, experience with acute placement of PD catheters and PD therapy itself is a critical factor for success. Acute abdominal processes would be a contraindication to using acute PD.
15. What is a cycler, and who can or should use it?
A cycler is a mechanized device made to assist in the administration and drainage of PD fluid. Use of the cycler to administer part or all of the PD prescription is termed APD. Although the cycler is primarily used by patients to administer their PD prescription at home, it can also be used in settings outside the home such as acute care, chronic care, or rehabilitation facilities to provide dialysis. The automation provided by APD allows a wider range of PD prescription options (both dwell volume and exchange frequency), allowing the ability to tailor prescriptions to accommodate individual patient needs. Accordingly, patients with “fast” peritoneal membrane transport characteristics have improved survival using APD compared with CAPD. Use of the cycler also reduces the number of manual connections the patient or care provider needs to perform. This lowers the risk for touch contamination. Patients treated with APD have similar or lower rates of peritonitis and similar or better technique survival compared with patients not using the cycler (e.g., CAPD).
16. What are the contents of PD solutions?
All PD solutions are sterile fluids containing physiologically balanced amounts of electrolytes and an osmotically active agent. The latter is needed to draw fluid across the peritoneal capillaries into the PD fluid (UF). There is a variety of commercial PD solutions available, with the main differences related to one of two things: the base buffer (and accompanying solution pH) and the osmotically active agent ( Table 53.1 ).
| Dianeal PD1 | Dianeal PD2 | Dianeal PD4 | Physioneal 35 | Physioneal 40 | Extraneal | Nutrineal | Plasma (Adult) | |
|---|---|---|---|---|---|---|---|---|
| Electrolytes (mmol/L) | ||||||||
| Sodium | 132 | 132 | 132 | 132 | 132 | 133 | 132 | 136–145 |
| Calcium | 1.75 | 1.75 | 1.25 | 1.75 | 1.25 | 1.75 | 1.25 | 1.12–1.32 |
| Magnesium | 0.75 | 0.25 | 0.25 | 0.25 | 0.25 | 0.25 | 0.25 | 0.65–1.05 |
| Chloride | 102 | 96 | 95 | 101 | 95 | 96 | 105 | 98–107 |
| Buffer (mmol/L) | ||||||||
| Lactate | 35 | 40 | 40 | 10 | 15 | 40 | 40 | 0.6–1.7 |
| Bicarbonate | — | — | — | 25 | 25 | — | — | 21–30 |
| pH | 5.5 | 5.5 | 5.5 | 7.4 | 7.4 | 5.5 | 6.7 | 7.4 |
| Osmotic Agent, Osmolarity (mOsm/L) | ||||||||
| 1.36% glucose | 347 | 345 | 344 | 345 | 344 | — | — | — |
| 2.27% glucose | 398 | 396 | 395 | 396 | 395 | — | — | — |
| 3.86% glucose | 486 | 484 | 483 | 484 | 483 | — | — | — |
| 7.5% icodextrin | — | — | a | — | — | 284 | — | — |
| 1.1% amino acids | — | — | — | — | — | — | 365 | — |
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