Peritoneal dialysis (PD): overview

Fundamentals

• Sterile dialysate fluid is introduced into the peritoneal cavity via a catheter and exchanged at intervals
  • Typically, the dialysate is infused, allowed to dwell for a period of time, and then drained; this cycle is repeated a number of times
    • Exception: continuous flow peritoneal dialysis
• Rather than using an extracorporeal dialyzer/filter, the patient’s peritoneal membrane serves as the semipermeable membrane
  • The presence of small pores aquaporin channels allow water to move through the membrane.
  • The peritoneum is highly vascular and the presence of capillaries allows for transfer of solutes.

Peritoneal membrane as filter

Why it works

• Within the peritoneal membrane (between the abdominal wall and the peritoneal cavity) there are capillaries, lymphatic vessels, and interstitial spaces
  • These enable the absorption of particles from the dialysate (in the peritoneal cavity) into the rest of the body
  • The peritoneal capillaries enable the movement of water (by
    ultrafiltration
    ) into the peritoneal cavity
• Transport in PD is an interaction of diffusion,
  ultrafiltration
  , and fluid absorption

Peritoneal transport: “three pore” conceptual model

• Large pores (>20 nm [>200 Å] diameter)
  • <10% of the pores in the peritoneal membrane
  • Permit transport of proteins (e.g., immunoglobulins) and other large molecules
  • Can be increased with significant inflammation such as peritonitis
• Small pores (4-6 nm [40-60 Å] diameter)
  • ~90% of pores in the peritoneal membrane
  • Transport most small molecules
• Ultra small pores (0.3-0.5 nm [3-5 Å] aquaporins)
  • 1-2% of pores in the peritoneal membrane
  • Responsible for nearly half of water transport

Diffusion in PD: key factors

• Concentration gradient of solute: dialysate/plasma (D/P) ratio
• Mass transfer area coefficient (MTAC)
  • Effective peritoneal surface area (analogous to the K₀A in
    HD
    )
    • Surface area of the peritoneal membrane
    • Vascularity: how many peritoneal capillaries are in the peritoneum
  • Diffusive characteristics of membrane for solute in question (permeability)
    • Varies from patient to patient
    • Changes in individuals over time
• Particle size:
  • Small molecules diffuse rapidly
  • Large molecules take a longer time to diffuse across the peritoneal membrane

Ultrafiltration in PD: key factors

• Osmotic gradient: fluid removal (
  ultrafiltration
  ) is accomplished using osmotic pressure rather than hydrostatic pressure
  • The osmotic gradient is created by including an osmotic agent (e.g., dextrose, icodextrin) in the dialysate
    • Increasing the concentration of the osmotic agent generates a greater osmotic gradient and results in more fluid removal
    • Using dialysate with higher dextrose concentration generates a greater osmotic gradient and results in more fluid removal
• Reflection coefficient: how resistant a particle in the dialysate is to being absorbed (i.e., how well it stays in the dialysate), on a scale of 0 to 1
  • An osmotic agent with a reflection coefficient of 1 would be an ideal osmotic agent: it would draw water into the peritoneal cavity without being absorbed
• Ultrafiltration coefficient (of the membrane): depends on the surface area and the number of pores
• Hydrostatic pressure
  • Filling the peritoneal cavity with dialysate increases the hydrostatic pressure
• Oncotic pressure gradients
  • Related to changes of protein levels inside the bloodstream as water is removed from the peritoneal cavities

Fluid absorption

• Peritoneal fluid, including dialysate, is absorbed during peritoneal dialysis:
  • Direct lymphatic absorption
  • Tissue absorption (into the interstitium) of peritoneal fluid
• Fluid absorption limits
  ultrafiltration
  and mass transfer
  • Higher levels of peritoneal absorption reduces the net
    ultrafiltration

Comparison with HD

• PD is generally the preferred chronic dialysis modality in pediatrics
  • Depends on patient and family preference, institutional practice and availability

Advantages of PD

• Vascular access is not required
• Better outcomes (vs
  HD
  ) in the US
  • Gentler fluid removal over longer time period avoids cardiac stunning seen in
    hemodialysis
  • Better preservation of remaining kidney function
  • Better outcomes after kidney transplantation
  • Better growth
  • Improved fluid balance
    • Also reduces need for antihypertensive medications
• Daily clearance
  • Less accumulation of toxins (e.g., phosphorus, potassium) between sessions
• Better quality of life
  • Home treatment (less frequent travel to the dialysis unit)
  • Self control of the therapy
  • No needles
  • Facilitates school attendance/employment
  • Extended travel is possible
  • Fewer dietary restrictions
• Less technically difficult, especially in babies
  • Teaching usually takes 3-5 days

Disadvantages of PD

• Risk of infection: peritonitis, exit site, and tunnel infections
• Hernias
• Decreased appetite
• Body image disturbance
• Labor intensive
• Drawbacks of home PD:
  • Caregiver burden
  • Requires a lot of physical space to keep supplies
  • Requires a savvy family
    • However, even with poor educational background (e.g., illiteracy) can be trained to be competent

Contraindications to PD

• Anatomical
  • Multiple prior abdominal surgeries with lots of scarring
  • Compromised diaphragm with communication between abdominal and thoracic cavities (e.g., diaphragmatic hernia, surgical complication)
  • Other congenital abnormalities: omphalocele, gastroschisis, bladder exstrophy
• ≥2 prior episodes of peritonitis
• Obliterated peritoneal cavity
  • E.g., from prior surgeries, trauma, infections
• Peritoneal membrane failure

Relative contraindications

• Presence of ileostomies/colostomies
  • Ideally would locate as far away as possible from the PD exit site to avoid contamination
• Infants with significant organomegaly
• Impending abdominal surgery
• If expecting transplant soon (typically <3-6 months), usually prefer to initiate
  HD
  • Significant time investment of planning surgery, waiting for tract to mature, training of patient/family
• Lack of appropriate caregiver(s)
• Lack of appropriate home environment