PD Membrane Characteristics

Peritoneal Dialysismedium

A nephrology fellow is reviewing peritoneal membrane physiology before starting a patient on peritoneal dialysis. Which of the following statements about the peritoneal membrane and fluid transport is correct?

  1. A.Ultrasmall pores (aquaporins) transport sodium along with free water
  2. B.The parietal peritoneum provides about 80% of the effective exchange surface area
  3. C.The risk of encapsulating peritoneal sclerosis exceeds 70% after 5 years on PD
  4. D.Net ultrafiltration is greatest at the beginning of a hypertonic dextrose dwellCorrect

Explanation

In the three-pore model, the peritoneal capillary endothelium contains large pores, small pores, and ultrasmall transcellular pores represented by aquaporin-1 channels. Hypertonic dextrose creates a crystalloid osmotic gradient that draws water from the peritoneal capillaries into the dialysate. The transcapillary ultrafiltration rate is greatest early in the dwell, when this osmotic gradient is maximal, and progressively declines as dextrose is absorbed and the gradient dissipates. Cumulative net ultrafiltration initially rises but may later plateau or decrease during a prolonged dwell because fluid absorption continues. Aquaporin-1 channels transport solute-free water without sodium. This causes dilution of dialysate sodium during the early dwell, producing the characteristic transient sodium dip known as sodium sieving. Small interendothelial pores account for most transport of sodium and other low-molecular-weight solutes, whereas the relatively few large pores permit movement of macromolecules such as proteins. The parietal peritoneum does not provide 80% of the anatomical surface area. Most of the anatomical peritoneal surface is visceral, although the effective exchange area is determined mainly by the number and perfusion of capillaries in contact with dialysate rather than anatomical area alone. Encapsulating peritoneal sclerosis is a rare but serious complication—generally affecting only a small percentage of PD patients—and its risk increases substantially with longer PD duration; it does not approach 70% after 5 years.

Why each option

A.
Incorrect. Aquaporins conduct free water only; sodium is excluded, which is why early-dwell sodium sieving occurs.
B.
Incorrect. The visceral peritoneum, not the parietal peritoneum, contributes most of the effective exchange surface area.
C.
Incorrect. Encapsulating peritoneal sclerosis is uncommon (about 1-3% at 5 years), increasing with longer PD duration.
D.
Correct. The dextrose osmotic gradient is maximal at the start of the dwell, so net ultrafiltration is greatest early and declines as dextrose is absorbed.

Reference: Rippe B. A three-pore model of peritoneal transport. Perit Dial Int. 1993;13(Suppl 2):S35-S38; Devuyst O, Rippe B. Water transport across the peritoneal membrane. Kidney Int. 2014;85(4):750-758.

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