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Anatomy and Physiology of Peritoneal Dialysis

Anatomy and Physiology of Peritoneal Dialysis. Peritoneal Membrane Anatomy Key Points. Serosal membrane with area equivalent to body surface area, I.e. 1 to 2 metres 2 80% is visceral peritoneum and gets its vascular supply via the mesenteric arteries and portal veins

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Anatomy and Physiology of Peritoneal Dialysis

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  1. Anatomy and Physiology of Peritoneal Dialysis

  2. Peritoneal Membrane AnatomyKey Points • Serosal membrane with area equivalent to body surface area, I.e. 1 to 2 metres2 • 80% is visceral peritoneum and gets its vascular supply via the mesenteric arteries and portal veins • 20% is parietal peritoneum and gets its vascular supply via arteries and veins of abdominal wall • Lymphatic drainage of peritoneal cavity is mainly via diaphragmatic stomata

  3. Peritoneal Membrane AnatomyKey Points • Peritoneal cavity is lined by a mesothelial monolayer which produces a lubricating fluid • Under the mesothelium is a gel-like interstitium containing connective tissue fibres, capillaries and lymphatics • The effective surface area is critical for dialysis and depends on the vascularity of the peritoneum as well as its surface area

  4. The Normal Peritoneal Membrane Mesothelial cell monolayer Interstitium Peritoneal vasculature

  5. The Normal Peritoneal Membrane

  6. Mesothelium Pathways for Peritoneal Transport Capillaries Macro molecules Water Small solutes Endothelium Colloidosmosis Crystalloidosmosis Glucose Interstitium Peritoneal tissue layer Dialysate

  7. Peritoneal transportTwo clinical end-points • Clearance of solutes (by diffusion and convection) • Fluid removal (transcapillary UF – fluid absorption)

  8. Peritoneal TransportThree Distinct Processes • Diffusion • Ultrafiltration • Fluid Absorption

  9. What Happens with Solute Removal During a CAPD Dwell? • Diffusion is at a maximum, and urea and creatinine equilibration are fastest, in the first hour but become slower as the gradient lessons with time • By 4 hours, urea is >90% and creatine > 65% equilibrated in most patients • Dialysate to plasma (D/P) ratios measure degree of equilibration at a given dwell time (e.g. D/P Urea, D/P Creatine)

  10. Peritoneal Equilibration Test

  11. DiffusionHow to Increase It • Maximize concentration gradient - More frequent exchanges (e.g., APD) - Larger dwell volumes • Increase effective peritoneal surface area - Larger dwell volumes

  12. UltrafiltrationWhat are the key factors? • Osmotic gradient (e.g. for glucose) • Reflection and UF coefficients (NB – not discussed during this course) • Hydrostatic and oncotic pressure gradients (NB – not discussed during this course)

  13. Peritoneal Fluid Absorption • Occurs directly via lymphatics • Also absorption into tissues with subsequent removal via lymphatics and capillaries • Difficult to measure but is about 1 to 2 mls per minute (250-500 mls in 4 hours)

  14. Net Ultrafiltration • Net UF is actual UF minus fluid absorption e.g. 1000mls – 200mls = 800 mls Net UF • Clinically we can only influence Net UF by: - altering the osmotic gradient (e.g. from 1.36% to 2.27%), or by - changing the osmotic agent (e.g. from glucose to icodextrin)

  15. Capillary Glucose Peritoneal Space Pathways of Glucose Flow Glucose transporter mediated: minimal Intercellular: >90%

  16. Membrane Model Membrane PERITONEAL DIALYSATE BLOOD

  17. What Happens to Fluid Removal with a 2L 4.25% PD Dwell? Note: I/P = Intraperitoneal or inside peritoneal cavity • UF is maximal at the start of the dwell, approx. 15 ml/min • It quickly lessons as glucose diffuses out of the dialysate into the blood and as the UF dilutes the glucose • I/P volume increases until about 3 hours when UF rate falls to equal the constant fluid absorption rate of 1-2 ml/min • After this, the I/P volume reduces until it is less than 2L after 8-10 hours, leading to net fluid retention

  18. Small Solute Clearance in PD Patients Clearance is the quantity of plasma from which solute is cleared per unit time • In PD: > total clearance = peritoneal + residual renal • Peritoneal clearance depends on: > diffusion + UF – fluid absorption and so varies during the course of the dwell period • Daily peritoneal clearance = > daily dialysate drain volume x D/P ratio (for the solute concerned over that day)

  19. Determinants of Clearance Achieved on PD • Residual renal function • Body size (Volume or Body Surface Area) • Peritoneal solute transport rate • The prescription

  20. What About Protein? • Protein losses occur via large pores, are greatest in high transporters and average 6 to 10 g/day • About 50% of losses are albumin and there is an inverse relationship to serum albumin • Fluid absorption during a dwell prevents losses being greater • Losses are not much affected by PD prescription, but increase during peritonitis

  21. Total Removal of Protein in Different Transport Groups 3000 2500 2000 H 1500 Total removal of protein, mg H-A 1000 L-A 500 L 0 0 60 120 180 240 300 360 Time, min Wang et al. Nephrol Dial Transplant 13: 1242-49, 1998

  22. Conclusion • A knowledge of peritoneal anatomy and physiology is important in the management of PD patients • In particular, it helps to solve problems with clearance and ultrafiltration • It also improves understanding of the impact of new technologies such as cyclers, larger dwell volumes, new PD solutions, etc.

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