peritoneal dialysis anatomy and physiology of peritoneal dialysis

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  • Slide 1
  • Peritoneal Dialysis Anatomy and Physiology of Peritoneal Dialysis
  • Slide 2
  • 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 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
  • Slide 3
  • Peritoneal Dialysis 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 Peritoneal Membrane Anatomy Key Points
  • Slide 4
  • Peritoneal Dialysis Peritoneal vasculature The Normal Peritoneal Membrane Mesothelial cell monolayer Interstitium
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  • Peritoneal Dialysis The Normal Peritoneal Membrane
  • Slide 6
  • Peritoneal Dialysis Pathways for Peritoneal Transport Endothelium Capillaries Mesothelium Small solutes Glucose Macro molecules Crystalloid osmosis Colloid osmosis Water Interstitium Peritoneal tissue layer Dialysate
  • Slide 7
  • Peritoneal Dialysis Peritoneal transport Two clinical end-points Clearance of solutes (by diffusion and convection) Fluid removal (transcapillary UF fluid absorption)
  • Slide 8
  • Peritoneal Dialysis Peritoneal Transport Three Distinct Processes Diffusion Ultrafiltration Fluid Absorption
  • Slide 9
  • Peritoneal Dialysis 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)
  • Slide 10
  • Peritoneal Dialysis Peritoneal Equilibration Test
  • Slide 11
  • Peritoneal Dialysis Diffusion How to Increase It Maximize concentration gradient - More frequent exchanges (e.g., APD) - Larger dwell volumes Increase effective peritoneal surface area - Larger dwell volumes
  • Slide 12
  • Peritoneal Dialysis Ultrafiltration What 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)
  • Slide 13
  • Peritoneal Dialysis 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)
  • Slide 14
  • Peritoneal Dialysis 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)
  • Slide 15
  • Peritoneal Dialysis Pathways of Glucose Flow GlucoseCapillary Peritoneal Space Intercellular: >90% Glucose transporter mediated: minimal
  • Slide 16
  • Peritoneal Dialysis Membrane Model BLOOD Membrane PERITONEAL DIALYSATE
  • Slide 17
  • Peritoneal Dialysis 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
  • Slide 18
  • Peritoneal Dialysis 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)
  • Slide 19
  • Peritoneal Dialysis Determinants of Clearance Achieved on PD Residual renal function Body size (Volume or Body Surface Area) Peritoneal solute transport rate The prescription
  • Slide 20
  • Peritoneal Dialysis 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
  • Slide 21
  • Peritoneal Dialysis 0 500 1000 1500 2000 2500 3000 Total removal of protein, mg 060120180240300360 Time, min L L-A H-A H Wang et al. Nephrol Dial Transplant 13: 1242-49, 1998 Total Removal of Protein in Different Transport Groups
  • Slide 22
  • Peritoneal Dialysis 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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