Peritoneal Dialysis

Anatomy and Physiology of Peritoneal Dialysis

Peritoneal Dialysis

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

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 AnatomyKey Points

Peritoneal Dialysis

Peritoneal vasculature

The Normal Peritoneal Membrane

Mesothelial cell monolayer

Interstitium

Peritoneal Dialysis

The Normal Peritoneal Membrane

Peritoneal Dialysis

Pathways for Peritoneal Transport

Endothelium

Capillaries

Mesothelium

Small solutes

Glucose

Macro molecules

Crystalloidosmosis

Colloidosmosis

Water

Interstitium

Peritoneal tissue layer

Dialysate

Peritoneal Dialysis

Peritoneal transport

Two clinical end-points

Clearance of solutes (by diffusion and convection)

Fluid removal (transcapillary UF – fluid absorption)

Peritoneal Dialysis

Peritoneal Transport

Three Distinct Processes

Diffusion

Ultrafiltration

Fluid Absorption

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)

Peritoneal Dialysis

Peritoneal Equilibration Test

Glucose

00.10.20.30.40.50.60.70.80.9

1

0 1 2 3 4Hours

D/D

O

Creatinine

0

0.2

0.4

0.6

0.8

1

0 1 2 3 4Hours

D/P

Low

Low ave

Ave

High ave

High

Peritoneal Dialysis

DiffusionHow to Increase It

Maximize concentration gradient

- More frequent exchanges (e.g., APD)

- Larger dwell volumes

Increase effective peritoneal surface area

- Larger dwell volumes

Peritoneal Dialysis

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)

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)

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)

Peritoneal Dialysis

Pathways of Glucose Flow

Glucose

CapillaryCapillary

Peritoneal SpacePeritoneal Space

Intercellular: >90%Intercellular: >90%

Glucose transporter Glucose transporter mediated: minimalmediated: minimal

Peritoneal Dialysis

Membrane Membrane MModelodelMembrane Membrane MModelodel

BLOOD

Mem

bran

e

PERITONEAL DIALYSATE

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

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)

Peritoneal Dialysis

Determinants of Clearance Achieved on PD

Residual renal function

Body size (Volume or Body Surface Area)

Peritoneal solute transport rate

The prescription

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

Peritoneal Dialysis

0

500

1000

1500

2000

2500

3000

Tot

al r

emov

al o

f p

rote

in, m

g

0 60 120 180 240 300 360

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

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.