The process of peritoneal dialysis involves the introduction of dialysis fluid into the peritoneal cavity via a catheter placed through the abdominal wall.
Two interactions take place between the dialysis fluid and the extracellular body fluids:
Dialysis
Ultrafiltration
Dialysis
Dialysis is the process whereby solute diffuses down a concentration gradient. The
dialysis fluid contains sodium, chloride, calcium and magnesium in concentrations aimed at
maintaining the concentrations of these ions in the normal range in the extracellular
fluid.
The fluid does not normally contain substances such as potassium, phosphate, urea and
creatinine and these can diffuse down a concentration gradient, across the dialysis
membrane, which consists of the peritoneal membranes and the capillary endothelium, and
pass into the dialysis fluid.
Patients with chronic renal failure also become acidotic, because of a failure to
replenish the bicarbonate buffer pool. During dialysis this bicarbonate must be
replaced. Bicarbonate could be added to the dialysis fluid, but tends to precipitate
out with the calcium in the fluid. Instead lactate has used and this is metabolised
in the liver to bicarbonate. More recently dialysis fluids have been developed which
contain bicarbonate and these are less damaging to the peritoneal membrane. The
dialysis bags are made of two compartments, one with the calcium-containing solution and
the other the bicarbonate. Immediately prior to use, the barrier between the two
compartments is broken and the solutions mixed.
Ultrafiltration
As well as needing to remove solutes and replace bicarbonate, it is also necessary to
remove water. This is achieved by creating an osmotic gradient across the dialysis
membrane, which acts as a semi-permeable membrane. The osmotic gradient is usually
achieved by adding dextrose to the dialysis fluid. This creates a hypertonic
solution which can drag water from the extracellular compartment.
The dialysis fluid is most effective when it is fresh and its ability to dialyse and ultrafiltrate will decrease with time. The fluid therefore needs to be regularly drained and replaced. Two basic types of peritoneal dialysis are used:
Continuous ambulatory peritoneal dialysis (CAPD)
Continuous cycling peritoneal dialysis (CCPD) or Automated peritoneal dialysis (APD)
CAPD
This method is more commonly used in adults and involves replacing the fluid four or
five times a day, leaving the fluid to dwell for about 4 hours and then a single overnight
dwell.
CCPD
This method is favoured in children and relies on a machine to cycle the dialysis
fluid into and out of the peritoneal cavity overnight. The patient is connected to
the machine when they go to bed and disconnected in the morning. The fluid is
replaced approximately every hour, giving 8-10 cycles overnight.
Below are some web sites which provide some more information about peritoneal dialysis:
http://kidney.niddk.nih.gov/kudiseases/pubs/peritoneal/index.htm
http://www.kidney.org/atoz/content/peritoneal.cfm