All You Ever Wanted To Know
Previous: Introduction to CAPD
Typically, Peritoneal Dialysis is delivered as 4 exchanges in 24 hrs. The day-time exchanges are 4 hrs each with a long night-time exchange.
The strength of the bag (i.e. 1.5% or 2.5% or 4.25%) is decided initially by trial and error based on the amount of ultrafiltration achieved.
Commonly, 2 litres of fluid is let inside the abdomen and when it is drained out at the end of the dwell time, the volume is measured and recorded. The 'extra' fluid that drains out is the amount of ultrafiltration for that exchange.
For example, if 2 litres of 1.5% solution is used and the drain volume at the end of 4 hrs is 2.5 litres, the ultrafiltration achieved is 500 mL.
This test tells us the type of peritoneal membrane in the individual.
The ultrafiltration in CAPD is driven by osmotic force exerted by glucose in the solution. Glucose is freely permeable across the capillary, meaning it can move from the dialysis solution into the patient's blood circulation reducing the glucose concentration in the PD fluid.
This exchange varies from person to person. Broadly there are four categories of 'exchangers';
The 'Low exchangers' do not lose the glucose concentration in the PD fluid and therefore the osmotic force exerted by glucose persists longer. This translates into a good volume of ultrafiltration.
Conversely, the 'High exchangers' lose the glucose in the PD fluid to the patient's blood and the osmotic force is therefore reduced translating into low volumes of ultrafiltration.
The other aspect of the membrane type is its ability to excrete waste products like creatinine into the PD fluid. It is easy to understand that 'low exchangers' will excrete less and 'high exchangers' will excrete more.
The Peritoneal Equilibration Test (PET) is a simple procedure where concentration of creatinine is measured at timed intervals in both, the blood and the PD fluid. Ratio is calculated and a high fluid to blood creatinine ratio means that the membrane is 'high exchanger' and low ratio means a 'low exchanger' membrane.
The amount of clearance achieved is measured by two variables, the KT/V and creatinine clearance.
KT/V is amount of urea clearance achieved corrected for the body size and the target value is 1.7 per week.
Creatinine clearance is the amount of blood cleared of creatinine every week. The target value is 60 L/week.
These variables are calculated by collecting drains for entire 24 hrs and measuring urea and creatinine in the fluid. A complex formula computes it for the entire week.
If the patient has urine output, similar calculations are done in the urine and the two values ( PD fluid clearance + urinary clearance ) are added up.
If clearance targets are not achieved, following steps can be taken:
Add a night-time exchange if not being given
Increase the volume of exchange (e.g. 2.5 lit )
Increase the tonicity of the fluid ( e.g. if all four exchanges are of 1.5%, change two of them to 2.5%)
Increase the number of cycles to 5 or sometimes even more. A cycler machine can be used in the night for frequent low volume exchanges ( e.g. 1 lit exchanges every one hour for 8 hrs ! )
Finally, one session of hemodialysis can be added every week to boost the amount of clearance achieved by CAPD alone.
This is the only common major complication associated with CAPD. Infection in the Peritoneal membrane is called peritonitis.
It is easy to understand that contaminated hands will definitely transfer infected material into the peritoneal cavity. This is the reason why proper hand cleaning is a must. (click here to watch a video on hand-washing technique).
The other common mode of acquiring infection is from the intestines. A constipated individual can acquire infection in this manner when he strains for defecation. Therefore straining is strictly 'prohibited' for patients on CAPD.
Acceptable frequency of infection is once in two years. Anything more frequent than this warrants a complete 'investigation' into the technique of exchange and other factors.
Peritonitis is easily treatable and is treated by adding antibiotics to the CAPD fluid. Sometimes, if the organism is resistant to various antibiotics, intravenous antibiotic treatment may be needed.
Sequele to a peritonitis episode is change in membrane characteristics resulting in reduced solute clearances which may lead to under-dialysis.