Keywords: CAPD, Peritoneal dialysis, Dialysis adequacy, Peritonitis, Swan neck catheter, Catheter Survival, Patient Survival.
|How to cite this article:|
Khanna R, Nolph KD. CAPD - An Overview. Saudi J Kidney Dis Transpl 1994;5:23-7
| The global picture|| |
At the end of 1993, nearly 80,000 patients were estimated to be on chronic peritoneal dialysis worldwide  , which represents about 14% of the dialysis population. The percentage of patients on chronic peritoneal dialysis in different countries range widely from 6% in Japan to 93% in Mexico  . Over two-thirds of them are on continuous ambulatory peritoneal dialysis (CAPD). Approximately 20% of the chronic peritoneal dialysis patients are on some form of cycler based automated peritoneal dialysis system  . The proportion of patients on cycler therapy is growing at a rate faster than CAPD.
| Survival on CAPD|| |
A number of studies indicate that the actuarial survival rates, in comparable populations, are similar with CAPD and haemodialysis (HD) ,,,,,,,,,,,, . However, Maiorca and colleagues have observed that the relative risk of death may be higher in elderly patients on HD than on CAPD  . They also observed a more rapid increase in the relative risk of death for patients over the age of 53 years on HD than on CAPD  . On the contrary, a Cox proportional hazard model analysis of 4387 new endstage renal disease patients in the United States found a 14% greater risk of mortality for patients on CAPD than those on HD. The increased risk of death on CAPD was entirely accounted for by an increased risk of death for older diabetics on CAPD  . Interestingly, younger diabetics on CAPD had a lower risk of death when compared to HD. The analysis did not categorize in detail the severity of cardiac risk and thus, older diabetics on CAPD might have had more severe cardiac disease than older diabetics on HD. There was no consistent difference in death rates at different ages as a function of modality choice (HD vs. CAPD) for primary renal diseases other than diabetes  . Several studies in the past few years have not revealed modality choice as a significant risk factor ,, .
Technique survival generally implies the percentage of patients alive on their original dialysis treatment;death, transplantation and recovery of renal function are considered a loss to risk. In essence, a technique failure represents transfer to another form of dialysis treatment. Technique survivals on CAPD have usually been lower than HD primarily because of a high incidence of peritonitis in the era of standard connectology ,,,,,,, . The recent improvement in the incidence of peritonitis, due mainly to the introduction of Y-set, has resulted in a significantly improved technique survival rate of 77.4% for CAPD patients  . An Italian study made a similar observation and speculated that elimination of peritonitis should eventually result in comparable technique survivals for both HD and CAPD patients  .
The swan neck catheter with a permanent bend in the intercuff segment, has shown a survival rate of 73% at three years, a rate considerably better than the 30% reported previously by the United States CAPD registry  . The swan neck catheter was designed to provide a downward exit hole to reduce the risk of exit-site infection and a caudal direction to the intraperitoneal. segment of the peritoneal catheter on insertion to reduce the risk of catheter tip migration out of the pelvic space. Additionally, the permanently moulded bend in the catheter eliminates the resilient force, and thus lowers the risk of external cuff extrusion. Use of swan neck catheters has essentially eliminated these above mentioned complications. A recent survey indicates its use of over 30 percent of all the new catheters inserted world over  .
Peritoneal membrane stability
The recent peritoneal biopsy registry report suggests that the peritoneal membrane undergoes minimal or no morphological change for up to seven years, in the absence of peritonitis in patients on CAPD  . The changes noted vary from minimal changes to loss of mesothelium with hyalinized stroma and a striking acellular structure (cellular desert). The collagen in the stroma undergoes a change in texture and orientation. The peritoneum attains brownish discoloration with a leathery appearance. Cellular infiltration is sparse, distributed in the stroma and around the perivascular location, and the microvilli are lost. The rough endoplastic reticulum is hyperplastic and the occurrence of peritoneal fibrosis is an all or none phenomenon. The diabetiform changes in the peritoneal capillary basement membrane in non-diabetic patients appear only in those who experience peritonitis. It is hypothesized that the breakdown of the mesothelial barrier during peritonitis results in an increased permeability to high concentrations of glucose in the peritoneal interstitium and the glycosylation of the capillary basement membranes. The technique of autologous mesothelial cell transplantation in animals and patients is an attempt to re-mesothelialize the peritoneum after peritonitis  . Sequential measurements of solute clearances and mass transfer co-efficients in patients on CAPD for up to five to six years have shown no changes from baseline measurements  .
| Peritoneal dialysis adequacy and nutrition|| |
To achieve comparable clinical outcomes, greater small solute clearances are required by haemodialysis than CAPD. To explain this difference, the peak concentration hypothesis proposes that for comparable protein intake by HD and CAPD patients, weekly HD requires at least one and a half times more weekly urea clearance than does CAPD, in order to maintain the peak pre- HD blood urea nitrogen (BUN) levels below the steady state BUN levels of CAPD , . With the same protein intake, protein catabolic rate (PCR) and weekly urea clearances, the BUN levels in patients on three-times-weekly HD would be higher than CAPD steady state levels. In that case, and if BUN were a marker of small molecular weight toxin that suppresses appetite, then HD patients are clearly under-dialyzed and should manifest loss of appetite and a lower PCR unless weekly urea clearances were increased well above those of CAPD. The peak concentration hypothesis is supported by several recent studies that found at the same Kt/V, the urea clearance normalized to total body water, CAPD is indeed associated with a higher PCR than HD ,,, . We, at the University of Missouri try to maintain a PCR per normalized body weight of 0.9 or higher per day in CAPD patients. Each CAPD patient's weekly urea clearance is followed along with a weekly creatinine clearance. We have found that in most patients, to maintain the target PCR of 0.9gm/kg/day, a weekly urea Kt/V of 1.7 and a weekly creatinine clearance of 50 liters per 1.7 m2 body surface area is required. It is important to add the contribution of the residual renal function to the total solute clearance. Thus, the average of renal urea and creatinine clearances is added to the dialysate clearance to estimate the total clearance.
The monitoring of dialysis adequacy in each patient includes initial as well as periodic assessment of peritoneal membrane transport characteristics using the peritoneal equilibration test (PET)  and weekly urea Kt/V and creatinine clearance. Adjustment in dialysis prescriptions are made to meet the adequacy standards as residual renal function declines and peritoneal membrane function changes.
| Peritonitis and exit-site infection|| |
Recent introduction and widespread use of disconnect devices have resulted in consistent reduction in the incidence of peritonitis to about one episode every 24 patient months  . The key innovation that has contributed to such dramatic improvement in the peritonitis rate is the Y-set wherein, a flushing of the system with dialysis solution followed by drainage from the peritoneal cavity after connection is performed, before fresh solution is instilled. The use of in line disinfectant is optional in this system. It is currently believed that the simple straightline spike system is being used by approximately 27%, a disposable disconnect system by about 31%, an ultraviolet device by 12%, a reusable Y-set by 48%, an ultraviolet disconnect system by 3% and a twin bag by about 10% of the people.
The United States Renal Data System report indicates that the risk of peritonitis has been reduced by 40% with the Y-set use  . The relative risk of first peritonitis was 0.6 compared to the standard system. The relative risk for standard ultraviolet system was 0.75. The average number of months between episodes of peritonitis was 9.0 with the standard system, 15 with the Yset, and 13.9 with the standard ultraviolet system.
A detailed classification of exit-site infection has been recently proposed  . Identification of early acute infection and traumatized exit-site may permit timely initiation of treatment and effective cure. Catheter immobilization and sterile dressings in the first 6 weeks after catheter insertion may favor better healing of exit-site by permitting in growth of epidermis into the sinus tract towards the superficial cuff. The nasal carriage of Staphylococcus aureus may predispose to exit-site infections. Prophylactic therapy with agents such as rifampicin may reduce the incidence of exit-site infections in these nasal carriers ,,, .
| Preservation of residual renal function|| |
Several retrospective and prospective studies suggest that residual renal function is preserved for a period longer in CAPD patients than HD , . This may relate to rapid decline in residual nephron function in HD patients due to fluctuations in glomerular capillary pressure and the action on the glomerular epithelium of inflammatory mediators released into the blood by the blood membrane interaction.
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Department of Medicine, MH 436 School of Medicine, University of Missouri-Columbia, Missouri