Peter C. Farrell

Membrane phenomena and mass transfer kinetics in peritoneal dialysis

Abstract Continuous ambulatory peritoneal dialysis (CAPD) is a process for the treatment of chronic renal failure in which metabolic waste products and excess body water are removed through the peritoneum, an intricate membrane-like tissue that lines the internal abdominal walls and covers the liver, intestine and other internal organs. As the newest of the widely-utilized modalities for chronic renal disease, CAPD is the most rapidly growing and, in many respects, the most subtle and poorly understood treatment. The peritoneum is not a simple barrier between two phases but rather a heterogeneous mucopolysaccharide hydrogel containing a labyrinthine vasculature through which blood flows as it equilibrates with a stagnant pool of dialysate residing within the peritoneal cavity. Despite the complexity of this transport medium, investigators have found it not only possible but quite useful to characterize the peritoneum in terms analogous to the mass transfer properties of a planar membrane separating well-mixed pools of blood and dialysate. This review begins with a summary of peritoneal dialysis and its contemporary role in the treatment of kidney failure. Measurements of equivalent peritoneal hydraulic permeability and of sieving coefficients and diffusive permeabilities as functions of molecular weight are subsequently tabulated from the literature and compared and contrasted to similar values for hemodialysis membranes. In addition, several published kinetic models, both analytical and numerical, which use a knowledge of peritoneal barrier properties and baseline clinical parameters for predicting rates of toxin and fluid removal during CAPD, are described and critically evaluated.

Long-Term Continuous Ambulatory Peritoneal Dialysis

Long-term mass transfer and nutritional and metabolic stability of end-stage renal disease patients maintained on continuous ambulatory peritoneal dialysis (CAPD) continue to be of concern. This study longitudinally monitored 43 Japanese CAPD patients (29 males, 14 females) from three centres within the Tokyo Metropolitan Area for an average period of 15 ± (SD) 8 months. The mean time for patients on CAPD at study initiation was 18 ± 15 months. Monitored parameters included urea and creatinine mass transfer coefficients, clearances and blood levels, ultrafiltration, lipid levels, dietary protein intake, and weight. Lipid data were also gathered retrospectively from patient records from the time of CAPD initiation. The results were analyzed using regression growth curve analysis and analysis of variance. Statistically significant linear rises with time were apparent only for the creatinine mass transfer coefficients, although this was not considered clinically significant in terms of changes either in peritoneal creatinine clearances or ultrafiltration. Serum cholesterol levels were found to rise significantly above pre-dialysis levels 11 months after CAPD onset, thereafter returning to levels not significantly above baseline levels. In summary, CAPD provided stable, acceptable treatment over the study period.

The Stability and Kinetics of Peritoneal Mass Transfer

Adequacy of peritoneal dialysis is dependent upon optimal solute and water transfer from the capillaries to peritoneal dialysate within the peritoneal cavity. Mass transfer is governed by the permeability of the capillary wall, the peritoneal interstitium and the mesothelial layer. Long-term exposure of these tissues to the processes and complications of CAPD may have an adverse effect on mass transfer, deleteriously affecting CAPD efficacy.

Mixed-Mode Therapy: Kinetic Analysis and Acute Clinical Evaluation

A mixed therapeutic modality was devised in which patients with chronic renal failure were treated with a combination of continuous ambulatory peritoneal dialysis (CAPD; two daily 4-hour exchanges per day; 16 h dry belly) and hemodialysis (1 session per week). Kinetic modeling analysis indicated that a time-averaged urea concentration equivalent to CAPD could be obtained with a Kt/V value of 1.2-1.6, depending on patient parameters, for the single-weekly hemodialysis. The therapy format was acutely evaluated in a 2-week clinical trial on 4 patients. Excursions in small-solute concentration were virtually equivalent to those predicted from theory. Adequate fluid removal was obtained in the 2 CAPD exchanges and blood pressure was well controlled. As a result of the success of the acute trials, and since this format may offer potential lifestyle advantages to patients who possess dual access, a chronic trial of mixed-mode therapy seems advised.