Osmotic Conductance of the Peritoneum in Capd Patients with Permanent Loss of Ultrafiltration Capacity

Abstract

ObjectiveTo compare the effectiveness of osmotic pressure in the induction of fluid flow during continuous ambulatory peritoneal dialysis (CAPD) in patients with permanent loss of ultrafiltration capacity (UFC) and clinically stable patients.DesignEstimation of osmotic conductance in individual CAPD patients using data from single CAPD dwell studies.PatientsTwenty clinically stable CAPD patients with normal ultrafiltration rate (NUR group); 8 CAPD patients with permanent UFC loss due to high diffusion rate for small solutes [high diffusion rate (HDR) group]; 3 CAPD patients with permanent loss of UFC dueto high absorption rate (HAR) of peritoneal dialysate (HAR group).DesignSix-hour dwell studies were carried out in each patient using 2 L of Dianeal 3.86% dialysis fluid. Dialysate volume and the peritoneal absorption rate were measured using radioiodinated serum albumin as a marker. The dialysate volume over dwell-time curves were examined using three mathematical models of fluid transport for solutions with a crystalloid osmotic agent: model P, based on a phenomenologically derived exponential function of time; model OS, based on the linear relationship between the rate of net volume change (Qv) to the difference of osmolality in dialysate and blood; and model G, based on the linear relationship between Qv and the difference of glucose concentration in dialysate and blood.ResultsAll three models provided an accurate description of the measured dialysate volume over time curves. The osmotic conductance, defined as the coefficient of proportionality between the rate of ultrafiltration and the osmolality (or, alternatively, glucose) gradient between dialysate and blood plasma, was 30% lower in the HDR group than in the NUR group, but close to the normal value in the HAR group.ConclusionIn the HDR group the changes in the peritoneal membrane, which resulted in the increased diffusion rate of small solutes, also yielded a decrease of osmotic conductance. In contrast, the changes in the membrane in the HAR group, which resulted in increased peritoneal absorption, did not change the osmotic con ductance or the solute diffusion rate. The detailed pathophysiological mechanisms for these two different types of UFC loss failure are still unknown.