OSMOTIC CONDUCTANCE OF THE PERITONEUM IN CAPD PATIENTS WITH PERMANENTLOSS OF ULTRAFILTRATION CAPACITY

Objective: To compare the effectiveness of osmotic pressure in the ind uction of fluid flow during continuous ambulatory peritoneal dialysis (CAPD) in patients with permanent loss of ultrafiltration capacity (UF C) and clinically stable patients. Design: Estimation of osmotic condu ctance in individual CAPD patients using data from single CAPD dwell s tudies. Patients: Twenty clinically stable CAPD patients with normal u ltrafiltration rate (NUR group); 8 CAPD patients with permanent UFC lo ss due to high diffusion rate for small solutes [high diffusion rate ( HDR) group]; 3 CAPD patients with permanent toss of UFC due to high ab sorption rate (HAR) of peritoneal dialysate (HAR group). Design: Six-h our dwell studies were carried out in each patient using 2 L of Dianea l 3.86% dialysis fluid. Dialysate volume and the peritoneal absorption rate were measured using radioiodinated serum albumin as a marker. Th e dialysate volume over dwell-time curves were examined using three ma thematical models of fluid transport for solutions with a crystalloid osmotic agent: model P, based on a phenomenologically derived exponent ial function of time; model OS, based on the linear relationship betwe en the rate of net volume change (Q(v)) to the difference of osmolalit y in dialysate and blood; and model G, based on the linear relationshi p between Q(v) and the difference of glucose concentration in dialysat e and blood. Results: All three models provided an accurate descriptio n of the measured dialysate volume over time curves. The osmotic condu ctance, defined as the coefficient of proportionality between the rate of ultrafiltration and the osmolality (or, alternatively, glucose) gr adient between dialysate and blood plasma, was 30% lower in the HDR gr oup than in the NUR group, but close to the normal value in the HAR gr oup. Conclusion: In the HDR group the changes in the peritoneal membra ne, which resulted in the increased diffusion rate of small solutes, a lso yielded a decrease of osmotic conductance. In contrast, the change s in the membrane in the HAR group, which resulted in increased perito neal absorption, did not change the osmotic conductance or the solute diffusion rate. The detailed pathophysiological mechanisms for these t wo different types of UFC loss failure are still unknown.