Peritoneal Dialysis International, Vol. 24, pp. 240–251 Printed in Canada. All rights reserved. 0896-8608/04 $3.00 + .00 Copyright © 2004 International Society for Peritoneal Dialysis 240 PERITONEAL FLUID TRANSPORT IN CAPD PATIENTS WITH DIFFERENT TRANSPORT RATES OF SMALL SOLUTES Danuta Sobiecka, 1 Jacek Waniewski, 1,2 Andrzej Weryn ´ski, 1 and Bengt Lindholm 2 Institute of Biocybernetics and Biomedical Engineering, 1 Polish Academy of Sciences, Warsaw, Poland; Divisions of Baxter Novum and Renal Medicine, 2 Department of Clinical Sciences, Huddinge University Hospital, Karolinska Institutet, Stockholm, Sweden Correspondence to: J. Waniewski, Institute of Biocyber- netics and Biomedical Engineering, Polish Academy of Sci- ences, Ks. Trojdena 4, 02-109 Warsaw, Poland. jacekwan@ibib.waw.pl Received 4 February 2003; accepted 23 December 2003. ♦ Background: Continuous ambulatory peritoneal dialy- sis (CAPD) patients with high peritoneal solute transport rate often have inadequate peritoneal fluid transport. It is not known whether this inadequate fluid transport is due solely to a too rapid fall of osmotic pressure, or if the decreased effectiveness of fluid transport is also a con- tributing factor. ♦ Objective: To analyze fluid transport parameters and the effectiveness of dialysis fluid osmotic pressure in the induction of fluid flow in CAPD patients with different small solute transport rates. ♦ Patients: 44 CAPD patients were placed in low (n = 6), low-average (n = 13), high-average (n = 19), and high (n = 6) transport groups according to a modified peritoneal equili- bration test (PET). ♦ Methods: The study involved a 6-hour peritoneal dialy- sis dwell with 2 L 3.86% glucose dialysis fluid for each patient. Radioisotopically labeled serum albumin was added as a volume marker.The fluid transport parameters (osmotic conductance and fluid absorption rate) were estimated using three mathematical models of fluid trans- port: (1) Pyle model (model P), which describes ultrafiltra- tion rate as an exponential function of time; (2) model OS, which is based on the linear relationship of ultrafiltration rate and overall osmolality gradient between dialysis fluid and blood; and (3) model G, which is based on the linear relationship between ultrafiltration rate and glucose con- centration gradient between dialysis fluid and blood. Dif- fusive mass transport coefficients (K BD ) for glucose, urea, creatinine, potassium, and sodium were estimated using the modified Babb–Randerson–Farrell model. ♦ Results: The high transport group had significantly lower dialysate volume and glucose and osmolality gra- dients between dialysate and blood, but significantly higher K BD for small solutes compared with the other transport groups. Osmotic conductance, fluid absorption rate, and initial ultrafiltration rate did not differ among the transport groups for model OS and model P. Model G yielded unrealistic values of fluid transport parameters that differed from those estimated by models OS and P. The K BD values for small solutes were significantly differ- ent among the groups, and did not correlate with fluid transport parameters for model OS. ♦ Conclusion: The difference in fluid transport between the different transport groups was due only to the differ- ences in the rate of disappearance of the overall osmotic pressure of the dialysate, which was a combined result of the transport rate of glucose and other small solutes. Although the glucose gradient is the major factor influ- encing ultrafiltration rate, other solutes, such as urea, are also of importance. The counteractive effect of plasma small solutes on transcapillary ultrafiltration was found to be especially notable in low transport patients. Thus, glucose gradient alone should not be considered the only force that shapes the ultrafiltration profile during perito- neal dialysis. We did not find any correlations between diffusive mass transport coefficients for small solutes and fluid transport parameters such as osmotic conductance or fluid and volume marker absorption. We may thus con- clude that the pathway(s) for fluid transport appears to be partly independent from the pathway(s) for small sol- ute transport, which supports the hypothesis of different pore types for fluid and solute transport. Perit Dial Int 2004; 24:240–251 www.PDIConnect.com KEY WORDS: Osmotic conductance; fluid absorp- tion; mathematical models; peritoneal equilibration test. T he characteristics of solute and fluid transport during peritoneal dialysis (PD) have been inves- tigated in patients with (1,2) and without ultrafiltra- tion problems (3,4), but the osmotic effectiveness of the dialysis fluid has not been assessed in detail in patients with different rates of small solute trans- port. The general observation from clinical studies is that patients with high transport rates of small sol- utes have lower ultrafiltration from blood to the peri- toneal cavity because of fast dissipation of the osmotic gradient between dialysis fluid and blood. A clear demonstration of this fact may be found in a negative by on May 24, 2011 www.pdiconnect.com Downloaded from