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Influence of Convection on the Diffusive Transport and Sieving of Water and Small Solutes across the Peritoneal Membrane

Ramzana B. Asghar^*, Ann M. Diskin, Patrik Spanel^,, David Smith and Simon J. Davies^*,

^* University Hospital of North Staffordshire, Stoke on Trent, United Kingdom; Centre for Science and Technology in Medicine, Keele University, Keele, United Kingdom; and V. Cermak Laboratory, J. Herovsky Institute of Physical Chemistry, Academy of Sciences of the Czech Republic, Prague

Address correspondence to: Prof. Simon J. Davies, Department of Nephrology, University Hospital of North Staffordshire, Princes Road, Hartshill, Stoke-on-Trent, ST4 7LN, UK. Phone: 01782-554164; Fax: 01782-620759; E-Mail: SimonDavies1{at}compuserve.com

The three-pore model of peritoneal membrane physiology predicts sieving of small solutes as a result of the presence of a water-exclusive pathway. The purpose of this study was to measure the diffusive and convective components of small solute transport, including water, under differing convection. Triplicate studies were performed in eight stable individuals using 2-L exchanges of bicarbonate buffered 1.36 or 3.86% glucose and icodextrin. Diffusion of water was estimated by establishing an artificial gradient of deuterated water (HDO) between blood/body water and the dialysate. ¹²⁵RISA (radio-iodinated serum albumin) was used as an intraperitoneal volume marker to determine the net ultrafiltration and reabsorption of fluid. The mass transfer area coefficient (MTAC) for HDO and solutes was estimated using the Garred and Waniewski equations. The MTAC of HDO calculated for 1.36% glucose and icodextrin were similar (36.8 versus 39.7 ml/min; P = 0.3), whereas for other solutes, values obtained using icodextrin were consistently higher (P < 0.05). A significant increase in the MTAC of HDO was demonstrated with an increase in the convective flow of water when using 3.86% glucose (mean value, 49.5 ml/min; P < 0.05). MTAC for urea was also increased with 3.86% glucose. The identical MTAC for water using 1.36% glucose and icodextrin indicates that diffusion is predominantly through small pores, whereas the difference in MTAC for the remaining solutes is a reflection of their sieving. The increase in the MTAC of water and urea associated with an increase in convection is most likely due to increased mixing within the interstitium.

Copyright © 2008 by the American Society of Nephrology. Online ISSN: 1533-3450 Print ISSN: 1046-6673