Bidirectional Solute Transport in Peritoneal Dialysis

Abstract

Three transport components are involved in solute transport in peritoneal dialysis: diffusion, convective transport, and peritoneal reabsorption of dialysate (fluid and solutes). The relative impact of these components on measureable transport characteristics (dialysate-to-plasma concentration ratio, diffusive mass transport coefficient, unidirectional clearances) may depend on the direction of solute transport, that is, from blood to dialysate or vice versa. The application of the bidirectional characteristics for the assessment of fluid and solute transport in peritoneal dialysis is reviewed and evaluated. Theoretical analysis as well as computer simulations were applied to discuss available data from our own studies on peritoneal transport as well as from published clinical, experimental, and theoretical studies in the same field. Thirty-three relevant clinical and experimental studies as well as theoretical analyses derived from the literature were reviewed. Data were extracted to highlight current controversies in the literature concerning the assessment of peritoneal reabsorption rate based on transport of macromolecules, middle molecules, and small solutes. Peritoneal reabsorption is the main component of the transport of macromolecules infused into the peritoneal cavity, and these solutes are currently being used for the assessment of the rate of reabsorption. In contrast, diffusive transport and peritoneal reabsorption cannot be experimentally discriminated for small solutes which exhibit negligible sieving through the membrane in convective transport (i.e., solutes with sieving coefficient equal to 1). For middle molecules each transport component may be of importance and may have an independent impact on bidirectional transport characteristics. Middle molecules, with sieving coefficients substantially less than 1, may be applied for estimation of peritoneal reabsorption rate using bidirectional transport characteristics, as apparent diffusive mass transport coefficients or unidirectional clearances. However, an independent measurement of sieving coefficient is necessary for this method.