NET FLUID ABSORPTION UNDER MEMBRANE-TRANSPORT MODELS OF PERITONEAL-DIALYSIS

The effect of oncotic pressure and lymphatic flow on intraperitoneal d ialysate volumes in peritoneal dialysis is investigated under each of two membrane transport models: one assuming a homogeneous single-pore membrane and the other a heteroporous three-pore membrane. In both cas es, solute and fluid removal are assumed to occur via a mass transport model in which the peritoneum acts like a synthetic membrane separati ng two well-mixed compartments (body and dialysate). The homoporous ma ss transport model of Pyle and Popovich and the three-pore model of Ri ppe et al., although conceptually different, are shown to be equivalen t mathematically. This feature allows one to apply the analytical solu tions of Vonesh et al. to either model. It also enables one to apply p arameter estimates from one model to another, for example, one can app ly the lumped sum reflection coefficients of the three-pore model to a homoporous membrane model. A comparison is made between the use of em pirically estimated rejection coefficients computed under the homoporo us membrane model of Pyle and Popovich versus lumped-sum reflection co efficients calculated in accordance with the three-pore model of Rippe et al. The two models predict similar drain volumes provided the exch ange is conducted using glucose as the osmotic agent. However, one doe s see a significantly different contribution of protein oncotic pressu re and lymphatic drainage to fluid absorption under the two sets of os motic reflection coefficients. Moreover, for a simulated exchange empl oying an osmotic agent with a molecular weight of 20,000 daltons, the use of reflection coefficients calculated under the three-pore model y ields net ultrafiltration values which are more consistent and physiol ogical than results obtained using the empirically estimated rejection coefficients. Since estimates of 'lymphatic flow' will vary according to the quantity and quality of input parameter values (i.e., hydrosta tic pressure, protein concentrations, osmotic reflection coefficients) , it would be better to label these estimates as the sum of lymphatic and unmodeled net fluid absorption.