Effect of blood perfusion on diffusive transport in peritoneal dialysis

Abstract

Diffusive transport between blood and dialysate during peritoneal dialysis is evaluated in clinical and experimental studies by the diffusive mass transport coefficient, KBD. This global parameter depends on the local diffusive characteristics of the blood capillary wall (permeability) and the tissue, as well as on the density and distribution of capillaries within the tissue. It also depends on the rate of delivery (or washout) of solutes from the tissue with blood flow, that is, on the rate of tissue perfusion. However, the role of blood perfusion in peritoneal transport has not been theoretically evaluated. The relationship between the local characteristics of the peritoneal tissue and the global diffusive mass transport coefficient was studied using a new extended version of the distributed model for peritoneal transport, which included the effect of tissue perfusion and capillary surface area on the blood-tissue transport. The solute concentration profiles within the tissue were found to depend on the solute penetration depth, which is equal to the square root of the ratio of the solute diffusivity in tissue to the solute clearance from the capillary bed to tissue. It was shown that KBD might be interpreted as the dialysance of a capillary bed of a characteristic size that would be immersed directly in dialysate. A definition of the effective peritoneal blood flow (EPBF; the blood flow within the tissue layer) was formulated, and it was shown that EPBF depends on the local transport characteristics for the solute. Assuming typical values of the model parameters (known from physiological studies), the values of KBD and EPBF for urea, creatinine, glucose, and CO2 were calculated and compared with the measured values with good qualitative agreement. The transient initial increase of KBD values observed at the beginning of the peritoneal dialysis dwell was interpreted as a transient sixfold increase in tissue perfusion and a twofold increase in the capillary surface area. The distributed model can be useful as a theoretical tool for detailed physiological interpretations of changes in peritoneal transport associated with changes in peritoneal microcirculation and structure of the interstitium.