Predicting Risk in Peritoneal Dialysis

Abstract

The flux of solute and fluid across a dialyzer in hemodialysis is influenced by the flow rate of blood and dialysis fluid combined with the imposition of hydraulic pressure across the artificial membrane. The flux parameters are predictable given that the filters are mass produced and tightly regulated with respect to quality control. In contrast, patients on peritoneal dialysis (PD) use their own peritoneal membrane as a dialyzer. Similar to other biologic systems, the characteristics in one person can differ from another person; furthermore, the membrane characteristics can change over time.To measure the transport capability in a given patient on PD, tests have been devised to determine transmembrane transport of small solutes and water. In North America, the standard peritoneal equilibration test (PET) has been used for decades to measure the rate of small solute movement and ultrafiltration across the peritoneal membrane (1). A 2-L bag of 2.5% dextrose solution is instilled in the peritoneal cavity and allowed to dwell for 4 hours. During this dwell, creatinine will diffuse from the blood into the dialysis fluid, and glucose will diffuse in the opposite direction, both along their respective concentration gradients. The volume of dialysate drained at the end of the dwell indicates how much fluid was ultrafiltered during this time. The concentration of creatinine in the dialysis fluid (D) at 4 hours is divided by the plasma creatinine (P), generating the D/P creatinine. Another way to examine membrane transport is to measure how rapidly glucose leaves the dialysate and enters the bloodstream. However, D/P glucose is not a measure of this reverse flux, because the glucose entering the blood will be metabolized in real time. One way to deal with this is to measure the glucose remaining in the dialysis fluid at the end of the dwell and compare it with the amount of glucose present in the fluid at the beginning. This measure generates the D4/D0 glucose. In patients with rapid transmembrane solute flux, the D/P creatinine will be high, and the D4/D0 will be low. In those with slower rates of solute movement, the D/P creatinine will be lower, and the D4/D0 will be higher. Typically, rapid transporters have faster dissipation of the glucose-generated osmotic gradient for ultrafiltration and therefore, less fluid removal compared with slower transporters.The original intent of the PET was to aid in designing a PD prescription tailored to the membrane characteristics of the patient. A rapid transporter has faster dissipation of the glucose-generated osmotic gradient and could encounter problems with ultrafiltration, particularly during long dialysis dwells. In that case, shorter rather than longer dwells would serve the patient better. The slow or low transporter was postulated to have more successful dialysis by using longer dwells to allow sufficient time for solute flux.