Fluid and Electrolyte Transport across the Peritoneal Membrane during CAPD According to the Three-pore Model

Abstract

In the present review, we summarize the principles governing the transport of fluid and electrolytes across the peritoneum during continuous ambulatory peritoneal dialysis (CAPD) in “average” patients and during ultrafiltration failure (UFF), according to the three-pore model of peritoneal transport. The UF volume curves as a function of dwell time [V( t)] are determined in their early phase by the glucose osmotic conductance [product of the UF coefficient (LpS) and the glucose reflection coefficient (σg)] of the peritoneum; in their middle portion by intraperitoneal volume and glucose diffusivity; and in their late portion by the LpS, Starling forces, and lymph flow. The most common cause of UFF is increased transport of small solutes (glucose) across the peritoneum, whereas the LpS is only moderately affected. Concerning peritoneal ion transport, ions that are already more or less fully equilibrated across the membrane at the start of the dwell, such as Na+ (Cl–), Ca2+, and Mg2+, have a convection-dominated transport. The removal of these ions is proportional to UF volume (approximately 10 mmol/L Na+ and 0.12 mmol/L Ca2+ removed per deciliter UF in 4 hours). The present article examines the impact on fluid and solute transport of varying concentrations of Ca2+ and Na+ in peritoneal dialysis solutions. Particularly, the effect of “ultralow” sodium solutions on transport and UF is simulated and discussed. Ions with high initial concentration gradients across the peritoneum, such as K+, phosphate, and bicarbonate, display a diffusion-dominated transport. The transport of these ions can be adequately described by non-electrolyte equations. However, for ions that are in (or near) their diffusion equilibrium over the peritoneum (Na+, Ca2+, Mg2+), more complex ion transport equations need to be used. Due to the complexity of these equations, however, non-electrolyte transport formalism is commonly employed, which leads to a marked underestimation of mass transfer area coefficients (PS). This can be avoided by determining the PS when transperitoneal ion concentration gradients are steep.

Relationship Between Residual Renal Function and Serum Fibroblast Growth Factor 23 in Patients on Peritoneal Dialysis

“…The peritoneal transporter category (or D/P Cr 4 h) was not associated with serum FGF23 levels in the present study, and previous studies have shown conflicting results regarding the effect of peritoneal transporter category on Pi elimination (29,30). Several clinical studies showed that peritoneal transporter category was associated with Pi clearance; patients with a high D/P creatinine value had greater Pi elimination through the dialysate (31).…”

Potassium kinetics during hemodialysis

“…28 The latter assumption reflects that the potassium ion will be hydrated with a shell of water and act like a solute slightly larger than urea; this assumption is consistent with the lower mass transfer-area coefficient for potassium than urea across the peritoneum. 29 The ultrafiltration rate during the treatment was assumed constant and calculated as the predialysis body weight minus the postdialysis body weight divided by the treatment time; it was assumed that all fluid removed during the treatment was from the central distribution volume of potassium.…”

Indirect Ion Selective Electrode Methods Potentially Overestimate Peritoneal Dialysate Sodium Losses

“…After peritonitis , ultrafiltration failure is the next most common cause of patients transferring to HD. Water moves out of the peritoneal capillaries and through the peritoneal mesothelium via aquaporin channels . During a PET test, water initially moves faster into the peritoneal cavity due to the large osmotic gradient, created by the fresh hypertonic glucose dialysate, whereas convective sodium movement into the peritoneal cavity moves slower through so called “small pores” .…”

“…As such, sodium sieving is used as an assessment of peritoneal free water transport and is used to investigate patients with ultrafiltration loss. Loss of sodium sieving has been linked to loss of aquaporin channels . Sodium sieving is greatest during the first 90 min of the PET test .…”