A phenomenological interpretation of the variation in dialysate volume with dwell time in CAPD

Abstract

Intraperitoneal fluid volume (IPV) changes versus time were followed in patients undergoing continuous ambulatory peritoneal dialysis (CAPD) using a simple volume recovery method. In each patient dialysates containing 1.36 and 3.86 percent glucose as an osmotic agent were investigated. The patients' IPV versus time data were fitted to a function determined by four "arbitrary" coefficients, from which both the initial ultrafiltration (UF) rate immediately following intraperitoneal (i.p.) fluid instillation and the "final" peritoneal-to-blood fluid absorption rate could be assessed. The peritoneal osmotic conductance to glucose, that is, the peritoneal ultrafiltration coefficient (Kf), times the peritoneal osmotic reflection coefficient to glucose (sigma g), Kf sigma g, was determined using two related approaches. Kf sigma g is a major determinant of the transperitoneal volume exchange, and it was calculated to be 3.54 +/- 0.85 (+/- SE) and 3.81 +/- 0.52 microliters/min/mm Hg, respectively, depending on the assumption employed. Kf sigma g was further analysed according to a three-pore model of membrane permeability to determine the possible range of Kf and sigma g compatible with a peritoneal small solute sieving coefficient (phi) ranging from 0.3 to 0.61. According to these calculations both Kf and sigma g ranged from 0.043 to 0.081 (ml/min/mm Hg and dimensionless, respectively). The maximal peritoneal lymph flow (L) realistic according to this analysis, and compatible with a measured total peritoneal-to-blood fluid absorption rate of 1.25 +/- 0.14 ml/min, was 0.75 ml/min, the most plausible values, however, falling between 0.3 to 0.5 ml/min.

Water transport across the peritoneal membrane

“…d. In a conventional Icodextrin PD solution, ~25-30% of the molecules (~3 mM) act as a colloid, implying a reflection coefficient close to unity. It should be noted that 3 mM of high MW icodextrin will produce a colloid osmotic pressure of ~58 mmHg (3 x 19.3 mmHg), which is sufficient to counteract the plasma colloid osmotic pressure (22)(23)(24)(25)(26) exerted by approximately 1 mM of negatively charged plasma proteins. Note, that the partitioning of fluid flows among the different porous pathways in the TPM is now almost identical to that in the peritoneum occurring during high net UF conditions in the absence of crystalloid osmotic forces.…”

Threefold Peritoneal Test of Osmotic Conductance, Ultrafiltration Efficiency, and Fluid Absorption

“…For an osmotic agent, "osmotic conductance" (OsmCond) measures the amount of fluid ultrafiltration (UF) to the peritoneal cavity induced by a unit concentration of the osmotic agent. Although a precise assessment of OsmCond for glucose and the peritoneal absorption rate (PA) can be performed using rather sophisticated clinical measurements (3,4,7), assessments that are easier to perform and that could be applied in clinical practice are needed.…”

“…Fluid removal in PD depends on the characteristics of the dialysis fluid, especially the type and concentration of the osmotic agent, the temporal distribution of dialysis fluid exchanges, the characteristics of the peritoneal membrane, and the status of the patient. Assessment of the efficiency of fluid removal by osmosis in patients on PD is in itself a multifactorial task, and several different parameters must be applied for its holistic description (3)(4)(5)(6). For an osmotic agent, "osmotic conductance" (OsmCond) measures the amount of fluid ultrafiltration (UF) to the peritoneal cavity induced by a unit concentration of the osmotic agent.…”

Representations of Peritoneal Tissue – Mathematical Models in Peritoneal Dialysis

“…This is the result of the assumption that osmotic conductance is constant that generally is only a simplification (Stachowska-Pietka et al, 2010;Waniewski et al, 1996a). The fluid transport may be also described by a simple phenomenological formula proposed by Pyle et al (Figure 3 shows example of patient with ultrafiltration failure defined as net ultrafiltration volume at 4 hour of the dwell less than 400 ml), and applied also by other investigators (Stelin &Rippe, 1990 …”