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

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.