Peritoneal fluid and tracer albumin kinetics in the rat. Effects of increases in intraperitoneal hydrostatic pressure

Objectives: To study the peritoneal fluid loss rate, the clearance (Cl) of radioactive tracer albumin (RISA) eliminated from the peritoneal cavity (PC), as well as the peritoneal-to-plasma RISA clearance (Cl → P) during acute peritoneal dialysis (PD) at large elevations in intraperitoneal hydrostatic pressure (IPP). Design: Experimental study in anesthetized Wistar rats. Methods: The intraperitoneal volume (IPV) was assessed using RISA dilution, correcting for the RISA Cl from the PC. Volume recovery at termination of the dwells was obtained using graduated cylinders and preweighed gauze tissues. Measurements of Cl and Cl → P were obtained by repeated micro-sampling of dialysate and plasma, respectively. The IPP was continuously measured, and could be varied by external concentric abdominal compression using an inflatable cuff. On termination of the experiments, samples from tissues lining the PC were analyzed with respect to their content of RISA and edema, the latter being assessed from wet/dry weight ratios. Results: At 2 mm Hg of IPP (control) the RISA Cl was 27.1 ± 2.0 (± SE) μL·min^-1, whereas Cl → P was only 8.07 ± 0.67 μL·min^-1, at a total fluid loss rate of 10.1 ± 5.4 μL·min^-1 for 1.36% Dianeal. At an IPP of 14 mm Hg, the Cl increased to 55.3 ± 4.1 μL·min^-1 and the peritoneal fluid absorption rate was 34.4 ± 5.6 μL·min^-1, whereas Cl → P was just moderately increased as compared to control (11.2 ± 1.4 μL·min^-1). Furthermore, a pleural effusion of 1.16 ± 0.08 mL was detectable at elevated IPPs. The degree of edema formation in the anterior abdominal muscles (AAM) and the diaphragm (DIA) was largely insignificant during 150 min at 2 mm Hg of IPP, but increased markedly at 14 mm Hg, as did the RISA uptake to the AAM and DIA. The discrepancy between Cl and Cl → P was largely accounted for by tracer entrance Into tissues lining the peritoneal cavity, mainly the AAM. Conclusions: At a nearly unchanging capillary Starling equilibrium, the losses of fluid and of RISA from the PC were markedly elevated at increased IPPs. However, the RISA clearance to the plasma appeared to be only moderately altered at elevated IPP and represented only a minor fraction of the RISA clearance out of the PC. Tissues lining the PC apparently act as a variable 'sink' for intraperitoneal proteins and fluid during peritoneal dialysis (PD).