Intraperitoneal fluid volume changes during peritoneal dialysis in the rat: Indicator dilution vs. volumetric measurements

In order to validate the single injection RISA (¹²⁵I human serum albumin) indicator diluation technique for assessing the alterations in intraperitoneal (i.p.) dialysate volume (IPV) which occur vs. time V(t)J during peritoneal dialysis (PD), the RISA dilution technique was compared to V(t) determinations using a direct volume recovery method in Wistar rats. Sixteen milliliters of cither 1.36 or 3.86% Dianeal® or 0.9% NaCl were used as dialysis fluids in exchanges lasting between 1 and 360 min. Approximately 4% (4.41 ± 0.59 (SE; n = 8) for 1.36% Dianeal and 4.07 ± 0.72 (n = 4) for 3.86% Dianeal) of the RISA dose given intraperito- neally was lost from the dialysate during the first 1(—1.5) min after instillation, conceivably due to rapid tracer adsorption to peritoneal surfaces. Following the initial instant tracer loss and RISA dilution due to a residual volume (3.07 ± 0.18 ml; n= 12), RISA disappeared at a fractional rate (FDR) of 2.10 ± 0.14 x 10^-3 min^-1 and 1.67 ± 0.09 x 10-³ min^-1, during the first 30 min for 1.36 and 3.86% Dianeal, respectively. The overall FDR was 1.33 ± 0.10 x 10^-3and 0.707 ± 0.082 x 10^-3 min^-1 for 1.36% Dianeal (0-150 min) and 3.86% Dianeal (0-360 min), respectively, while the overall (0- 150 min) FDR for the NaCl exchanges was 1.40 ±0.21 x 10^-3 min^-1. These values correspond to RISA clearances out of the peritoneal cavity (K_E) of 29.2 ± 1.8,22.1 ± 1.6, and 25.7 ± 2.4 µl x min^-1 for 1.36 and 3.86% Dianeal and 0. 9% NaCl, respectively. The K_E value for 3.86% Dianeal was significantly (p< 0.05) lower than for the two dialysates with lower osmolality. The slightly enhanced FDR of RISA during the first 30 min was partly due to the presence of nonprotein-bound free iodine in the RISA preparation used, and also to an enhanced disappearance of albumin during the first portion of the dwell. V(t) data from individual experiments using the RISA dilution technique (RISA curves) were analyzed by computer-aided nonlinear least-squares regression analysis. The RISA curves seemed to overestimate the directly mea-sured V(t) curves when the RISA FDR was set constant during the dwell at the overall (0-150 or 0- 360 min) rate constants given above. A complete overlap of the RISA curves and V(t) curves was obtained, however, if the RISA FDR during the first 30 min was set nearly twice as high as during the rest of the dwell. It is concluded that the RISA technique can be used with reasonable accuracy for assessing V(t) curves after correcting for peritoneal losses of the mac- romolecular tracer, the latter being enhanced during the first 30 min of the dwell. For practical purposes, however, it may be sufficient to assume that the peritoneal tracer dose is lost in a monoexponential fashion. Not correcting for RISA adsorption and for an initially en-hanced RISA disappearance rate will lead to a slight overestimation of the intraperitoneal residual vol-ume and of the initial UF rate.