FLUID AND SOLUTE TRANSPORT IN CAPD PATIENTS USING ULTRALOW SODIUM DIALYSATE

Transcapillary ultrafiltration during CAPD is determined by the ultrafiltration coefficient of the peritoneal membrane and by Starling forces, the latter being mainly determined by the osmolality of the dialysate. Dialysate sodium concentration decreases during a dwell, implying that: (1) sodium passes the peritoneal membrane to a lesser extent than H2O, and (2) more H2O than sodium is removed in overhydrated patients. We therefore compared two dialysate solutions with similar osmolality, but different sodium concentration (Na+ 129 mmol/liter and 102 mmol/liter). Two peritoneal permeability tests (2 x 6 hrs, dextran 70 as volume marker) with an interval of two days were performed in 10 CAPD patients. Transcapillary ultrafiltration rate was higher with ultralow sodium dialysate (USD) than normal sodium dialysate (NSD): 1.80 +/- 0.16 ml/min versus 1.58 +/- 0.18 (P < 0.01). It was especially higher during the last two hours of the dwell: 0.49 +/- 0.12 ml/min (USD) versus 0.27 +/- 0.13 (NSD). The effective lymphatic absorption rate was not different: 1.01 +/- 0.12 ml/min (USD) versus 1.05 +/- 0.09 (NSD). Using two different kinetic models, the reflection coefficients for glucose, sodium and chloride were 0.032, 0.029 and 0.027 (for the convection model) and 0.033, 0.030 and 0.027 (for the diffusion model). As a consequence the decline in osmotic pressure was more gradual during the exchange with USD. The peritoneal membrane characteristics, that is the effective peritoneal surface area and the peritoneal restriction coefficient, were not altered by the composition of the dialysate. The mass transfer area coefficient (MTC) of sodium (23 D) was 8.3 +/- 1.1 ml/min and of chloride (35.5 D) 9.4 +/- 1.0 ml/min, both after correction for Gibbs-Donnan equilibrium. It implies that these electrolytes were transported at a lower rate than expected on basis of their molecular weight. This phenomenon is most likely caused by interaction of these ions with H2O molecules. The unexpected high MTC for potassium may be due to the release of potassium from the cells after the intraperitoneal instillation of acid, hyperosmolar dialysate. Sodium and chloride loss increased with US dialysate: Na+ 71 +/- 5 mmol (USD) versus 21 +/- 6(NSD) (P < 0.001) and Cl- 78 +/- 6 mmol (USD) versus 26 +/- 6 (NSD) (P < 0.001). As a consequence, plasma sodium, chloride and osmolality decreased with USD. The decline in plasma volume with USD was probably caused by a combination of a decreased plasma osmolality and a larger ultrafiltration. It is concluded that USD may be beneficial in clinical practice for CAPD patients with fluid overload and sodium excess.