Effect of dialysis and ultrafiltration on osmolality, colloid osmotic pressure, and vascular refilling rate

The effect of regular dialysis and isolated ultrafiltration on plasma osmolality, plasma colloid osmotic pressure (COP), plasma volume, and vascular refilling rate was evaluated in maintenance dialysis patients. Nineteen patients underwent regular dialysis and 11 isolated ultrafiltration. Blood samples from these subjects were obtained from arterial or venous dialysis system ports and peripheral veins. For any decrease in plasma volume, there was an increment in COP with each procedure in both venous and arterial ports and the presence of a progressive decrease of plasma osmolality was observed only during regular dialysis. Second, five additional patients underwent 2 hrs of regular dialysis and isolated ultrafiltration in separate sessions removing comparable amounts of fluid (approximately 2.5 liter); after 2 hrs, there were no differences in the changes of plasma volume and COP, but again plasma osmolality decreased only during regular dialysis. These studies demonstrate that moderate changes in plasma osmolality do not affect COP. Furthermore, the ability of the plasma to recruit fluid and generate vascular refilling (as assessed by COP) is similar during regular dialysis and isolated ultrafiltration, provided the rates of ultrafiltration are the same.