Preservation of plasma volume during hemodialysis depends on dialysate osmolality

Hypotension is a major complication of hemodialysis and has been variously attributed not only to plasma volume loss which results from ultrafiltrative fluid removal, but also to extracorporeal blood circulation, osmolar changes or hemodynamic abnormalities which complicate the procedure. The present investigation demonstrates that plasma volume loss during ultrafiltrative fluid removal is reconstituted by mobilization of extravascular fluid, and that the rate of mobilization depends on dialysate osmolality. 7 subjects underwent 2 sessions of hemodialysis differing only in the NaCl concentration and osmolality of the dialysate, each session with ultrafiltration of fluid averaging about 1,800 ml over 3 h. Higher dialysate osmolality resulted in (1) less net decrease in plasma volume, -160 ml vs. -590 ml; (2) more mobilization of extravascular fluid, 1,670 ml vs. 1,150 ml, and (3) better control of hypotension. These results suggest that higher dialysate osmolality improves hypotension and dysequilibrium during hemodialysis by altering the movement between body fluid compartments and preserving normal hemodynamic mechanisms.