Effect of high and low ultrafiltration volume during hemodialysis on relative blood volume

Achieving an optimal posthemodialysis hydration status may be difficult because objective criteria for dry weight are lacking. Both relative blood volume changes (Delta RBV) at the end of hemodialysis and Delta RBV normalized for ultrafiltration volume (Delta RBV/UF ratio) have been reported to indicate posthemodialysis volume status. A parameter for volume status should not be influenced by variations in ultrafiltration volume. However, if the volume that has to be ultrafiltrated to reach dry weight varies as a result of variations in prehemodialysis weight, either Delta RBV or the Delta RBV/UF ratio (or both) must change. To elucidate the relation between intradialytic ultrafiltration volume versus Delta RBV and its derivative, the Delta RBV/UF ratio, we studied the effect of a relatively high (mean +/- SD, 2.7 +/- 0.51) and low (1.5 +/- 0.31) intradialytic ultrafiltration volume on these parameters in eight patients. Posthemodialysis weight was comparable in low and high ultrafiltration volume sessions. The average end-hemodialysis Delta RBV did not differ between high (-6.7 +/- 2.5%) and low ultrafiltration volume sessions (-7.3 +/- 1.0%; NS), but the intraindividual variation was considerable. The Delta RBV/UF ratio differed markedly (p <0.001) between high (-2.4 +/- 0.8 %/l) and low (-4.9 +/- 1.3%/l) ultrafiltration volume sessions. In conclusion, the considerable random intraindividual variation of Delta RBV and the systematic change of the Delta RBV/UF ratio with variations in intradialytic ultrafiltration volume limit the use of these parameters as an aid to assess hydration status in hemodialysis patients