Effect of High and Low Ultrafiltration Volume during Hemodialysis on Relative Blood Volume

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Achieving an optimal posthemodialysis hydration status may be difficult because objective criteria for dry weight are lacking. Both relative blood volume changes (ΔRBV) at the end of hemodialysis and ΔRBV normalized for ultrafiltration volume (ΔRBV/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 ΔRBV or the ΔRBV/UF ratio (or both) must change. To elucidate the relation between intradialytic ultrafiltration volume versus ΔRBV and its derivative, the ΔRBV/UF ratio, we studied the effect of a relatively high (mean ± SD, 2.7 ± 0.5 l) and low (1.5 ± 0.3 l) intradialytic ultrafiltration volume on these parameters in eight patients. Posthemodialysis weight was comparable in low and high ultrafiltration volume sessions. The average end-hemodialysis ΔRBV 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 ΔRBV/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 ΔRBV and the systematic change of the ΔRBV/UF ratio with variations in intradialytic ultrafiltration volume limit the use of these parameters as an aid to assess hydration status in hemodialysis patients.

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