Linking Intradialytic Blood Volume Dynamics to Extracellular Fluid Status: Toward Personalized Fluid Assessment in Hemodialysis

<b>Background</b>: Accurate assessment of volume status remains a central challenge in hemodialysis (HD). Although bioimpedance spectroscopy (BIS) can quantify fluid compartments, it is time-consuming and requires a lot of personnel. Modern HD machines provide continuous relative blood volume (RBV) monitoring. We examined whether intradialytic RBV dynamics reflect pre-dialysis extracellular fluid (ECW) status to inform personalized fluid management. <b>Methods</b>: In an ancillary, monocentric, prospective study of the SkInDialysis trial (DRKS00036332), 11 maintenance-HD patients underwent three standardized dialysis sessions with simultaneous measurement of RBV and BIS. BIS was performed at five time points per session (pre-HD; 20, 80, and 160 min after the start of HD; and post-HD). Ultrafiltration (UF), RBV, total body water (TBW), ECW, and intracellular water (ICW) were recorded. <b>Results</b>: Mean total UF was 2809 ± 894 mL/session. RBV declined to 94.7 ± 3.1% at 20 min and to 87.6 ± 5.5% by the end of the session. TBW decreased by 2.9 ± 2.7%, driven by ECW reduction (-3.15 ± 2.9%) over ICW (-1.1 ± 1.65%). Cumulative UF correlated with declines in TBW (R² = 0.18; <i>p</i> = 0.02) and ECW (R² = 0.23; <i>p</i> = 0.01) and more modestly with ICW (R² = 0.16; <i>p</i> = 0.04). In contrast, ΔRBV (pre- vs. post-HD) did not correlate with UF, weight loss, or compartmental water changes. Early steady-state RBV at 80 min correlated with pre-HD ECW (R² = 0.19; <i>p</i> = 0.02) and more strongly with the pre-HD ECW/ICW ratio (R² = 0.34; <i>p</i> = 0.001). <b>Conclusions</b>: In this small, repeated-measures cohort, absolute early steady state RBV levels were associated with pre-dialysis ECW and the ECW/ICW ratio, whereas RBV change (ΔRBV) did not track absolute fluid removal. Our data support a time-anchored RBV level as a pragmatic, device-embedded indicator of the pre-dialysis extracellular reservoir.