Longitudinal patterns of fluid overload, blood volume and vascular refilling: a prospective study in patients on maintenance hemodialysis

Introduction. Patients on maintenance hemodialysis accumulate excess fluid between treatments. Intradialytic removal of fluid via ultrafiltration is partly compensated by vascular refilling from the interstitial space. Associations between whole-body fluid status and blood volume were previously investigated on the population level. The aim of this observational cohort study was to assess longitudinal changes in fluid compartment volumes on an intra-patient level. Methodology. Pre-dialysis bioimpedance spectroscopy measurements and absolute blood volume estimations were conducted in maintenance hemodialysis patients during 14 consecutive dialysis treatments over 5 weeks. Blood volume was determined using the dialysate bolus method. Longitudinal changes were evaluated using linear mixed models. Correlations were analyzed with repeated measures correlation coefficients (ρ_rm). Results. Twenty-five patients were included in the final analysis [88% male, median (quartile 1, quartile 3) age and dialysis vintage of 66.0 years (48.0, 74.0) and 23.5 months (13.5, 34.5), respectively]. Pre-dialysis fluid overload significantly decreased from the first to the third treatment within the week (β = −0.38, P < .01) with no significant within-week changes in euvolemic body mass (β= −0.04, P = .78) or absolute blood volume at treatment start (β = −0.06, P = .65). Fluid overload did not correlate with absolute (ρ_rm = 0.10, P = .65) or specific blood volume (ρ_rm=0.06, P = .78) at treatment start on an intra-patient level, but correlated moderately with refilling volume (ρ_rm = 0.46, P < .01). Conclusions. The observed lack of intra-patient correlations between pre-dialysis fluid overload and blood volume suggests that excess fluid may not necessarily accumulate proportionally in the interstitial and intravascular space, thus challenging previous assumptions regarding within-week changes in fluid compartments.