Slope analysis of blood volume and calf bioimpedance monitoring in hemodialysis patients.

BACKGROUND: Continuous intradialytic bioelectrical impedance spectroscopy of the calf (cBIS) monitors changes in calf extracellular fluid volume (cECV), thus allowing estimation of hydration in end-stage renal disease patients. Blood volume monitoring (BVM) during hemodialysis (HD) provides information about relative changes in intravascular volume, which indirectly reflects plasma refilling. We hypothesize that the rate of plasma refilling changes when cBIS-determined dry weight (BIS-DW) is reached.

METHODS: Post-HD weight was reduced from baseline (BL) in 15 patients until dry weight was reached according to cBIS criteria (BIS-DW). The slopes of cBIS and BVM curves were analysed during the first 30 and last 20 min in 31 BL treatments, which were compared to the slopes during 31 treatments when BIS-DW was reached.

RESULTS: During BL treatments, BVM slopes did not differ between the first 30 and last 20 min (-0.112 ± 0.157%/min versus -0.089 ± 0.036, P = n.s.), while cBIS slopes were generally steeper at the beginning than at the end of HD (-0.184 ± 0.139%/min versus 0.10 ± 0.127, P < 0.01). During BIS-DW treatments, BVM and cBIS slopes were steeper at the beginning than at the end (BVM: -0.131 ± 0.122 versus -0.064 ± 0.051, P < 0.01; cBIS: -0.192 ± 0.129 versus -0.035 ± 0.012, P < 0.001) and the cBIS slopes were steeper than BVM slopes at the beginning of HD. This relationship is inverted at the end of HD, when BIS-DW is reached (beginning: -0.192 ± 0.129 versus -0.131 ± 0.122, P < 0.05, end: -0.035 ± 0.012 versus -0.064 ± 0.051, P = 0.05).

CONCLUSIONS: This study demonstrates that cECV changes faster at the beginning than at the end of HD. A reversal steepness of the cBIS slope in relation to BVM slope is observed at the time when BIS-DW is reached. Therefore, combined analysis of cBIS and BVM aiming at clinical end points may be useful to assess the relationship between plasma refilling and tissue hydration during dialysis.