Impact of Diffusion, Ultrafiltration, and Posture on Total Body Electrical Resistance in Patients on Hemodialysis.

Background Monitoring of hydration in patients on hemodialysis (HD) by currently available bioelectrical impedance analysis (BIA) methods is hampered by limited accuracy. This may be caused by changes in total body electrical resistance (TBER) that are induced by processes other than ultrafiltration (UF). To identify these sources of error we examined the impact of UF, diffusion, and postural change (PC), separately. Methods Extracellular TBER (TBERe) was measured by bioimpedance spectroscopy every 30 min in 23 patients on HD, for 2 hours during diffusion-only (DO), followed by 2 hours UF-only (UFO). The impact of PC from upright to semi-recumbent position was assessed by a 2-hour TBERe measurement on the day after HD. Results TBERe increased by 23.5 ± 12.4 Ω (P < 0.001) during DO and by 40.0 ± 16.2 Ω (P < 0.001) during UFO. PC, evaluated on a separate day, was associated with an increase in TBERe of 27.6 ± 26.0 Ω (P < 0.001). TBERe changes during DO were mainly attributed to PC and to a lesser extent to electrolyte exchange. Extrapolation of the data to a conventional 4-hour HD session indicates that about 32% of the total increase in TBERe is not related to UF. Conclusions A significant part of the increase in TBER during HD is not related to UF but can be attributed to other processes such as the effects of PC and diffusion-related electrolyte exchange. These factors have to be taken into account when TBER-guided UF is considered.