Monitoring hemodialysis changes with bioimpedance. What do we really measure?

Changes in electrical resistance (delta R), water (delta H2O), and blood variables (Na+, K+, Cl-, protein [Pr], osmolality [Osmo], hemoglobin [Hb], and hematocrit [Hct]) were monitored during hemodialysis in 20 subjects. Average delta H2O was 1.69 +/- 0.91 liters. Blood, R, and circumferences (C) were measured for the upper limb (UL), lower limb (LL), and total body (TB) before, midway, and at the end of dialysis. Segmental volumes (V) and specific resistivity (rho) were computed. delta Hct and delta Pr were stronger predictors of delta H2O than delta R. Forward stepwise regression analysis using delta R at 5, 50, and 548 kHz, delta Pr, and delta Hct showed that delta Hct and delta Pr alone accounted for 77% of the variance in delta TBW. Adding delta R increased this at most to 81%. Both the UL and LL had significant increases in rho. The volumetric change of the UL was nonsignificant, whereas that of the LL accounted for over one half of the delta H2O. Single frequency bioimpedance cannot be used to monitor dialytic delta H2O unless one accounts for fluid composition changes. Monitoring bioimpedance changes over only a single body segment is also questioned.