Extracellular fluid redistribution during hemodialysis: Bioimpedance measurement and model

Intradialytic fluid redistribution may cause hypotension. We hypothesized that measuring extracellular fluid volumes (ECV) with segmental bioimpedance analysis (SBIA) could test a simple, volume-driven model of redistribution among the arm, leg and trunk compartments. Patients (22, 5 females/17 males, with ages 60.2 ± 9 years, weights 80.7 ± 15 kg, heights 174 ± 9 cm) were studied during 30 HD treatments on different days. Hypotensive symptoms (Hypo+) were observed in eight patients. ECVs in the arm, trunk and leg, respectively V_A, V_T and V_L, were measured at initiation of, and throughout, dialysis. Two variables λ_A and λ_L were defined as V_A/V_T and V _L/V_T. System dynamics, assuming initial equilibrium, are then described by two rate coefficients k_RL and k_RA and two constants β and γ. These were obtained using a Marquardt-Levenberg least-squares algorithm. Significant differences (Hypo+ versus Hypo-) for λ_L (0.55 ± 0.13 versus 0.84 ± 0.3, *p < 0.05) and λ_A (0.17 ± 0.032 versus 0.23 ± 0.07, **p < 0.01) were found. The small value of λ_L might indicate that less leg volume predisposes to hypotension, larger peripheral volume mitigates hypotension. Observed transport ratios indicated that the ratio of limb to trunk volume stabilized during dialysis after an initial adjustment. These data imply encumbered movement of water from the interstitial components around skeletal muscle in the arm and leg to those of the trunk and are useful in predicting anatomical or situational predispositions to hypotension.