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 lambda(A) and lambda(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 beta and gamma. These were obtained using a Marquardt-Levenberg least-squares algorithm. Significant differences (Hypo+ versus Hypo-) for lambda(L) (0.55 +/- 0.13 versus 0.84 +/- 0.3, *p < 0.05) and lambda(A) (0.17 +/- 0.032 versus 0.23 +/- 0.07, **p < 0.01) were found. The small value of lambda(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.