MODELING OF DYNAMIC CHANGES IN BLOOD VOLUME AND SEGMENTAL EXTRACELLULAR VOLUME DURING HEMODIALYSIS

Abstract

Previous studies have shown that blood volume (BV) measurements can be used to estimate vascular refilling from the interstitial compartment during hemodialysis (HD). However, BV changes cannot predict extracellular volume (ECV) conditions. A further parameter is needed to define ECV. A four-compartment model (BV, Vtrunks, Varm and Vleg) that can be parameterized by segmental bioimpedance analysis (SBIA) is used to develop a relationship between BV and ECV (see Fig. 1).Fig. 1Intravascular refilling is postulated to occur by fluid transfer from two main compartments (central [trunk] and peripheral [arm and leg]). Intercompartment transfers are modeled as volume driven, assuming relatively fixed compartmental compliances. A group of differential equations are used to predict the fluid shifts during HD. SBIA is used to detect ECV changes in the trunk and the periphery. Ten patients were studied during HD: ECV in the arm, trunk and leg were measured by SBIA and BV was measured by an ultrasound technique. Results indicate that refilling from the arms and legs may have an exponential trend. A transfer coefficient (k, k1 and k2) can be varied to obtain a best fit to the data. The point at which the refilling curve tends to plateau seems to describe the best fit for dry body weight. A similar relationship is not seen in the refilling curve for the central compartment.