Modeling of change in blood volume and extracellular fluid volume during hemodialysis

Knowledge of dynamics of shift of fluid volume between intra- and extravascular compartments during hemodialysis (HD) is important for managing HD treatment to help patients approach dry weight without hypotension. The Relative blood volume (RBV) monitor indicates change in plasma volume based on the difference between ultrafiltration rate (UFR) and plasma refilling rate (PRR) during HD. However, the absolute value of PRR cannot be obtained from RBV. The aim of this study was to investigate whether fluid transport from the interstitial to blood spaces can be quantitatively analyzed with a two compartments model. 14 patients (30 measurements) were studied. RBV using a blood volume monitor (BVM, Fresenius) and calf extracellular volumes (ECV) by calf bioimpedance device (Hydra 4200, Xitron) were continuously measured during HD. A mathematic model was established with unknown transport coefficients (k₁, k₂, α, β, γ, δ) and these coefficients were estimated using a Least Squares Optimization algorithm by fitting from experimental data. A high correlation (R²>0.8) between experimental data and calculation by the model were observed in both RBV and ECV measurements. Coefficients k₁ and δ significantly differed with different degree of hydration. This model provides parameters which can used to understand relationships between degree of hydration and refilling rate.