Determination of Fluid Shifts During Chronic Hemodialysis Using Bioimpedance Spectroscopy and an In-Line Hematocrit Monitor

Abstract

Effective ultrafiltration in hemodialysis requires a plasma refilling rate (PRR) sufficient to support blood volume. Knowledge of PRR and compartment shifts helps in understanding intradialytic events and may improve fluid removal. Simultaneous measurements with in-line hematocrit and bioimpedance spectroscopy were done in eight patients (17 runs) and extended for 15–60 min beyond the end of hemodialysis to determine fluid shifts and evaluate the relationship between changes in blood volume, extracellular fluid volume, and PRR. Absolute blood volumes, plasma refilling rates, and interstitial fluid volumes were calculated. Significant correlations were noted between different compartment volumes and the change in weight and estimates of plasma refilling. Pooling all runs, the change in interstitial fluid volume from beginning to end of hemodialysis correlated with the change in weight (r = 0.63, p = 0.01). Changes in PRR tracked changes in interstitial fluid volume for patients with two or more runs. In conclusion, the combination of in-line hematocrit and bioimpedance spectroscopy allows the quantification of different compartment volumes and PRR. Interstitial fluid volume may be a major determinant of PRR. This information would permit the development of strategies to reduce intradialytic morbidity.