On-Line Identification of Hemodynamic Variables by Dilution of Ultrapure Dialysate During Hemodialysis

The removal of fluid during hemodialysis (HD) affecting both blood volume and cardiac output (CO) is one of the main causes for intradialytic hypotension. Information on hemodynamic variables obtained during HD may help to detect and to prevent this risk. It was the aim to develop a technique and a model for simple bedside identification of these variables. The 4008H HDF dialysis machine (Fresenius Medical Care, Bad Homburg, Germany) has the capability to inject defined volumes of ultrapure dialysate at correct temperatures into the extracorporeal bloodline at the relatively slow rate of 150 mL/min, which can be used for the purpose of indicator dilution measurements. However, the classic bolus approach to calculate CO fails in the setting of long and slow infusions. Dilution curves were therefore analyzed by a two-compartment model where the exchange between central ( V1) and peripheral (V2) compartments was determined by systemic blood flow (qsys). A blood volume monitor (BVM, Fresenius Medical Care, Bad Homburg, Germany) was used for on-line measurement of changes in blood water concentration (BWC) caused by the dilution. Extracted dilution curves were then used to fit the two-compartment model to the experimental data. The model produced plausible parameters where qsys determined in two patients and for repeated measurements (7.6±0.6 L/min) was close to CO obtained by the reference technique (7.1±1.6 L/min). The system has the potential for complete automatization when equipped with appropriate control inputs to the BVM and to the HDF-module of the HD machine.