In vitro Tests and Modelization of Bicarbonate Kinetics and Mass Transfers During Online Hemodiafiltration

This paper proposes an in vitro hemodiafiltration (HDF) model in which the patient is represented by a 2 L bag of fresh heparinized bovine blood circulated by a 4008H monitor through a 0.6 m² hemodialyzer to investigate kinetics of bicarbonate (HCO₃^-) during online post-dilution HDF. Five tests were carried out, with three ultrafiltration rates, zero (HD test), 30 and 50 ml/min. Blood gases, pH, HCO₃^- hematocrit and electrolytes were measured with an ABL77 (Radiometer) blood gas analyzer, and HCO₃^- was simultaneously measured with a biochemistry analyzer. The variation over time of plasma HCO₃^- concentrations was also calculated using mass conservation and the model of Legallais et al (JMS 168, 2000, 3–15). Agreement between theoretical and measured concentrations was good during the first 25 minutes of each test, corresponding to the time necessary to dialyze the blood. In hemodialysis (HD), there was an HCO₃^- mass transfer into blood through the membrane due to diffusion which vanished after 40 minutes, while in HDF tests, blood lost HCO₃^- due to ultrafiltration after 10 minutes. With reinjection, the net HCO₃^- mass flow rate to the “patient” decayed, from 1.8 mmol/min at t=0 to zero at the end of the test (t=60 min), and was higher in HD than in HDF. “Patient” dialysance, taking into account reinjection, was positive in all tests, and decayed from about 110 ml/min to 40 ml/min at the end of dialysis. These data confirmed that online HDF automatically corrects acidosis without creating alkalosis when HCO₃^- dialysate concentration is around 30 mmol/L.