Validity of the hydrogen ion mobilisation model during haemodialysis with time-dependent dialysate bicarbonate concentrations

Background: The hydrogen ion (H + ) mobilisation model has been previously shown to accurately describe blood bicarbonate (HCO 3 ) kinetics during haemodialysis (HD) when the dialysate bicarbonate concentration ([HCO 3 ]) is constant throughout the treatment. This study evaluated the ability of the H + mobilization model to describe blood HCO 3 kinetics during HD treatments with a time-dependent dialysate [HCO 3 ]. Methods: Data from a recent clinical study where blood [HCO 3 ] was measured at the beginning of and every hour during 4-h treatments in 20 chronic, thrice-weekly HD patients with a constant (Treatment A), decreasing (Treatment B) and increasing (Treatment C) dialysate [HCO 3 ] were evaluated. The H + mobilization model was used to determine the model parameter (H m ) that provided the best fit of the model to the clinical data using nonlinear regression. A total of 114 HD treatments provided individual estimates of H m . Results: Mean ± standard deviation estimates of H m during Treatments A, B and C were 0.153 ± 0.069, 0.180 ± 0.109 and 0.205 ± 0.141 L/min (medians [interquartile ranges] were 0.145 [0.118,0.191], 0.159 [0.112,0.209], 0.169 [0.115,0.236] L/min), respectively; these estimates were not different from each other ( p = 0.26). The sum of squared differences between the measured blood [HCO 3 ] and that predicted by the model were not different during Treatments A, B and C ( p = 0.50), suggesting a similar degree of model fit to the data. Conclusions: This study supports the validity of the H + mobilization model to describe intradialysis blood HCO 3 kinetics during HD with a constant H m value when using a time-dependent dialysate [HCO 3 ].