Acid-base balance during hemodiafiltration on-line: Effects of diffusion and convection

HDF with on-line production of substitution fluid from ultrapure bicarbonate dialysate, is a new dialysis modality which simplifies the procedure and reduces the cost of treatment. To evaluate the changes in Ph and blood gases we studied 13 patients during two sessions, one of Hd and the other of HDF on-line. During HD plasma HCO3- rose from 22.6 +/- 2.1. pre-HD to 27.8 +/- 1.9 mEq/l post-HD (p < 0.007). The bicarbonate gain (BG) was 135 +/- 77.2 mEq and H+ generation (GH+) was 105.4 +/- 61.2 mEq. During HDF on-line plasma HCO3- increased from 23.1 +/- 1.8 to 28.4 +/- 1.3 mEq (p < 0.001). BG was 128.3 +/- 48.5 mEq and GH+ 131.1 +/- 50.2 mEq. In a second phase we studied II of these patients treated with HDF on-line. Blood pH, pCO(2), HCO3-, Hct and total protein concentration were determinated simultaneously at dialyzer inlet and outlet and after the reinfusion at 30' and 150'. Bicarbonate concentration was also determinated in reinfusion liquid by the Henderson-Hasselbach equation, with the values for pK and solubility coefficient of CO2 replaced by those for saline bicarbonate solution. Blood pH drops significantly during the passage through the filter and after reinfusion. pCO(2) and HCO3- concentration increased both during blood passage through filter and after reinfusion. Bicarbonate flux (Jbic) through the filter was -1.45 +/- 0.72 and -2.31 +/- 0.54 mEq/min. (p < 0.01) at 30' and 150' respectively. However the Jbic after reinfusion was 1.99 +/- 0.55 and 7.20 +/- 0.62 mEq/min (p < 0.01) at 30' and 150' respectively. There were no differences between HD and HDF on-line. The acidosis was corrected effectively with bicarbonate concentration, aproximately 23 mEq/l before HDf and 28 mEq/l after session. High ultrafiltration rates will tend to decrease Jbic through the Filter. However the convective bicarbonate loss was adequately counterbalanced by the bicarbonate content of dialysis fluid used as reinfusion.