A comparison of bicarbonate kinetics and acid-base status in high flux hemodialysis and on-line post-dilution hemodiafiltration.

OBJECTIVES

To compare bicarbonate kinetics and acid base status in HD and HDF for the same patient; and to investigate the effect of patient physiologic parameters on these kinetics.

METHODS

In order to monitor HCO3- kinetics during dialysis, acid-base parameters, pH, blood gases partial pressures, and HCO3- concentrations were recorded during 3 regular dialysis (HD) and 3 on-line post-dilution HDF sessions performed on 12 patients, using same dialysis fluid with a 38 mmol/l HCO3- concentration. HCO3- mass transfers through the hemodialyzers membranes and into the patients were continuously calculated during the sessions from HCO3- concentrations, together with HCO3-dialysance. The"apparent" HCO3-gain was calculated by integrating over time the instantaneous mass transfer from dialyzer and re-infusion fluid to the patient. A second method consisted in calculating the patient apparent bicarbonate space (ABS) and HCO3- mass (ABS times plasma concentration) at beginning and end of session.

RESULTS

No significant differences were observed between acid base parameters at the end of HD and HDF sessions. In contrast to urea clearances, HCO3- dialysances decayed with time during sessions from 110 to 140 ml/min to about 40 ml/min after one hour. The net HCO3- gain was taken as the difference between final and initial HCO3-masses. This net gain was in average 63% of apparent gain in HD and 74% in HDF.

CONCLUSIONS

Uremic acidosis was well corrected without risk of alkalosis. An unexpected result was the continuous decay of bicarbonate dialysance both in HD and HDF during runs.