A Comparison of Bicarbonate Kinetics and Acid-base Status in High Flux Hemodialysis and On-Line Post-Dilution Hemodiafiltration

Abstract

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.