On the CO2 Gain in On-Line Hemodiafiltration.

Background The dialysis bath holds up to 90 mmHg carbon dioxide (CO2) in order to keep pH low and salts in their soluble forms. CO(2)crosses the dialyzer membrane and diffuses to patients. In post-dilution on-line hemodiafiltration (HDF) many liters of CO2-containing dialysis bath - in the form of infusate - are delivered directly to patients bypassing the filtering membrane, but the precise amount of CO(2)delivered is unknown. Methods To gain insights on this issue 18 outpatients undergoing their regular on-line HDF were investigated by means of blood gas analysis. Results Arterial pre-dialysis samples show slight hypocapnia (35.40 +/- 3.22 mmHg) consistent with the secondary compensatory response to metabolic acidosis. In blood coming back to patients (venous line of extracorporeal circuit) pCO(2)doubled, amounting to 69 +/- 5.5 mmHg (P < .0001 with respect to pre-dialysis values) hence in on-line HDF a CO(2)gain does occur. Turning off the infusate flux pump, pCO(2)decreased to 63.1 +/- 5.8 mmHg (P = .004) meaning that delivery of infusate in post-dilution mode significantly contributes to CO(2)gain, albeit by a small amount. Conclusion On-line HDF is featured by CO(2)delivery to patients, in part dragged by the infusate.