On the CO2 gain in on-line hemodiafiltration.

ABSTRACT

BACKGROUND: The dialysis bath holds up to 90 mmHg carbon dioxide (CO2 ) in order to keep pH low and salts in their soluble forms. CO2 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 CO2 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) pCO2 doubled, amounting to 69 ± 5.5 mmHg (P < .0001 with respect to pre-dialysis values) hence in on-line HDF a CO2 gain does occur. Turning off the infusate flux pump, pCO2 decreased to 63.1 ± 5.8 mmHg (P = .004) meaning that delivery of infusate in post-dilution mode significantly contributes to CO2 gain, albeit by a small amount. CONCLUSION: On-line HDF is featured by CO2 delivery to patients, in part dragged by the infusate.