Carbon dioxide removal in acetate hemodialysis: effects on acid base balance

Studies were performed in patients on maintenance acetate hemodialysis to assess the quantity and processes involved in the removal of carbon dioxide (CO2) during the treatment. For this purpose the CO2 losses from whole blood were evaluated in vivo using a mass balance technique. The data demonstrated that the CO2 recovered in the dialysate exceeded the amount calculated to have left the blood in the same period. This observation suggested that CO2 may be generated by the blood cells as they go through the dialyzer. In vitro studies confirmed this observation and established that the uremic blood cells can generate CO2 when exposed to a low PCO2 and/or HCO3. The net effect of this CO2 generation may be hydrogen ion gain by the patient. The contribution of ultrafiltration to the losses of CO2 depends on the volume of ultrafiltrate and the plasma HCO3 concentration. The dialysance of total CO2 was found to be equal to that of urea nitrogen. Treatments with high urea dialysance may interfere with the acute and chronic correction of the acid base balance in these patients. The data presented suggest that multiple factors related to the removal of CO2 during acetate dialysis may be responsible in part for the low plasma bicarbonate observed in patients on chronic maintenance hemodialysis.