Extracorporeal CO2 removal with hemodialysis (ECBicCO2R): how to make up for the bicarbonate loss?

Hemodialysis is a powerful tool for extracorporeal CO2 removal, because CO2 can be eliminated both as gas and as bicarbonate with blood flow rates as low as 10-15 ml/kg/min. An unsolved problem remains, however: how to make up for the bicarbonate loss. In an animal model we investigated three methods of realkalinisation: a) indirect alkalinisation with salts of organic anions (acetate, lactate, citrate, pyruvate, fumarate, succinate, malate) b) direct realkalinisation with hydroxyl ions (NaOH) c) direct alkalinisation with TRIS as "CO2-buffer". a) The decrease of pulmonary CO2 elimination depended on metabolism: acetate and lactate were metabolized at a rate of 1.8-3.5 mmol/min, thus allowing a steady-state elimination of 40-75 mmol CO2/min (25-40% of CO2 production). The other organic acids were not metabolized sufficiently to achieve a measurable reduction of pulmonary CO2 elimination. CO2 removal was quantitatively the same as during routine acetate hemodialysis and could not be increased using other organic acids. b) NaOH alone, through theoretically the best substitute for NaHCO3, had serious side effects and led to an increase in pulmonary artery pressure. c) with TRIS at a rate of 5 mmol/min, all metabolic CO2 could be removed for up to seven hours without clinical side effects, but not for longer periods. We conclude that a combination treatment for realkalinisation has to be worked out to compensate for the bicarbonate loss.