Abstract

In patients on hemodialysis, the reduced alkali urinary loss makes metabolic acidosis less severe. Unexpected is the large occurrence of respiratory alkalosis and acidosis.
During the therapy, the convective/diffusive inward fluxes of CO₂ and bicarbonate and the loss of organic anions affect acid-base homeostasis.
In bicarbonate-dialysis, the neutralization of acids by bicarbonate and gain of gaseous CO from the dialysate cause an increase of CO₂ content in the body water, which requires an increase in lung ventilation (>10%) to prevent hypercapnia. In on-line hemodiafiltration, the infusate drags additional CO₂ into bloodstream, while in acetate – free biofiltration the dialysate is CO₂ – free and this prevents any addition of CO₂.
Bicarbonate and acetate diffuse into extracellular fluid according to their bath-to-blood concentration gradients. The initially large bicarbonate flux decreases rapidly because of the rapid increase in blood concentration. The smallest acetate flux is instead constant with time providing a constant source of alkali.
Rapid alkalinization elicits H⁺ mobilization that consumes most of the bicarbonate added. Some H⁺ are originated by back-titration of body buffers, but others are originated by new organic acid production, a maladaptive event that wastes metabolic energy. In addition, organic anions diffuse into dialysate causing a substantial increase in net acid production.
A novel dialysis protocol prescribes a low initial bath bicarbonate concentration and a stepwise increase during the therapy. Such a staircase protocol ensures a smoother increase of blood bicarbonate concentration avoiding the initial rapid growth and reducing the rate of organic acid production, thus making the treatment more effective.

Keywords: Acid-base, Bicarbonate, Carbon Dioxide, Dialysate, Hemodialysis