In
bicarbonate-dialysis the
dialysate is
acid, thus allowing
salts to remain in their soluble form, as a result of the chemical reaction of
bicarbonate with any
acid that yields
carbon dioxide (CO2). The residual
anion, commonly
acetate or more rarely
citrate, reaches the patients' bloodstream. CO2 also spreads to the patients and ventilation needs therefore to be increased to avoid
hypercapnia. In addition, during on-line haemodiafiltration in post-dilution mode, the
dialysate - in the form of infusate - carries CO2 (and
acetate) to the patient, bypassing the filtering membrane. On the contrary, in
Acetate-Free Biofiltration (AFB) the
dialysate is free of
acid and, uniquely, is also a CO2-free bath. Despite the infusion of
bicarbonate in post-dilution mode, the blood coming back from the extracorporeal circuit does not carry any burden of CO2. As a result, AFB is the recommended
renal replacement therapy for patients affected by
lung disease and those with CO2 retention (
respiratory acidosis). Patients with some degree of ventilatory dysfunction may in fact experience acute
hypercapnia (
acidosis by
dialysate) at the beginning of the treatment if
bicarbonate-dialysis or on-line HDF is performed (and regardless of whether
acetate-containing or
citrate-containing bath is employed).
Acidosis by
dialysate is characterized by respiratory symptoms first and by haemodynamic instability later, which make it look very similar to
acetate intolerance. To discriminate between these two conditions, blood gas analysis is mandatory. The presence of
hypercapnia can be revealed by using the Very Simple Formula (expected pCO2 =
bicarbonate + 15), thus identifying those patients that may take the most advantage of AFB.