In bicarbonate-dialysis the dialysate is acidic, 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.
KEYWORDS: acetate-free biofiltration, carbon dioxide, hypercapnia