Acetate free biofiltration (AFB) : from theory to clinical results

Acetate free biofiltration (AFB) is a hemodiafiltration technique based on a buffer-free dialysate and bicarbonate infusion in the postdilution mode. The performance of AFB requires a dialysis machine equipped with an automatic control system to balance the infusion rate to that of ultrafiltration. The filters employed are usually polyacrylonitrile hollow-fiber hemodialyzers. A 145 mEq/l sodium bicarbonate solution is generally used and the infusion rate is regulated at about 8-10 liters per session to ensure optimal convective removal of toxins as well as to compensate for the bicarbonate lost in the dialysate. During AFB bicarbonate transfer results from the balance between diffusive and convective bicarbonate losses in the dialyzer and the amount of bicarbonate infused in the venous return. Thus bicarbonate supply can increase along with the rise in plasma bicarbonate concentration until a steady state is reached when the rate of infused bicarbonate equals bicarbonate losses into the dialyzer. A mild alkalosis may sometimes occur which can be avoided by slightly reducing bicarbonate concentration and/or infusion rate during the session. In spite of the large amount of sodium infused and the unusual high chloride concentration in the dialysate, no difference in the postdialysis plasma sodium levels nor in chloremia has been observed between AFB and bicarbonate dialysis. This is essentially due to the very large removal of these anions by convection (chloride and sodium) and by diffusion (sodium) into the dialyzer. Similarly the significant convective losses of calcium suggest a high dialysate calcium concentration to avoid negative intradialytic calcium balance. Polyacrylonitrile membranes, regularly employed in AFB, allow the passage of endotoxin fragments to the blood circuit in a lesser extent than other membranes. Coupled with the fact that a buffer-free dialysate and a sterile bicarbonate infusion are used AFB can be considered a highly biocompatible dialysis technique. As compared to conventional dialysis AFB allows adequate removal of small molecules and better removal of larger molecules such as β 2 -microglobulin. In the short run AFB is characterized by an increase in cardiovascular stability: it improves dialysis symptoms and the subjective well-being of patients. A better acid-base correction is regularly reported together with a rise in some nutritional indices like serum albumin levels. The reasons for these favorable results are not well defined yet. A number of multicenter studies on the effects of AFB have been published with quite similar results, but most of them are non-randomized, and use historical controls. Only one prospective, cross-over study comparing bicarbonate dialysis with AFB in diabetic dialysis patients is available. It concludes that in a six-month observation period with AFB it is possible to better control some metabolic aspects and to improve both treatment tolerance and patients' life quality.