Low
dialysate to blood flow rate ratios are a unique characteristic of continuous arteriovenous
hemodiafiltration (CAVHDF) that should allow complete saturation of dialysis fluid with small-molecular-weight blood solutes. The aim of the investigation was to evaluate the performance of different hemofilters in CAVHDF. In 10
critically ill patients with
acute renal failure, the efficiency of four hollow-fiber hemofilters,
polyamide 0.6 m(2),
polyacrylonitrile (PAN) 0.3 and 0.6 m(2),
acrylonitrile sodium methallylsulfonate (AN69HF) 0.6 m(2), has been evaluated. For comparison,
dialysate flow rates (Q(di)) were standardized to 16.6 and 25 ml/min. Samples for
urea nitrogen were obtained from the arterial blood line (C(bi)) and from the
dialysate exit port (C(do)) within 24-hour running time. Outflowing
dialysate (Q(do)) was also measured at the same time. Blood flow (Q(b)) was calculated by the bubble transit time technique. Diffusive and total
urea clearances were determined. AN69HF and PAN hemofilters provided higher clearances than the
polyamide hemofilter. Despite the smaller surface area, PAN 0.3 m(2) had a total
urea clearance comparable to that of PAN 0.6 m(2) and AN69HF at Q(di) = 16.6 ml/min. While at Q(di) = 16.6 ml/min equilibrium between blood and
dialysate (C(do)/C(bi) congruent with 1) occurred with the AN69HF and PAN hemofilters, at Q(di) = 25 ml/min the equilibrium was obtained only with the AN69HF hemofilter. In conclusion, almost complete
urea saturation of dialysis fluid has not been obtained with all hemofilters tested here. In our experience, membrane characteristics play an important role in determining diffusive efficiency in CAVHDF.