Sodium removal during pre-dilution haemofiltration.

Background: Cardiovascular instability still affects a large percentage of uraemic patients undergoing extracorporeal substitutive treatments. Post-dilution haemofiltration has been reported to be a method for improving cardiovascular stability; however, the limited removal of small molecular weight solutes together with the need for high blood flow from the fistula greatly restrict the use of this treatment. To increase the solute clearances and to partially resolve the necessity for high blood flow, the replacement solution, in a quantity about double that used in post-dilution mode, can be administered in pre-dilution mode. A high vascular stability has also been observed for pre-dilution haemofiltration. Since the lower morbidity may be due to less sodium removal when compared with haemodialysis, it would be important to characterize the sodium transport in this kind of treatment.
Methods: Nine patients underwent nine pre-dilution haemofiltration treatments (one for each patient) with on-line prepared substitution fluid.
Results: As mean values, total (NaF(pw)) and ionized (NaE(pw)) plasma water sodium concentrations increased from 149.4 +/- 2.8 mEq/l to 151.1 +/- 2.4 mEq/l, and from 143.1 +/- 2.8 to 144.5 +/- 1.2 mEq/l, respectively, during the treatment, suggesting a hypotonic concentration of net ultrafiltrate. Plotting the difference between final and initial ionized plasma water concentrations (fNaE(pw) - iNaE(pw)) against the difference between initial plasma water values and ionized sodium concentration in the reinfusate (iNaE(pw) - NaE(R)), a significant negative correlation was found, with the regression line that intercepts the abscissa at the (iNaE(pw) - NaE(R)) value of 8.8 mEq/l; this means that to avoid changes in NaE(pw) in our patients, the NaE(R) should be lower than the iNaE(pw) by this amount. This is quite different from the theoretical value of approximately 4 mEq/l necessary to avoid changes in NaE(pw) during haemodialysis. The ratio between the total sodium concentration in the ultrafiltrate (NaF(uf)) and NaF(pw) (alpha) at the post-reinfusion site was 0.96 and decreased to 0.94 when NaF(pw) values at the pre-reinfusion site were considered. This last value is quite close to the theoretical alpha value of post-dilution haemofiltration.
Conclusion: As for post-dilution haemofiltration, less sodium removal, compared with haemodialysis, can partly explain the improved cardiovascular stability during pre-dilution haemofiltration.