Search

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 ∼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 (α) 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 α 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.

The concept of dialysis adequacy has to be widened to include medium size and large molecule removal in addition to urea kinetics. The HEMO study found a non-significant trend toward a beneficial effect on mortality of high-flux dialysis compared with low-flux dialysis. In that study, the beneficial effect of convection could have been attenuated by the fact that 'internal filtration' in high-flux haemodialysis (HD) is lower than that expected by convection in haemofiltration (HF) or haemodiafiltration (HDF). To explore the putative beneficial effect of convection, this Italian multicentre study was planned, comparing on-line convective treatments (HF and HDF) with standard, low-flux HD. The enrolled patients will be evaluated prospectively on their usual treatment for 2 months (baseline period) and subsequently randomized to continue either with low-flux HD (50%) or to start on-line convective treatment (50%), HF or HDF according to a 1:1 ratio. The primary end point of the study will be cardiovascular stability and blood pressure control. As secondary aims of the study, the impact on symptoms, morbidity and mortality will be assessed. Feasibility and patient compliance during HF and HDF treatments will also be evaluated. The experimental phase of the study, of at least 2 years, is divided into a 3-month adaptation period and a subsequent evaluation period. A recruitment period of 1 year is planned. The study design has adequate power to detect an absolute reduction of 3% hypotensive episodes with the experimental convective treatments compared with standard low-flux HD.

The current renewed interest in haemofiltration (HF) is due mainly to its potential advantages in reducing morbidity and mortality.
We analysed the data obtained from eight patients treated with pre-dilution HF and compared them with the calculated post-dilution HF and haemodialysis (HD) data in order to quantify the depurative performance of HF and to assess whether its claimed ability to improve cardiovascular stability is at least partially related to less sodium removal.
The urea clearance (as a marker of small molecules) of pre-dilution HF was equivalent to that of standard low-flux HD and ∼20% higher than that of post-dilution HF. For a calculated increase of 117% in the dialytic clearance of β₂-microglobulin using high-flux HD, the increase in β₂-microglobulin removal with HF was only 30%. The intradialytic decrease in plasma water β₂-microglobulin concentrations was 73% (vs 53% in HD) followed by a post-dialytic rebound of 85% (vs 42% in HD). The interdialytic concentrations of β₂-microglobulin were always lower. At the start of the subsequent session, the β₂-microglobulin concentrations observed in HF-treated patients were 92% of those in the patients treated with HD. Sodium removal was similar using both HF modalities at the same total dialysate and reinfusate sodium concentrations, and lower than in the case of HD.
HF makes it possible to obtain constantly lower plasma water β₂-microglobulin concentrations whose long-term clinical relevance has to be verified. Under usual operational conditions, sodium removal is less with pre-dilution HF than with HD, thus raising some doubts about the intrinsic capacity of HF to improve cardiovascular stability by a mechanism other than less sodium removal.