Extracorporeal dialysis: techniques and adequacy

Abstract

Introduction and Aims: There is no consensus on the best method to quantify hemodialysis and the optimal dosage of hemodialysis. Urea is the substance most often measured as a surrogate for the clearance of small molecules. In particular, Kt/ V, that is the dialyzer clearance of urea, multiplied by the length of the dialysis session, divided by the volume of distribution of urea, is the most widely accepted indicator of adequacy of hemodialysis. Most commonly Kt/V is estimated, by means of Daugirdas formula, from the values of BUN ( postdialysis/predialysis), ultrafiltration volume and postdialysis body weight. The aim of this study was to evaluate the possibility to calculate directly Kt/V as the total dialyzer urea clearance during the dialysis session, indexed for the volume of total body water of the patient, measured by means of electrical body impedance analysis (BIA). Methods: Seventy eigth patients (54 males), aged 34-86 years (mean 65.1), height 140-180 cm (mean 165), dry body weight 43-130 kg (mean 72.8), in maintenance hemodialysis treatment since 1-31 years (mean 6.5); 14 patients were diabetic. All patients were on a schedule of treatment with 3 dialysis (3h-4h15min length)/week, with bicarbonate dialysis (n=55), hemodiafiltration (n=16), or acetate free biofiltration (n=7). Different dialysis monitors were employed: Fresenius (n=37), Hospal (n=24), Gambro or others (n=17). BIA was measured using a single frequency tetrapolar impedance analyzer (ST-BIA, Akern, Firenze) at the end of the dialysys session. Total body water (TBW) was calculated from the values of resistance and reactance obtained with BIA, combined with body height and weight. Kt/VBIA was calculated as total blood clearance of urea during dialysis session over TBW. First, urea extraction ratio (UER) was calculated as ( predialysis-postdialysis)/predialysis values of urea, measured with standard auto-analyzer technique. The total volume of blood flow (QB, L) during dialysis was recorded from each monitor at the end of dialysis session. Then, Kt/V was calculated as: UER x QB / TBW; where UER x QB is the total dialyzer blood clearance of urea during the dialysis session. For comparison, Kt/V was calculated with the standard Daugirdas formula (spKt/V). Results: A very high correlation coefficient (r = 0.818) was found between the results of Kt/V measured with the two formulas. The slope was 1.13 x and the intercept non significantly different from 0. The mean difference sp Kt/V (Daugirdas) Kt/V ( present study) was -0.03, statistically not significant. The range of agreement ( 1.96 SD around the mean difference) between the two measurements was between -0.31 and + 0.37. The correlation between the two formulas was even better when clustering the patients in groups according to the model of dialysis monitor. In fact, correlation coefficient r ranged 0.853-0.912. This results indicate that the accuracy of the measurement of blood flow may be different according to the different monitors. Conclusions: It seems possible to assess the adequacy of dialysis from the direct calculation of Kt/V as total blood clearance of urea during dialysis session over total body water measured with BIA.