DIRECT DETERMINATION OF BLOOD RECIRCULATION RATE IN HEMODIALYSIS BY ACONDUCTIVITY METHOD

Blood recirculation is one of the key factors of decreasing dialysis e fficiency. Determination of recirculation rate (R) is necessary to opt imize effective dialysis delivery and to monitor vascular access funct ion. R can be directly measured by a conductivity method in paired fil tration dialysis (PFD), a double-compartment hemodiafiltration system that permits direct access to plasma water via the ultrafiltration str eam. Measurement of R, in this system, involves the first of two condu ctivity sensors integrated in a urea monitor (UMS, Bellco-Sorin, Miran dola, Italy), and two saline injections. The rise in conductivity (Del ta C1) induced by a 2.7 ml bolus of hypertonic saline 20% (mg/dl) in t he arterial line serves for calibration, and is followed by an equival ent injection into the venous line, giving rise to Delta C2. The ratio Delta C2/Delta C1 equals R. A comparison between R values obtained wi th this method and with the low-flow technique in 32 chronic dialysis patients during 138 PFD sessions is reported. Mean R +/- SD by the con ductivity method was 5.1 +/- 2.0 and 5.7 +/- 2.0% after 65 and 155 min utes of PFD (correlation coefficient, r = 0.75), whereas it was 6.4 +/ - 4.9% and 5.5 +/- 4.6% after 30 sec of low blood pump flow for urea a nd creatinine markers, respectively (r = 0.35). After 120 sec of low f low, mean R increased to 9.0 +/- 5.1 and 8.8 +/- 4.6% for urea and cre atinine, respectively (r = 0.45). Considerable discrepancies were foun d in R values measured simultaneously with the two blood markers. Stat istically significant differences were found between the two measureme nt modalities (blood-side and conductivity); the correlation coefficie nts (r) varied between 0.28 and 0.41. The observed differences in mean R results do not seem considerable from a clinical perspective. The b est agreement between blood-side and conductivity R measurements was o btained with Rcreat after 30 sec of low flow. Overall, a wider distrib ution was found in R values from blood-side determinations, most likel y consequent to variability in the dosing method. The conductivity met hod appears move accurate and simple in assessing total R, and can be readily automated and integrated into the dialysis machine. The author s, therefore, recommend evaluation of R using methods not based on che mical blood concentration values.