UREA DISTRIBUTION VOLUME ANALYSIS IN HEMODIALYSIS

The urea kinetic model is used to monitor the dialysis and dietary protein prescriptions. Because patient urea distribution volume (V) is difficult to measure, it is usualyasumed to be 58 % of body weight although total body water may vary between 35 and 75 % of body weight. In this study we have calculated V by a precise method, by direct quantitation of urea in dialysate and have-determined whater dialysis characteristics modify V. Twenty patients, ten male and 10 female, on thrice weekly dialysis (3 hours/session) were studied. All dialysate was collected in a graduated 120 litertank. Urea concentration in blood and dialysate was measured hourly and V calculated. In each patient we performed 6 dialysis in which blood flow (250 or 350 ml/hr), ultrafiltration (0.5 or 1.5 l/h), membrane or buffer (bicarbonate or acetate) was varied. At the end of haemodialysis V varied between 43 and 72 % of body weigh, but in each patient this value was practically constant. V increased during haemodialysis, 41.7 +/- 7.1 % in the first hour, 50.4 +/- 7.5 in second hour and 55.5 +/- 7.9 % in the third hour. This data suggest a bi or multicompartmental urea kinetic model. The blood flow, ultrafiltration, membrane or buffer did not influence the results. V was significantly increased in males as compared to females, 60.5 +/- 6.1 % versus 50.9 +/- 6.1 % (p < 0.01).