ULTRAFILTRATION (UF) PULSE TECHNIQUE TO MEASURE ABSOLUTE BLOOD VOLUME (V) DURING HEMODIALYSIS (HD) REVISITED

Abstract

The measurement of relative blood volume (RBV) by on-line techniques during HD has been of limited value in preventing intradialytic morbid events because of variable RBV thresholds (RBV*) where patients become symptomatic. Additional information on V could he helpful in detecting more reliable RBV*. V was measured early (V1, 25 min) and late (V2, 120 min) into HD analyzing the rate of RBV changes before and after a step-change in UF-rates based on a model presented previously (Schallenherg et al., Life Support Systems, 1987). Contrary to previous assumptions the technique provides initial V(0), independently of the timing of the UF-pulse when unmodified RBV curves are used. This value is theoretically correct for a stable distribution of red blood cells between different parts of the circulation measured as the ratio of whole body hematocrit to macro hematocrit (Fcells). However, the assumption of a constant Fcells is violated with increased degrees of hemodynamic compensation. Rescaling the RBV curves to the time (t) of the UF-pulse yields V(t). RBV(t) and V(t) were measured in 3 patients during HD. Comparison of RBV(2,1) (a) obtained by on-line monitoring to V2/V1 (b) obtained from V(t) revealed significant differences (dRBV=a-b) between volume changes which can be explained by changes Fcells, d positive (negative) difference indicating an increase (decrease) in Fcells (Tab. 1).The differences between real and apparent blood volume changes (dRBV) may account for the variability in observed RBV*.