Relative Blood Volume Changes Underestimate Total Blood Volume Changes during Hemodialysis

Background: Measurements of relative blood volume changes (RBV) during hemodialysis (HD) are based on hemocon-centration and assume uniform mixing of erythrocytes and plasma throughout the circulation. However, whole-body hemat-ocrit (Ht) is lower than systemic Ht. During HD, a change in the ratio between whole-body to systemic Ht (F cell ratio) is likely to occur as a result of a net shift of low Ht blood from the microcirculation to the macrocirculation. Hence, RBV may differ significantly from total blood volume changes (TBV). Therefore, this study compared RBV and TBV during HD. Design, setting, participants, and measurements: Plasma and erythrocyte volumes were measured using 125I- and 123I-radioiodinated albumin and 51Cr-labeled erythrocytes, respectively. After validation of the standardized method in two patients on a nondialysis day, seven patients completed the protocol during HD. 125I-albumin and 51Cr-labeled erythrocytes were administered 20 min before the start of HD. 123I-albumin was administered at 160 min into the HD session to quantify and correct for 125I-albumin leakage. RBV was measured continuously throughout HD. The F cell ratio was derived from whole-body and systemic Ht