Relative blood volume changes underestimate total blood volume changes during hemodialysis

Background: Measurements of relative blood volume changes (ARBV) during hemodialysis (HD) are based on hemoconcentration and assume uniform mixing of erythrocytes and plasma throughout the circulation. However, whole-body hematocrit (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, Delta RBV may differ significantly from total blood volume changes (Delta TBV). Therefore, this study compared Delta RBV and Delta TBV during HD. Design, setting, participants, and measurements: Plasma and erythrocyte volumes were measured using I-125- and I-123-radioiodinated albumin and Cr-51-labeled erythrocytes, respectively. After validation of the standardized method in two patients on a nondialysis day, seven patients completed the protocol during HD. I-125-albumin and Cr-51-labeled erythrocytes were administered 20 min before the start of HD. I-123-albumin was administered at 160 min into the HD session to quantify and correct for I-125-albumin leakage. ARBV was measured continuously throughout HD. The F cell ratio was derived from whole-body and systemic Ht. Results: Total ultrafiltration volume was 2450 +/- 770 ml. TBV declined from 5905 +/- 824 to 4877 +/- 722 ml during HD. Thus, TBV declined 17.3 +/- 4.4%, whereas the RBV decline was only 8.2 +/- 3.7% (P = 0.001). The F cell ratio increased from 0.896 +/- 0.036 to 0.993 +/- 0.049 during HD (P = 0.002). Conclusions: ARBV significantly underestimates ATBV during HD. The rise in F cell ratio strongly suggests that during HD, blood translocates from the microcirculation to the macrocirculation, probably as a cardiovascular compensatory mechanism in response to hypovolemia.