Can Bioimpedance Analysis Be Used to Estimate Absolute Blood Volume in Hemodialysis Patients?

Introduction: Under physiological conditions, blood volume and extracellular volume are in a ratio of 1-3, even in hemodialysis patients. The question therefore arises: can blood volume be inferred from the bioimpedance analysis data? The aim of the study was to compare the blood volumes calculated from extracellular volume determined by bioimpedance analysis data to the actually measured blood volumes.

Methods: Immediately before treatment, extracellular volume and volume overload were evaluated by bioimpedance spectroscopy. The actual blood volume was determined by indicator dilution, using an on-line infusate bolus and subsequent calculation with the data from the relative blood volume monitor. Alternatively, blood volume was calculated from extracellular volume divided by 3 and compared to the measured blood volume.

Findings: Overall, there were no significant differences between measured (5.56 ± 1.47 L) and calculated (5.79 ± 1.30 L) blood volumes. However, intra-individually, there were very large discrepancies with a range of -1.409 to 1.450 L. Median absolute deviation was 382 mL corresponding to 6.2 mL/kg. The differences between measured and calculated blood volumes correlated significantly (r = -0.98; p < 0.001) with the blood to extracellular volume ratio.

Discussion: In almost half of patients, blood volume can be inferred from bioimpedance data with sufficient certainty. But the greater the deviation from the physiological blood to extracellular volume ratio of 1-3, the more the calculated blood volumes differ from the measured values. For this reason, bioimpedance data should not be used uncritically to set the ultrafiltration.