The electrical impedance of blood is used in biomedical applications such as impedance cardiography for monitoring blood flow. Impedance cardiography assumes a constant value for the conductivity of blood. However, this assumption has been shown to be invalid for the case of flowing blood since the conductivity is affected by flow induced changes in the orientation of red blood cells. A number of previous studies have modeled the conductivity of blood in constant flow. This study investigates the conductivity changes due to pulsatile flow as experienced during the cardiac cycle. This is achieved through the development of a theoretical model of the conductivity of pulsatile blood flowing through rigid tubes. Conductivity waveforms of pulsatile blood were generated by incorporating realistic physiological flow and cell orientation dynamics into previously reported steady flow conductivity models. Results show that conductivity correlates with the spatial average blood velocity and that features of the velocity waveform are reproduced in the conductivity signal. Conductivity was also shown to be dependent on the shape of the velocity profile. The modeled conductivity change is comparable with previously published experimental results for pulsatile blood flow, supporting the reliability of the model.

It can be concluded that impedance measurements above a frequency of 30 kHz decrease sensitivity to extracellular fluid and are not reliable for early detection of lymphedema.

Background: Bioimpedance spectroscopy (BIS) is a non-invasive method used to measure fluid volumes. In this report, we compare BIS measurements from patients with heart failure (HF) to those from healthy adults, and describe how these point-of-care fluid volume assessments may be applied to HF management.Methods and results: Fluid volumes were measured in 64 patients with NYHA class II or III HF and 69 healthy control subjects. BIS parameters including extracellular fluid (ECF), intracellular fluid (ICF), total body water (TBW), and ECF as a percentage of TBW (ECF%TBW) were analyzed. ECF%TBW values for the HF and control populations differed significantly (49.2 ± 3.2% vs. 45.2 ± 2.1%, respectively; p < 0.001); both distributions satisfied criteria for normality. Interquartile ranges did not overlap (46.7–51.0% vs. 43.8–46.4%, respectively; p < 0.001). Subgroup analyses of HF patients who underwent transthoracic echocardiography showed that impedance measurements correlated with inferior vena cava size (Pearson correlation −0.73, p < 0.0001). A case study is presented for illustrative purposes.Conclusions: BIS-measured ECF%TBW values were significantly higher in HF patients as compared to adults without HF. We describe three strata of ECF%TBW (normal, elevated, fluid overload) that may aid in clinical risk stratification and fluid volume monitoring of HF patients.Clinical Trial Registration: COMPARE – www.ClinicalTrials.gov; IMPEL – www.ClinicalTrials.gov; Heart Failure at Home – www.ClinicalTrials.gov, identifier: NCT02939053; NCT02857231; NCT04013373.

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