In Vivo Blood Characterization From Bioimpedance Spectroscopy of Blood Pooling

Characterization of blood impedance properties is important to estimate clinical diagnostic indexes such as hematocrit, glucose level, and hydration. Current in vivo bioimpedance spectroscopy methods are performed on a body appendage and thus represent a combined measurement of all tissues in the measurement field, rather than the blood individually. This paper describes a novel in vivo measurement technique to calculate bioelectrical properties of blood while excluding the disturbances from surrounding tissues, based on analysis of the impedance changes caused by blood accumulation. The forearm was modeled as a cylinder containing anatomical structures such as skin-fat layer, muscles, and bones. Blood volume was modeled as the inner cylinder. A tetrapolar electrode system was applied to a human forearm, and the impedance curves measured with and without blood pooling were processed to calculate the impedance parameters of arterial blood. The bioelectrical parameters of blood were estimated by fitting the blood curve to a Cole-Cole model using the Levenberg-Marquardt (LM) nonlinear curve-fitting method. The proposed approach was verified using an experimental phantom, an equivalent circuit model, and a preliminary human experiment. Results show that electrical properties of blood and surrounding tissues can successfully be separated. Of Cole-Cole parameters, the characteristic frequency fc is the most reliable parameter to characterize blood bioelectrical properties. This method may allow a simplified measurement of blood characteristic parameters for many biomedical and clinical monitoring applications.