Electric properties of flowing blood and impedance cardiography

An effective resistivity is defined for axisymmetric flow through a circular tube with a uniform electric field in the longitudinal direction. The resistivity of flowing blood is found to be a function of the shear rate profile. Under axisymmetric conditions shear rate profiles are a function of a single parameter: the reduced average velocity, which is the average velocity divided by the radius of the tube. The resistivity of human blood was investigated while the blood was in laminar flow in a circular tube with different constant flow rates. The relative change in resistivity in % is given by: -0.45.H.(1 - exp[-0.26.((v)/R)0.39]); where H is the packed cell volume in % and (v)/R is the reduced average velocity in s-1. In accelerating flow the resistivity change is synchronous with the change in flow rate, but in decelerating flow there is an exponential decay characterized by a relaxation time tau. For packed cell volumes of 36.4% and 47.5% tau was estimated to be 0.21 s, for a packed cell volume of 53.7% tau was estimated to be 0.29 s. The resistivity changes in elastic tubes are influenced by both velocity changes and changes in diameter, but in opposite directions.