Thoracic resistivity for stroke volume calculation in impedance cardiography.

Blood resistivity (rho) values used in the Kubicek formula for stroke volume (SV) calculation by impedance cardiography are bench derived and do not take into account complex blood velocity movements and dynamic hematocrit changes in systole. In this study, the relevance of rho has been questioned. Thoracic resistivity (rho tau) has been calculated in dogs from the rearranged Kubicek formula: rho tau = (SV . ZO2)/(L2 dZ/dtmax.T), where SV was measured by the electromagnetic flowmeter (EM). Hematocrit (Hct) in the dog was varied by hemorrhage and infusion. In contrast to the direct and exponential bench rho-Hct relationship, rho tau varies inversely with Hct, but by no more than +6.3 omega . cm (at Hct 26%) and -11.8 omega . cm (at Hct 66%) about a mean rho tau of 135 +/- 1.0 omega . cm (at Hct 40%). Impedance SV calculated using rho tau over a wide range of SV bears a linear relationship to EM values with a 95% prediction limit for a single SV estimate of +/- 0.84 about a mean value of 26.9 ml. The findings suggest that rho tau is virtually constant during a variety of physiological disturbances.