Thoracic resistivity for stroke volume calculation in impedance cardiography

Area of Science:

• Cardiovascular Physiology
• Biomedical Engineering
• Medical Instrumentation

Background:

• Blood resistivity (rho) in impedance cardiography (ICG) is typically bench-derived.
• Existing rho values do not account for dynamic physiological changes like blood velocity and hematocrit.
• This limits the accuracy of stroke volume (SV) calculations using the Kubicek formula.

Purpose of the Study:

• To investigate the relevance of bench-derived blood resistivity (rho) in ICG.
• To determine the in-vivo behavior of thoracic resistivity (rho tau) under varying hematocrit conditions.
• To assess the impact of rho tau on stroke volume calculations.

Main Methods:

• Rearranged Kubicek formula to calculate thoracic resistivity (rho tau) in dogs.
• Measured SV using an electromagnetic flowmeter (EM).
• Varied hematocrit (Hct) through hemorrhage and infusion.

Main Results:

• Thoracic resistivity (rho tau) showed an inverse relationship with Hct, with minimal variation (+6.3 to -11.8 omega.cm) around a mean of 135 +/- 1.0 omega.cm.
• Calculated SV using rho tau correlated linearly with EM-measured SV (95% prediction limit +/- 0.84 ml).
• rho tau demonstrated virtual constancy across diverse physiological disturbances.

Conclusions:

• Thoracic resistivity (rho tau) is a more reliable parameter than bench-derived rho for ICG.
• The stability of rho tau supports its use in accurate stroke volume calculations.
• This study refines ICG methodology by accounting for dynamic physiological variations.