A new technique to measure and track blood resistivity in intracardiac impedance volumetry.

Objective: To propose and verify a technique by which blood resistivity can be measured continuously and instantaneously with a conductance catheter used to measure ventricular volume by intracardiac impedance volumetry.

Methods: Intracardiac impedance volumetry involves the measurement of ventricular blood volume using a multi-electrode conductance catheter. Ventricular volume measurement with the conductance catheter requires the value of blood resistivity. Previously, blood resistivity has been determined by drawing a sample of blood and measuring resistivity in a separate measuring cell. A new technique is proposed that allows the resistivity of blood to be measured with the conductance catheter itself. Two adjacent electrodes of the catheter are chosen to establish a localized electric field. With a localized field, the resistance measured between the adjacent electrodes bears a constant ratio (resistivity ratio) to the resistivity of blood. Finite element cylindrical models with exciting electrodes were created to determine the resistivity ratio. Blood resistivity was determined by dividing the resistance found due to the localized electric field by the resistivity ratio. The proposed scheme was verified in cylindrical physical models and in in vivo canine hearts.

Results: Finite element simulations showed the resistivity ratio to be 1.30 and 1.43 for two custom-made catheters (Ohmeda Inc. and Biosensors Inc., respectively). The resistivity ratio remained constant as long as the cylindrical volume of blood around the adjacent electrodes had a radius larger than the electrode spacing. In addition, this ratio was found to be a function of electrode width. The new technique allowed us to measure saline resistivity with an error, -0.99+/-0.25% in a physical model, and blood resistivity with an error, -0.625+/-2.75% in an in vivo canine model.

Conclusion: The new in vivo technique can be used to measure and track blood resistivity instantaneously and continuously without drawing blood samples.