| Popis: |
Objective. To propose and verify a technique by which blood resistivitycan be measured continuously and instantaneously with a conductance catheterused to measure ventricular volume by intracardiac impedance volumetry.Methods. Intracardiac impedance volumetry involves the measurement ofventricular blood volume using a multi-electrode conductance catheter.Ventricular volume measurement with the conductance catheter requires thevalue of blood resistivity. Previously, blood resistivity has beendetermined by drawing a sample of blood and measuring resistivity in aseparate measuring cell. A new technique is proposed that allows theresistivity of blood to be measured with the conductance catheter itself.Two adjacent electrodes of the catheter are chosen to establish a localizedelectric field. With a localized field, the resistance measured between theadjacent electrodes bears a constant ratio (resistivity ratio) to theresistivity of blood. Finite element cylindrical models with excitingelectrodes were created to determine the resistivity ratio. Bloodresistivity was determined by dividing the resistance found due to thelocalized electric field by the resistivity ratio. The proposed scheme wasverified in cylindrical physical models and in in vivo canine hearts.Results. Finite element simulations showed the resistivity ratio to be 1.30and 1.43 for two custom-made catheters (Ohmeda Inc. and Biosensors Inc.,respectively). The resistivity ratio remained constant as long as thecylindrical volume of blood around the adjacent electrodes had a radiuslarger than the electrode spacing. In addition, this ratio was found to be afunction of electrode width. The new technique allowed us to measure salineresistivity with an error, −0.99 ± 0.25% in a physicalmodel, and blood resistivity with an error, −0.625 ±2.75% in an in vivo canine model. Conclusion. The new in vivotechnique can be used to measure and track blood resistivity instantaneouslyand continuously without drawing blood samples. |
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