Analysis of the Spatial Sensitivity of Conductance / Admittance Catheter Ventricular Volume Estimation
The Geselowitz relation is a volume-integral formulation of impedance measured between electrodes used to identify the most important influences on the measurement. Cross sectional plots of the formulation show the spatial distribution and relative importance of contributions to the total measurement. This formulation was used to analyze the admittance method, which is a technique used to separate blood and muscle signals.
Conductance catheters are used to estimate blood volume in the largest chamber of the heart, the left ventricle. Four inline electrodes are used to perform the measurement where the outer two carry an electric current and inner two measure the resulting voltage. The measured conductance of the blood indicates the volume in the left ventricle. Parallel structures also contribute to the measurement, and the most significant is cardiac muscle. The admittance method uses the capacitive component of admittance to identify the muscle signal, which is due solely to the complex muscle properties, so that it can be eliminated.
The admittance method was applied on a spatial basis using the Geselowitz method to arrive at a sensitivity plot of the measurement with the muscle portion of the signal removed. Visualization of the signal contributions is useful to identify the origin of signals measured by impedance devices.