EFFECTS OF PHYSICAL PARAMETERS ON THE CYLINDRICAL MODEL FOR VOLUME MEASUREMENT BY CONDUCTANCE

Despite its undisputed utility for determining changes in ventricular pressure-volume relationships, the conductance catheter technique has not been proven reliable for measuring absolute volume. This limitatio n is due to violations of the assumptions inherent in the cylindrical model on which the method is based (i.e., homogeneous electric field a nd no leakage current). The purpose of this investigation was to relat e cylindrical model correction factors to the physical environment of the catheter and to the cylindrical equation. Physical measurements of saline-filled, nonconductive cylinders using a four-electrode conduct ance catheter were compared with a three-dimensional finite element mo del of the physical apparatus. These measurements were incorporated in to a parallel conductance model to relate physical parameters to corre ctions in the cylindrical equation for volume measurement. Excellent a greement between measured and modeled data was found Results demonstra ted a nonlinear relationship between the field nonhomogeneity correcti on factor (alpha) and cylinder diameter. The relationship between alph a and diameter was consistent with a theoretical extrapolation of cyli nder diameter toward infinity. An inverse relationship between alpha a nd the parallel conductance volume (V-p) was also clarified. The paral lel conductance model was able to demonstrate opposite effects of the physical presence of the catheter body and electrodes, which tended to cancel out any net effect on measured conductance. Results of this in vestigation and the developed finite element model clarify the nature of the correction terms in the cylindrical model and may lead to great er application of the conductance technique.