Effects of physical parameters on the cylindrical model for volume measurement by conductance

Abstract

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 limitation is due to violations of the assumptions inherent in the cylindrical model on which the method is based (i.e., homogeneous electric field and no leakage current). The purpose of this investigation was to relate 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 conductance catheter were compared with a three-dimensional finite element model of the physical apparatus. These measurements were incorporated into a parallel conductance model to relate physical parameters to corrections in the cylindrical equation for volume measurement. Excellent agreement between measured and modeled data was found. Results demonstrated a nonlinear relationship between the field nonhomogeneity correction factor (alpha) and cylinder diameter. The relationship between alpha and diameter was consistent with a theoretical extrapolation of cylinder diameter toward infinity. An inverse relationship between alpha and the parallel conductance volume (Vp) was also clarified. The parallel 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 investigation and the developed finite element model clarify the nature of the correction terms in the cylindrical model and may lead to greater application of the conductance technique.

Measurement of Coronary Lumen Area Using an Impedance Catheter: Finite Element Model and in Vitro Validation

“…It has been shown that a substantially homogenous field is provided by (1) the placement of detection electrodes equidistant from the excitation electrodes and (2) maintaining the distance between the detection and excitation electrodes comparable with the vessel diameter. 12 We confirmed these results in the present study and α is consequently taken as one in the foregoing analysis in the absence of blood cells. At any given position along the vessel, G p is a constant.…”

“…optimal spacing between the detection and excitation electrodes, etc. 12 The same group of investigators showed that the nonuniformity effect is very significant, however, under in vivo conditions 13 . A number of reasons were cited for the discrepancy between in vivo and in vitro studies including the changes in blood resistivity due to shear-induced alignment of red cells, blood hematocrit, catheter position, etc.…”

Novel method for measurement of medium size arterial lumen area with an impedance catheter: in vivo validation

“…It is agreed that the nonhomogeneity of the electric field is much less of a problem when applying the conductance technique in the aorta than in the left ventricle because the vessel more closely adheres to the cylindrical model (11). In blood vessels, Hettrick et al (12) showed that the value of ␣ is near unity for small cylinders (Ͻ10 mm in diameter) in a finite-element simulation and in in vitro measurements. Finally, the value of ␣ deviates from unity primarily under in vivo conditions because of the presence of blood cells (13).…”

“…THE CONDUCTANCE CATHETER METHOD, which has been used previously to measure ventricular volume (4 -5, 7, 14 -16, 29, 30, 33), has recently been adapted to determine the crosssectional area (CSA) of the aorta (11)(12)(13). The conductance catheter technique is based on a cylindrical model by measuring the electrical impedance of the blood with two outer electrodes for excitation and two inner electrodes for detection to yield the CSA of the chamber of interest.…”

Impact of surrounding tissue on conductance measurement of coronary and peripheral lumen area

“…The parallel conductance or electric current leakage through surrounding tissue is one of the most challenging issues in determination of arterial CSA by electric conductance measurements. Although the capability of conductance catheter technique has been extensively investigated using computational simulations, the majority of the vessel-tissue models were idealized to be homogeneous and axisymmetric around blood vessel [7,9,12,29]. A systematic assessment of the role of non-uniform surrounding tissue configuration on the parallel conductance in non-axisymmetric geometries was the focus of this study.…”