Finite element model determination of correction factors used for measurement of aortic diameter via conductance.

Abstract

Traditional methods for estimating the slope alpha and offset volume Vp for determining real-time chamber volume by the conductance catheter technique are not suited to measurements made in the aorta due to the relatively low resistivity of the aortic wall. We developed three distinct three-dimensional finite element models of the conductance catheter and surrounding tissues in order to predict alpha and Vp and to examine the nature of the electric field near the aortic wall. A heterogeneous isotropic model of the catheter, aorta and surrounding tissues accurately predicted the values of alpha and Vp. A homogeneous anisotropic model was developed to examine the effects of anisotropy of blood and the layers of the aortic wall on measured values of resistance, alpha and Vp. This model demonstrated that anisotropy of blood and aortic wall tissue can increase the values of both alpha and Vp. Finally, a three-dimensional homogeneous isotropic rectangular model allowed examination of the effects of catheter position. This model showed small effects of catheter position on measured resistance (9.7% increase) and larger effects on alpha (21.2% decrease) and Vp (41.9% increase). We conclude the following: the FEA models may lead to accurate estimate values of alpha and Vp in vivo. The unique anisotropic conductive properties of the layers of the aortic wall contribute to the high observed values of alpha and Vp in the aorta. Finally, catheter position has a proportionately greater effect on alpha and Vp than on measured resistance. The results of this study should assist in the determination of aortic mechanical properties using conductance catheter measurements of vessel dimension.