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

Traditional methods for estimating the slope alpha and offset volume V-P fo r determining real-time chamber volume by the conductance catheter techniqu e are not suited to measurements made in the aorta due to the relatively lo w resistivity of the aortic wall. We developed three distinct three-dimensi onal finite element models of the conductance catheter and surrounding tiss ues in order to predict alpha and V-P and to examine the nature of the elec tric field near the aortic wall. A heterogeneous isotropic model of the cat heter, aorta and surrounding tissues accurately predicted the values of alp ha and V-P. A homogeneous anisotropic model was developed to examine the ef fects of anisotropy of blood and the layers of the aortic wall on measured values of resistance, alpha and V-P. This model demonstrated that anisotrop y of blood and aortic wall tissue can increase the values of both alpha and V-P. 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) a nd larger effects on alpha (21.2% decrease) and V-P (41.9% increase). We co nclude the following: the FEA models may lead to accurate estimate values o f alpha and V-P in vivo. The unique anisotropic conductive properties of th e layers of the aortic wall contribute to the high observed values of alpha and V-P in the aorta. Finally, catheter position has a proportionately gre ater effect on alpha and V-P than on measured resistance. The results of th is study should assist in the determination of aortic mechanical properties using conductance catheter measurements of vessel dimension. (C) 1999 Biom edical Engineering Society. [S0090-6964(99)00203-9].