Measurement of ventricular volume from blood conductance using two-dimensional finite element analysis.

We used finite element analysis to study the relationship between the intraventricular blood conductance and the right ventricular volume. Previous studies reported a quasi-linear dependence between these two quantities. We quantified the effects of the resistivities of the surrounding tissues (e.g. heart wall, lungs) on this relationship and performed simulations for four different right ventricular longitudinal sectional areas to assess the linearity of the relationship. The relationship was most significantly affected by the blood conductivity. However, the effects of the cardiac muscle and the lungs could not be neglected. The dependence of the intraventricular blood conductance on the ventricular volume was found to be non-linear. Although to some extent inaccurate, a linear approximation of this relationship is useful for the development of rate-responsive implantable cardiac pacemakers, where the pacing rate is adjusted based on the need for cardiac output. The cardiac output is computed from the product of the heart rate and the stroke volume. The stroke volume can be estimated by measuring the changes in the intraventricular blood conductance. The electrodes needed for the stroke volume estimation can be placed on the same catheter as those used for pacing. The use of this method for clinical monitoring or diagnosis has to be investigated further given that its errors in the estimation of the stroke volume are considerably larger than those corresponding to standard methods such as dye- or thermo-dilution.