A model study of volume conductor effects on endocardial and intracavitary potentials.

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Abstract
An idealized mathematical model was developed to study the effects of variations in conductive and geometric parameters on measured endocardial and intracavitary potentials. The model consists of a spherical multielectrode probe located eccentrically within a system of concentric spheres that represent a blood cavity, myocardium, lung region, and surface muscle layer. Solutions were found for endocardial and intracavitary probe potentials produced by two different configurations of equivalent myocardial sources: 1) multiple activation wave fronts oriented radially, representing global fronts in the myocardium; and 2) pairs of equal and opposite dipoles on a line oriented tangentially to the endocardial surface, representing cardiac sources during early ectopic activation. It was found that the complexities of the cardiac source configurations are reflected in the endocardial potential but not in the associated probe potential, which exhibits a smoothed-out, low-amplitude distribution. In addition, probe potential depends on probe size and location within the cavity. Furthermore, endocardial and probe potentials are influenced by variations in the conductivity of different regions; an increase in blood conductivity results in a decrease in both endocardial and probe potential magnitudes produced by either type of cardiac sources, and an increase in myocardial conductivity results in an increase in both potential magnitudes, whereas an increase in lung conductivity results in an increase in the magnitude of the potential produced by radial sources but a small decrease in the magnitude of the potential produced by tangential sources. The effects of variations in skeletal muscle conductivity are negligible. The volume conductor effects of myocardial anisotropy (9:1 anisotropy ratio) are to attenuate both endocardial and probe potentials by as much as 60% and 71%, respectively, for radial sources and by 96% and 85%, respectively, for tangential sources. In conclusion, volume conductor influences should be considered in the interpretation of measured cavity potentials. Multiple myocardial events are resolved in endocardial potentials but not in potentials measured by an intracavitary multielectrode probe. This observation indicates that for the purpose of resolving cardiac activity, efforts should be directed at inverse reconstruction of endocardial potentials from potentials measured with an intracavitary probe.