Body surface Laplacian mapping of cardiac electrical activity

Cardiac electrical activity is distributed over the 3-dimensional volume of the myocardium. Ideally one would like to be able to map the 3-dimensional distribution of cardiac electrical activity in the heart at each point in time during the cardiac cycle. The conventional electrocardiogram and vectorcardiogram provide a highly degenerate representation of cardiac electrical activity. The body surface potential map (BSPM) involves the measurement of the potential at many points on the body surface in an attempt to characterize the distributed nature of cardiac electrical activity.^1-7 Although the BSPM has been shown by a number of investigators to reliably identify major single cardiac events,⁸ its ability to resolve spatially separated bioelectrical events is limited^4-6 because of the smoothing effect of the torso volume conductor.^9,10. The fundamental limitation to interpreting the BSPM in terms of cardiac electrical events is the nonuniqueness of the electrocardiograph inverse problem.¹¹ One can generally represent the heart as a bioelectrical source in terms of a 3-dimensional distribution of electrical dipoles. One can prove that any given body surface potential distribution does not uniquely specify a specific 3-dimensional electrical dipole source distribution. Whereas the epicardial potential distribution has been estimated from the BSPM,^12,13 such inverse calculations require detailed geometric information¹⁴ and are mathematically ill-posed. Furthermore, the epicardial potential distribution itself does not specify the distribution of bioelectrical sources within the heart. The ability to image cardiac electrical activity noninvasively from the body surface could greatly enhance our ability to diagnose a wide range of cardiac abnormalities, specifically ischemia, infarction and cardiac rhythm disturbances. We have developed a new approach to mapping spatially distributed cardiac electrical events noninvasively by measuring the 2-dimensional Laplacian of the potential on the body surface.^15,16 The 2-dimensional Laplacian of the potential is the second spatial derivative of the electrical potential on a surface and provides a 2-dimensional-type projection image of the 3-dimensional distribution of cardiac electrical sources onto the body surface.¹⁶ The 2-dimensional Laplacian of the potential map may also be interpreted as an equivalent charge density distribution. A depolarization wavefront perpendicularly approaching a point on the chest wall will generate a positive equivalent charge density at that point; a depolarization wavefront tangential to the chest wall will generate a parallel double layer of positive and negative equivalent charge density overlying the wavefront. The purpose of the present study is to investigate body surface Laplacian maps (BSLMs) during normal and abnormal cardiac beats in humans and to test the hypothesis that body surface Laplacian mapping can relate body surface recordings to regional myocardial events.