Effects of the volume conductor on the apparent orientation of a known cardiac dipole.

The surface electrocardiogram (EKG) is dependent on two major factors: the cardiac generator and the volume conductor. This investigation assessed the effects of the volume conductor in man on the apparent orientation of a simulated cardiac dipole. The apparent orientation of the dipole was calculated from measured surface potentials from about 60 locations on the body of five patients with implanted cardiac pacemakers. The real orientation of the dipole (an implanted pacemaker) was determined radiographically. The effects of both inhomogeneity and boundary characteristics of the volume conductor on the apparent orientation of the dipole were assessed using a new inverse algorithm. The difference between the orientation of the real and the calculated dipoles averaged 30 degrees (range 15 degrees--40 degrees) when the torso was assumed to be an infinite-homogeneous volume conductor. When the configuration of the torso was accounted for, however, the difference between the orientation of the real and calculated dipoles was reduced to 9 degrees (range 5 degrees--13 degrees). Thus, by taking into account the geometry of the torso and neglecting the inhomogeneities in the volume conductor, it is possible to calculate the orientation of a dipole in the cardiac region with an accuracy of about 9 degrees. It is reasonable to assume that the orientation of real activation wave fronts from localized areas of the heart could be calculated with a similar degree of accuracy.