Non-dipolarity of heart potentials estimated by magnetocardiography in normal subjects.

We studied non-dipolarity characteristics during ventricular excitation in normal adults and children by magnetocardiography (MCG) by recording magnetic field on the thorax. The source and currents of the electrical dipole from the onset up to 60 ms of ventricular excitation were analyzed in 16 adults and 5 children. A single equivalent current dipole (ECD) was estimated by Sarvas' formula for the sphere model at 1 ms intervals. The non-dipole value (NDV) was calculated from the magnetic field strength at each recording point and theoretically estimated by ECD, representing an index for the non-dipolarity. At 32-34 ms from the beginning of QRS, the mean NDV was a minimum in all subjects suggesting at least a non-dipole component during this period. High NDV (over 5%) were present in most subjects in both the early and late phase compared to this period. Thirteen of 16 adults had a high NDV in the early phase (9.3 +/- 3.0%, mean +/- SD) and all 16 subjects had a high NDV in the late phase (21.5 +/- 10.5%). All 5 children had high NDV in both the early (10.5 +/- 5.4%) and late phases (16.8 +/- 7.9%). A single ECD estimation by MCG showed a relatively low non dipolar component and MCG could be applied to the clinical evaluation of cardiac excitation in both normal and pathological conditions.