Distortion Factors in the ECG
Fig. 18.1 The lead vectors of the standard 12-lead ECG in a finite, homogeneous torso model calculated from the model of Hyttinen (1989; 1993). Compare with the idealized lead vectors shown in Figure 15.9.
Fig. 18.2 The Brody effect. The spherical volume represents the more highly conducting intracavitary blood mass. Its effect on an applied uniform lead field shows an increased sensitivity to radial and decreased sensitivity to tangential dipoles in the heart muscle region.
Fig. 18.3 The electric heart vector of a dog in the consistent orthogonal and spherical coordinates of Appendix A. (M = magnitude, E = elevation angle, A = azimuth angle.)
These investigators noted that during the QRS-complex the electric heart vector exhibits three different peaks, which they named M1, M2, and M3. It is known that from these, the peaks M1 and M2 arise mainly from radial electric forces and the peak M3 arises mainly from tangential forces (though, unfortunately, they did not confirm this interpretation with intramural source measurements).
Millard modified the extent of the Brody effect by changing the volume of the left ventricle during the QRS-complex by venesection - that is, by removing blood with a catheter. As a consequence, the M2 peak decreased and the M3 peak increased. The effect was stronger as more blood was removed from the ventricle, as can be seen in Figure 18.4.
These experimental results are easy to explain. As mentioned, the M2 peak is formed from radial electric forces, which are enhanced by the Brody effect. If this effect is attenuated by venesection, the corresponding peak is attenuated. The peak M3 is formed from tangential forces, which are attenuated by the Brody effect. If the Brody effect is reduced by venesection, the corresponding M3 signal will be less attenuated (i.e., increased in magnitude).
Fig. 18.4 The effect of decreasing the ventricular volume on the electric heart vector amplitude. LVED = left ventricular end-diastolic.
Fig. 18.5 The effect of blood resistivity on the magnitude of the electric heart vector.
Although the Brody effect of the intracavitary blood is clearly demonstrated, the effect is diminished, when the remaining inhomogeneities are included.
Both abnormally low and high lung conductivities reduce the magnitude of surface potentials.
Low skeletal muscle conductivity enhances the surface potentials.
Increasing heart conductivity results in an increase in body surface potentials.
Fig. 18.6 The effect of inspiration on the electric heart vector during the QRS-complex and ST-T-wave. The ordinate plots the difference in magnitude [mV] between heart vector magnitude determined in midrespiration and full inspiration. The abscissa shows the QRS- or ST-T-interval divided into 10 equal points (so that the corresponding waveforms are time-normalized).
Fig. 18.7 Effect of inspiration on the elevation angle of the time normalized heart vector for the QRS-complex and T-wave (top and bottom, respectively) shown in the consistent coordinate system of Appendix A. The change in angle between midrespiration and full inspiration is shown.
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