A Clinical Application of the “Brody Effect”

Abstract

Brody described the importance of differences in resistivity between blood in the left
ventricular lumen and the myocardium, and the distance between the primary dipole in the
myocardium and the image dipole in the lumen. The image dipole existing in the ventricular
lumen influences the ECG voltages recorded from leads either radial or tangential to the left
ventricular mass. The intensity of the image dipole and the magnitude of the effect on the
body surface ECG are proportional to the differences in resistivity and the distance between
primary and image dipoles. Tacrolimus, a commonly-used immunosuppressant, decreases left
ventricular lumen and thickens the left ventricular free-wall tremendously; thus, although left
ventricular myocardial mass does not change, ECG voltages in leads facing the left ventricular
free-wall should decrease monumentally. The hypothesis of this study is that ECG voltages
in lead aVF facing the left ventricular free-wall and oriented radially from its mass would decrease
proportional with the decrease in luminal radius and thickening of the free-wall. ECG’s
and 2D-directed M-mode ECHO’s of the left ventricle were recorded from dogs before and after
receiving tacrolimus and developing drastic reductions in left ventricular luminal volumes.
R waves in lead aVF determined by the radial spread of depolarization from subendocardium
to subepicardium of the left ventricular free-wall decreased precipitously as the ratio of luminal
radius to wall thickness decreased. The hypothesis that ECG voltages would decrease as luminal
volume decreases and the wall thickens was accepted. This study demonstrates that factors
other than wall mass must be considered in electrocardiology.