The quantum of electrical energy reaching the surface of the chest wall varies widely .It depends upon myocardial mass, proximity to the chest wall and the thickness of chest wall.
Apart from this , the amount of blood within the left ventricle also determine the QRS voltage of ECG.
In dilated LV due to a regurgitant lesion , the LVEDV is increased . Since blood is a very good conductor of electricity , it amplifies the transmural activation front and results in high voltage QRS complex . This is referred to as Brody’s effect.
Where else , we can visualise the Brody effect ?
During excercise stress testing , when the heart rate and the LV diastolic volume increases .There is a significant increase in QRS voltage in leads facing LV, especially V5 and V6.
This is usually a benign response in healthy individuals. However in patients with preexisting CAD and LV dysfunction an increase in R wave amplitude may be a marker of exercise induced LV dilatation which could predict an adverse outcome .
Is there a reversed Brody effect , where Q waves get deepened on exercise ?
This has not been described in literature , but it is seen often in patients with post MI stress testing .Q gets deepened .If the q gets minimised* it could indicate presence of significant viable tissue , as it gets recruited during the excercise induced positive inotrpism mediated by catecholamine .Lengthening or deepening of Q indicate less viable tissue.