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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.

*Study in progress : Will  be referenced shortly .

Brody effect is a complex phenomenon.

Advanced readers follow the link for illustration on Brody effect

http://www.bem.fi/book/18/18.htm

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