A combination of low voltage qrs and high voltage qrs is a well known marker of dilated cardiomyopathy . classically patients with severe forms of dilated cardiomyopathy show high voltage qrs complex in V1 to V6 and significantly low voltage in limb leads.
Why this happens ?
This happens due to two reasons.
1 .We know , chest leads are unipolar and picks up the electrical activity directly beneath the lead. In dilated cardiomyopathy the enlarged heart (Usually more than 6 cm in diastole , may reach 9cm ) brings the myocardium closer to chest .This increases the electromotive forces reaching the lead.
2. The enlarged LV increases the residual end systolic and end diastolic volume , this increase in blood volume independently increases the electrical conductivity and inscribes a high voltage complex.
This is some times called as Brody effect .The same phenomenon occurs in physiological conditions as in stress testing where excercise increases the qrs voltage due to increased
Why limb leads do not show this high voltage ?
The limb leads are bi polar leads hence as a rule , they record a smaller voltage than chest leads.In many patients with cardiomyopathy , the muscle mass is replaced by fibrotic tissue (Interstitial fibrosis ) and this brings down the net electrical energy draining from the heart.
Note : In spite of this, a dilated LV records high voltage in precardial leads as explained above
When can limb leads record high voltage in cardiomyopathy ?
It should be realised conduction defects can cause an increase in qrs voltage irrespective of the status of the muscle .This happens due to LAFB,LBBB, non specific IVCD. Because , these conduction defects are very common in cardiomyopathy , there is very poor correlation of LV mass verses high voltage qrs .
What is the correlation of low voltage to LV muscle mass ?
This has better correlation a very poor voltage < 5 mm( the largest qrs ) in the limb leads predicts a very badly scarred LV .