Why LVH generates high voltage QRS in some and low voltage QRS or even q waves in others?
Posted Jul 21 2010 11:14am
LVH is one of the commonest ECG abnormality . We know the hall mark of LVH is increased QRS voltage .We also know , ECG is not a fool proof method to detect LVH .It has very good specificity , but little sensitivity , meaning that increase in QRS voltage is fairly accurate in predicting LVH but absence of which cannot exclude LVH.
Why Increased QRS voltage does not occur in many with LVH ?
Even though we think myocardial mass is the sole determinant of QRS voltage , in reality it is determined by many other factors.
Distance between the ECG lead , and the myocardium is an important factor. In classical concentric LVH , the LV cavity is not enlarged ,in fact it may shrink a little as the hypertrophy grow inwards and obliterate the LV cavity.(We do not know yet , how much of LVH grow out and how much muscle grow in ! )
The blood volume within LV is a very good conductor of electricity.A good volumed LV may augment a QRS voltage.
This can be observed in some of the patients with DCM , where high voltage QRS is recorded mimicking LVH.
But ,what really matters is the fine balance of blood volume and myocardial mass that determine the incidence and magnitude of LVH pattern in ECG.
QRS voltage as a tool to differentiate pathological from physiological LVH
We know QRS current is generated from within the myocytes .If the myocytes are uniformly hypertrophy without altering the basic mechanical and electrical architecture QRS complex will be amplified in a sm0oth manner and result in classical high voltage QRS of LVH.
If the hypertrophy occurs in a disorganised fashion, where in myocardial fibres slips out of plane with adjacent muscle bundles, the QRS voltage may not increase and even be slurred or notched as we see in many cases of LVH with non specific intravascular conduction defects
The classical disarray of myocardial fibers that occur in HCM causes pathological q waves.
* Other factors that determine LVH include bundle branch conduction delay or blocks which is not discussed here.(Ex: An incomplete LBBB can amplify the qrs without any LVH )
LVH with fibrosis
Fibrosis is not a standard feature of LVH. It occurs in few who are genetically predisposed , and mediated by heightened sensitivity to circulating growth factors.
Fibrosis can have wide impact on the electrical as well as mechanical function of heart.
Fibrotic heart has a potential to blunt the high voltage QRS complex.
It may even cause pathological q waves .It predispose to ventricular arrhythmia
It prevents regression of LVH , even after the loading conditions corrected.
Other conditions that attenuate LVH features in ECG
Diabetic hypertensive show less ECG voltage than isolated HT .
CKD patients often do not show ECG features of LVH inspite of LVH
Diagnosis of LVH by ECG is a simple clinical exercise , but we realise now , the underlying mechanism are too complex .
A simple question , why every one with LVH do not increase their QRS voltage ? exposes our ignorance on the subject!
But one thing is clear, physiological LVH (Meaning LVH , purely due to loading conditions including SHT/Aortic stenosis) more often result in high voltage , while in true pathological LVH(infested with fibrosis ) the increase in voltage is not consistent .