Here is a tracing of a 6 months old girl who was diagnosed as having a particular congenital heart disease. The ECG tracings shows some typical hints as to what CHD might be at play:
Looking at the tracing above, you see that there is apparently low voltage of the limp leads, as the QRS complexes appear really tiny. However, when looking at the P waves you find these of quite normal voltage. In lead II the P waves seem a bit steep and almost take the appearance of small R waves.
Also, there is left axis deviation. The latter is quite tricky to spot due to the low voltage QRS complexes.
There is one particular CHD that comes with low voltage QRS on limp leads, tall P waves and left axis deviation.
--> Ebstein anomaly.
In Ebstein anomaly the tricuspid valve leaflets have not developed properly. Thus, the septal leaflet remains attached to the myocardium and the anterior leaflet is positioned towards the right ventricular apex. Thereby the right atrium gains myocardial mass that former was ventricular myocardium. This is called "atrialisation" of the ventricle. This "atrialisation" gives rise to the enlarged steep P waves, which may look like R waves in severe cases of Ebstein anomaly.
Due to the decreased right ventricular myocardial mass, the QRS axis shifts towards left (i.e. left axis deviation).