Hi & Welcome.
I appreciate your concern for your beloved daughter.
Incomplete data like purpose of doing ECG, any associated symptoms related to
heart disease like breathlessness, fainting, increased heart beats, chest pain etc???
Based on the ECG findings she seems to have
left ventricular hypertrophy (enlargement of left ventricle of the heart).
The most common cause at this age could due to
hypertrophic cardiomyopathy which is a genetic disease that occurs when the heart muscle becomes abnormally thick, making it harder for the heart to pump blood.
It may also be due to
Aortic valve stenosis. This disease is a narrowing of the tissue flap (aortic valve) that separates the left ventricle from the large blood vessel leaving your heart (aorta). The narrowing of the aortic valve requires the left ventricle to work harder to pump blood into the aorta.
She needs to evaluated by a cardiologist and an
echocardiography may be required which can reveal thickened muscle tissue in the left ventricle, blood flow through the heart with each beat, and heart abnormalities related to left ventricular hypertrophy, such as aortic valve stenosis.
Treatment depends upon the underlying cause and the symptoms.
Hope these answer your queries.