Left atrial enlargment and low voltage qrs in teenager?

Cardiology. I am not sure what you are asking here. The question is not clear. But, it surely sounds unexpected and i would make sure this youngster is seeing a cardiologist.
See below. Obesity, chronic lung disease, and dilated cardiomyopathy are the 3 most likely causes. It's also possible that the individual is entirely normal as the ekg changes are not highly specific. An echocardiogram would sort this out easily, accurately, and non-invasively. Best wishes!