Michelle, my son was diagnosed with aml at 18, relapsed at 20, had a double u cord stem cell transplant including total body irradiation and is now being treated by a pediatric cardiologist for anthracycline induced cardiotoxicity. He has had echos and muga scans every 6 mos for the last 5 years.
Make sure you take your echo and ekg reports to the cardiologist as well as a list of any medication your son is being treated with or was treated with that can effect the heart. I am confident they will do another echo in the office as cardiologists have their own equiptment, their owned trained personnel and really dont trust other folks readings. They will view the echos themself and knowing your childs history they can zone in on what problems they may or may not expect.
There is a margin of error because an echo is performed by a person and is at the mercy of the eye of the reader. It is much better to have one done by specialists that are familiar with your child and while the cardiologist may not be familiar with your child at this point, they will do a history and physical, be able to assess your child for anything physical that may alter a reading such as pectus excavatum (Pigeon chest), or anything in their history that can cause a red flag such as particular medications, asthma etc.
Take a deep breath, I hope this is nothing, (i have seen discrepancies) but it is much better for a child of 6 to be followed long term since his little organs are growing and developing and there is concern regarding his leukemia and treatment..
If your child does have a slightly enlarged left ventricle, i doubt that there would be any recognizable symptoms but would be important for a cardiologist to evaluate and follow up on.