I work on a cardiac unit at the local hospital.
Usually chest x-ray is done to assess the size of the heart and also to see if fluid is building up in the lungs. We periodically get x-rays to assess patient condition. Mainly the lungs, because the size of the heart is not going to change much after medications are adjusted.
Echocardiogram is an ultrasound of the heart. It is the most important test to evaluate how the blood flows through the heart and its pumping function. They can literally "see" the valves and how the blood flows from the valves and if there are any issues. They also can calculate "ejection fraction" of the left ventricle (the most important chamber in the heart that pumps blood to the brain and the entire body). Ejection fraction (EF) is the % of blood that the ventricle pumps out of it with every beat. The normal EF for humans is 55-65%. If EF is lower that means the heart is not pumping the blood as it should and the patient is diagnosed with heart failure. The lower the EF calculated on echo the worst the prognosis is. There is also diastolic heart failure that has normal EF, but the ventricle doesn't relax to allow enough blood in.
When heart murmur develops, it usually means the heart valves are having issues. It can be due to the valve issues or due to heart failure. As we age, our valves can develops some mild problems and create a slight murmur. When the murmur is very pronounced, it usually starts impacting the heart pumping function.
Would I do echo on my dog? I am not sure that would impact how they treat the dog, unless you plan to fix valves, etc I wouldn't do it. If x-ray shows interstitial edema (fluid accumulation in the lungs), diuretics are prescribed. It is the same treatment regardless of what EF shows on echo. The treatment for heart failure is low sodium diet, cardiac/blood pressure medications, and diuretics.
I would talk to your vet and ask her/him why they would do an echo.
Olga
Remembering my angels Sebastian and Sofie.