Cardiologists treat and diagnose problems with the heart and blood vessels. Cardiologists focus on preventative care, while a cardiac surgeon is responsible for surgery. Usually, a primary care physician will refer a patient to a cardiologist if the PCP suspects something isn’t right with the heart and cardiovascular system. The cardiologist can then run tests and decide on a treatment plan. Some sub-specialties include invasive or noninvasive cardiologists. Invasive means that they perform tests that are internal, while noninvasive cardiologists only perform tests that are external, like echocardiograms.