One type is a full-thickness or transmural heart attack, meaning it involves the full thickness of the heart's muscular wall. Generally, this type of heart attack produces an injury that shows up on an ECG as an ST segment elevation. Cardiologists call this kind of attack an ST-elevation myocardial infarction, or STEMI.
A partial-thickness heart attack, or non-ST-elevation myocardial infarction (non-STEMI), produces different ECG changes -- or at least, it should. Instead of becoming elevated, the ST segment is depressed, or lowered.
To complicate matters, angina often produces exactly the same changes as a partial-thickness heart attack, and it can sometimes mimic a full-thickness heart attack. More often, however, the ECG abnormalities that accompany a heart attack are atypical, subtle, or even absent. That's why doctors always use blood tests to confirm a heart attack diagnosis.