To confirm the diagnosis of a heart attack (or myocardial infarction), a doctor will require blood tests and/or an EKG.
During a heart attack, the EKG shows a series of abnormalities. The first abnormality is called a hyperacute T wave. This T wave shows up on the test as taller and more pointed than a normal T wave. The abnormality only lasts for a short time. Then elevation of the ST segment occurs. Elevation of the ST segment is the hallmark abnormality of an acute myocardial infarction - a heart attack. The ST segment elevation happens when a lack of blood flow and oxygen is actually injuring the heart muscle. This is also called a current of injury. Not long after come T wave inversions. When the heart muscle cells actually die over time, these abnormalities are replaced with Q waves.
It is important to know that not everyone with a heart attack has an abnormal EKG. In as many as 40 percent of patients, the initial EKG may not show ST elevation in patients having a heart attack. When a Q wave develops after a heart attack, it is known as a Q-wave myocardial infarction and usually corresponds to a transmural myocardial infarction (this means the heart muscle wall?s entire thickness has died). When a Q wave does not develop after a heart attack, it is called a non-Q-wave myocardial infarction and usually corresponds to the less severe non transmural heart muscle death, also called a subendocardial myocardial infarction. This means heart muscle just below the inner lining of the heart has died.
EKGs can not only tell a doctor if a heart attack is present but can also show the approximate location of a heart attack - often including which artery is involved.