The electrocardiogram (ECG) is an important and central tool used to establish the diagnosis of myocardial ischemia or infarction. Abnormalities in the ST segment and T waves represent myocardial ischemia (and sometimes infarction) and may be followed by the formation of Q waves. However, the ECG may be normal or nonspecific in a patient with either ischemia or infarction. On the other hand, findings thought typical of acute myocardial infarction (MI) may occur in other conditions, such as myocarditis. (See "Clinical manifestations and diagnosis of myocarditis in adults".)
The use of the ECG for the diagnosis of myocardial ischemia and MI will be reviewed here. This topic includes important updates on the ECG criteria for MI, including those made in the 2012 Joint European Society of Cardiology/American College of Cardiology Foundation/American Heart Association/World Health Federation Task Force in the Third Universal Definition of Myocardial Infarction . (See "Criteria for the diagnosis of acute myocardial infarction", section on 'Third Universal Definition of MI'.)
Also included in this report are specific ST segment (including elevation and depression) and T wave changes that may represent myocardial ischemia (table 1), as well as changes in the QRS complex associated with prior MI (table 2). (See 'Localization of ischemia or infarction' below.)
The importance of the ECG in diagnosing ischemia and infarction was reinforced by the Myocardial Infarction Triage and Intervention (MITI) study . The MITI project reviewed data on 3027 patients: ECG abnormalities were an early sign of myocardial ischemia and could be identified on a prehospital ECG within 90 minutes of symptom onset . Persistent and transient ST segment and T or Q wave abnormalities on serial ECGs discriminated patients with from those without acute ischemia or infarction better than changes on a single ECG; dynamic changes in ST segment elevation between serial ECGs improved the overall sensitivity from 34 to 46 percent. When serial evolution of all ECG abnormalities (ST segment, T or Q waves, or left bundle branch block) were considered, the diagnostic sensitivity of the ECG increased from 80 to 87 percent with a fall in specificity from 60 to 50 percent.
It is also important to understand that ECG abnormalities are not the sole criteria for diagnosing MI. As will be described below, ECG changes may be absent or nonspecific, and when present may be due to ischemia without infarction or to non-ischemic disorders, such as acute pericarditis. Hence, infarction may be diagnosed by the appropriate combination of clinical and laboratory tests, in conjunction with (but not dependent upon) the presence of characteristic ECG changes.