The electrocardiogram (ECG) is a central tool used to establish the diagnosis of myocardial ischemia or infarction. New abnormalities in the ST segment and T waves represent myocardial ischemia and may be followed by the formation of Q waves. However, in some patients with ischemia or infarction, especially if the left circumflex coronary artery is involved, the standard 12 lead electrocardiogram may be normal or nonspecific.
The 2012 Joint European Society of Cardiology/American College of Cardiology Foundation/American Heart Association/World Heart Federation (ESC/ACCF/AHA/WHF) Task Force defined acute and prior MI and proposed new criteria necessary to secure the diagnosis . These include the presence of ST-T wave changes or a pathologic Q wave. (See "Criteria for the diagnosis of acute myocardial infarction", section on 'Third Universal Definition of MI'.)
The findings on the ECG depend upon several characteristics of the ischemia or infarction including:
- Duration — hyperacute/acute versus evolving/chronic
- Size — amount of myocardium affected
- Localization — anterior versus inferior-posterior
ACUTE MYOCARDIAL ISCHEMIA
An acute ST elevation myocardial infarction (MI) presents with a current of injury pattern characterized by elevation of the ST segment in different leads, depending upon the location of the MI. The earliest change, infrequently seen since it occurs very early in the course of an acute infarction, is hyperacute T waves, which are tall, peaked, and symmetric, in at least two contiguous leads. The hyperacute T wave reflects a localized increase in potassium concentration in the area of the acute infarction.