A 12-lead electrocardiogram (ECG / EKG) test is very important for the diagnosis of myocardial infarction and should be obtained as soon as possible after presentation (Figure 1188.1). This is because the test is noninvasive, inexpensive and rapid. ECG reading is obtained rapidly from ECG machine and treatment is started rapidly. ECG scan is very useful for the diagnosis, prognosis, and monitoring of myocardial infarction. General ECG changes of myocardial ischemia are ST elevation/depression and deep symmetric T wave inversion. Persistent elevation of ST segment differentiates STEMI from unstable angina and NSTEMI. Normal appearing ECG does not exclude cardiac ischemia or myocardial infarction.
ECG changes like ST segment elevation or depression and T wave inversion are not sufficient on their own for diagnosis of myocardial infarction. Revised criteria for the diagnosis of myocardial infarction (acute, evolving, or recent) have been proposed by European Society of Cardiology (ESC) and American Society of Cardiology (ACC) (2000).
Revised criteria for the diagnosis of myocardial infarction (ESC/ACC, 2000)
Anyone of the following is satisfactory for diagnosis of acute/evolving/recent myocardial infarction:
1. Typical myocardial necrosis-associated rise and fall of troponin or CK-MB PLUS at least one of the following:
- Symptoms of ischemia
- Pathologic Q wave on ECG
- ST-segment elevation or depression on ECG (indicative of ischemia)
- Coronary artery intervention (e.g. coronary angioplasty)
2. Pathologic features of an acute myocardial infarction
Necrosis of myocardial cells leads to the release of intracellular macromolecules from the cells into the blood-stream. Detection of significant amounts of these biochemical markers in blood under appropriate clinical setting can allow diagnosis of myocardial infarction to be made. Measurement of these markers can also distinguish between unstable angina and acute myocardial infarction.