The basic pathogenetic mechanism is atherosclerosis of a coronary artery; acute coronary syndrome results from rupture or erosion of an atheromatous plaque with subsequent superimposition of thrombosis (Figure 1201.1). Clinical presentation is decided by the degree of ischemia, amount of collateral circulation, myocardial oxygen demand, and certain other patient-specific determinants. As the condition is life-threatening, early recognition of acute coronary syndrome is essential so that proper and timely treatment can be instituted. The essential parameters for diagnosis are clinical features, electrocardiographic changes, and detection of biochemical markers released in blood from myocardial damage.
The term acute coronary syndrome is coined to distinguish between chronic stable angina from unstable angina and acute myocardial infarction.
Majority of patients with acute coronary syndrome have a prior history of effort angina or coronary artery disease. The usual clinical manifestation of myocardial ischemia is chest pain. The pain is typically located in the center or left side of the chest and variously described as pressure, squeezing, constriction, crushing, tightness or heaviness. The pain radiates to the left arm, shoulder, neck, and jaw. It develops after exertion, meals, or emotional stress. In angina pectoris, pain is relieved by rest and nitroglycerin. Pain is similar in acute myocardial infarction (necrosis of myocardium due to ischemia as shown in Figure 1201.2) but is more severe and persistent (>30 minutes), and not readily relieved by rest or nitroglycerin. In myocardial infarction, pain is often accompanied by other features such as sweating, nausea, vomiting, breathlessness, and palpitations. Acute myocardial infarction may be clinically silent (in 25% of cases, especially those with diabetes or hypertension).
Unstable angina can present with (i) new angina of severe onset, (ii) angina at rest, or (iii) recent increase in frequency or pattern of angina.
Diagnosis of Acute Coronary Syndrome
Previously, the diagnosis of acute myocardial infarction required any two of the following criteria (World Health Organization, 1974):
- Symptoms of myocardial ischemia, i.e. severe and prolonged chest pain,
- Unequivocal changes consistent with acute myocardial infarction on electrocardiogram (development of Q wave)
- Elevation of cardiac enzymes in the blood
A 12-lead electrocardiogram (ECG) is an important test for diagnosis and should be obtained as soon as possible after presentation (Figure 1201.3). This is because the test is noninvasive, inexpensive and rapid. The test is useful for 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 the European Society of Cardiology (ESC) and American Society of Cardiology (ACC) (2000).
Necrosis of myocardial cells leads to the release of intracellular macromolecules from the cells into the bloodstream. 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.
Revised criteria for the diagnosis of myocardial infarction (ESC/ACC, 2000)
Any one of the following is satisfactory for diagnosis of acute/evolving/recent myocardial infarction:
- 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)
- Pathologic features of an acute myocardial infarction