What is the definition of a myocardial infarction (MI) or heart attack?

Definition of Myocardial Infarction (Heart Attack)

Myocardial infarction is defined as myocardial cell death due to prolonged ischemia, which is the result of a perfusion imbalance between oxygen supply and demand. 1

Pathophysiological Definition

• Myocardial infarction reflects cell death of cardiac myocytes caused by ischemia, typically resulting from a perfusion imbalance between supply and demand 1
• Cell death is categorized pathologically as coagulation and/or contraction band necrosis, which usually evolves through oncosis, but can result to a lesser degree from apoptosis 1
• After the onset of myocardial ischemia, cell death is not immediate but takes time to develop (as little as 20 minutes in some animal models) 1
• Complete necrosis of all myocardial cells at risk requires at least 2-4 hours or longer, depending on collateral circulation, persistent or intermittent coronary occlusion, and individual oxygen demand 1

Clinical Features of Ischemia

• Ischemic symptoms typically include various combinations of chest, upper extremity, jaw, or epigastric discomfort with exertion or at rest 1
• The discomfort associated with acute myocardial infarction usually lasts at least 20 minutes 1
• Often, the discomfort is diffuse, not localized, not positional, not affected by movement, and may be accompanied by dyspnea, diaphoresis, nausea, or syncope 1
• Myocardial infarction may occur with atypical symptoms or even without symptoms, being detected only by ECG, biomarker elevations, or cardiac imaging 1

Diagnostic Criteria

• Myocardial infarction is diagnosed when blood levels of sensitive and specific biomarkers (cardiac troponin or CK-MB) are increased in the clinical setting of acute myocardial ischemia 1
• The preferred biomarker is cardiac troponin (I or T), which has nearly absolute myocardial tissue specificity 1
• Maximal concentration of troponin T or I exceeding the 99th percentile of values for a reference control group on at least one occasion during the first 24 hours after the clinical event 1
• In the absence of troponin or CK-MB assay, total CK (greater than two times the upper reference limit) may be used, though it is less satisfactory 1

Classification of Myocardial Infarction

The universal definition classifies MI into five types based on pathological, clinical, and prognostic differences 1:

1. Type 1 (Spontaneous MI): Related to atherosclerotic plaque rupture, ulceration, fissuring, erosion, or dissection with resulting intraluminal thrombus 1
2. Type 2 (Secondary MI): Due to ischemic imbalance from increased oxygen demand or decreased supply (e.g., coronary spasm, embolism, anemia, arrhythmias, hypertension, hypotension) 1
3. Type 3 (Sudden Cardiac Death): Cardiac death with symptoms suggestive of myocardial ischemia, new ECG changes or LBBB, but death occurring before biomarkers could be obtained 1
4. Type 4a: MI associated with percutaneous coronary intervention (PCI) 1
5. Type 4b: MI associated with stent thrombosis documented by angiography or autopsy 1
6. Type 5: MI associated with coronary artery bypass grafting (CABG) 1

Temporal Classification

Myocardial infarctions can be classified temporally according to pathological appearance 1:

• Acute MI: 6 hours to 7 days, characterized by the presence of polymorphonuclear leukocytes 1
• Healing MI: 7 to 28 days, characterized by mononuclear cells and fibroblasts, absence of polymorphonuclear leukocytes 1
• Healed MI: 29 days or more, manifested as scar tissue without cellular infiltration 1

Size Classification

MIs are classified by size 1:

• Microscopic (focal necrosis)
• Small (<10% of the left ventricular myocardium)
• Moderate (10-30% of the LV myocardium)
• Large (>30% of the LV myocardium)

Important Clinical Considerations

• The clinical and ECG timing of an acute ischemic event may not correspond exactly with the pathological timing 1
• ECG may still demonstrate evolving ST-T changes and cardiac biomarkers may still be elevated at a time when pathologically the infarct is in the healing phase 1
• Any amount of myocardial necrosis caused by ischemia should be labeled as MI, as even minimal damage detected by cardiac troponins implies an impaired clinical outcome 1
• Early recognition of symptoms and immediate action by calling emergency services (9-1-1) are crucial to ensure timely receipt of emergency care and improve survival chances 2