Classification and Diagnostic Criteria of Myocardial Infarction Types

1. Definitions of the Five MI Types

• Type 1 (spontaneous) MI – Caused by atherosclerotic plaque rupture, erosion, fissuring, or dissection that generates a thrombus occluding a coronary artery and leads to myocardial necrosis. 1
• Type 2 (supply‑demand mismatch) MI – Results from an imbalance between myocardial oxygen supply and demand without a primary coronary occlusion. 1
• Common precipitants of Type 2 MI – Severe anemia, respiratory failure, extreme hypertension or hypotension, tachyarrhythmia or bradyarrhythmia, coronary artery spasm, and coronary embolism. 1
• Therapeutic implication for Type 2 MI – Emergency catheterization or thrombolytic therapy does not improve outcomes; management focuses on correcting the underlying trigger (e.g., treating anemia, stabilising blood pressure, correcting arrhythmia). 1
• Type 3 MI (sudden cardiac death) – Sudden death with clinical and ECG features suggestive of MI, occurring before cardiac biomarkers can be measured; autopsy‑detected fresh coronary thrombus confirms the diagnosis. [1][2]
• Type 4a MI (PCI‑related) – MI that occurs during or immediately after percutaneous coronary intervention. Diagnosis requires a rise in cardiac troponin > 5 × the upper limit of normal (ULN) from a normal baseline plus at least one of: new ischemic symptoms, new ECG changes, angiographic complications, or imaging evidence of new myocardial loss. 1
• Type 4b MI (stent thrombosis) – MI caused by thrombus formation inside a previously implanted stent, documented by angiography or autopsy. [1][2]
• Type 5 MI (CABG‑related) – MI occurring during or after coronary artery bypass graft surgery. Diagnosis requires troponin > 10 × ULN together with new Q waves, new left‑bundle‑branch block, angiographic graft occlusion, or imaging evidence of new myocardial injury. 1

2. Pathological Timeline of Myocardial Infarction

All phases are described in the Circulation consensus. 1

3. Clinical Pitfalls and Interpretation Issues

• Discordance between ECG/biomarkers and pathology – Electrocardiographic changes may still appear “evolving” while troponin levels remain elevated during the healing phase, potentially misleading clinicians about the infarct stage. 1
• Troponin elevation is not synonymous with MI – Elevated troponin can arise from non‑ischemic myocardial injury (e.g., sepsis, heart failure, pulmonary embolism, renal failure, myocarditis, cardiotoxic drugs). Such elevations should not be managed as MI. 1

Myocardial Infarction Classification and Diagnosis

Types of Myocardial Infarction

• The American College of Cardiology defines five distinct types of myocardial infarction (MI), each with different pathophysiology, clinical presentation, and treatment approaches, including Type 1: Spontaneous Myocardial Infarction, Type 2: Myocardial Infarction Secondary to Ischemic Imbalance, Type 3: Myocardial Infarction Resulting in Death When Biomarker Values Are Unavailable, Type 4: Myocardial Infarction Associated with Revascularization Procedures, and Type 5: Myocardial Infarction Associated with CABG 3, 4, 5, 6
• Type 1 MI is related to atherosclerotic plaque rupture, ulceration, fissuring, erosion, or dissection with resulting intraluminal thrombus in one or more coronary arteries, leading to decreased myocardial blood flow or distal platelet emboli with ensuing myocyte necrosis 3, 7
• Type 2 MI occurs when conditions other than coronary artery disease contribute to an imbalance between myocardial oxygen supply and demand, with common causes including coronary endothelial dysfunction, coronary artery spasm, coronary embolism, tachy/brady-arrhythmias, anemia, respiratory failure, hypotension, and hypertension with or without left ventricular hypertrophy 4, 6
• Type 4a MI is associated with percutaneous coronary intervention (PCI) and is defined by elevation of cardiac troponin values >5× the 99th percentile upper reference limit in patients with normal baseline values, requiring additional evidence such as symptoms of ischemia, new ECG changes, angiographic complications, or imaging evidence of new loss of viable myocardium 5, 6
• Type 5 MI is associated with CABG and is defined by elevation of cardiac biomarker values >10× the 99th percentile URL in patients with normal baseline values, with additional criteria including new pathological Q waves, new LBBB, angiographic evidence of graft or native coronary artery occlusion, or imaging evidence of new loss of viable myocardium 6

Clinical Classification of Myocardial Infarction

• The American College of Cardiology recommends that MI be clinically classified based on ECG findings and biomarker results, including ST-Elevation MI (STEMI) and Non-ST-Elevation MI (NSTEMI) 3, 8
• Q-wave MI vs. Non-Q-wave MI can be distinguished by the presence or absence of Q waves on ECG, with Q waves representing transmural infarction and being associated with larger infarct size 3, 4

Clinical Pitfalls and Considerations

• The American Heart Association notes that MI may present with atypical symptoms or even without symptoms, especially in women, elderly, diabetics, and post-operative or critically ill patients, and that differentiation between true MI and other causes of myocardial injury is crucial 3, 4, 9
• The American College of Cardiology recommends prompt recognition of STEMI equivalents, including hyperacute T-wave changes, true posterior MI, multilead ST depression with ST elevation in lead aVR, and characteristic diagnostic criteria in the setting of left bundle branch block 9