What is the classification of myocardial infarction (MI)?

Classification of Myocardial Infarction

Myocardial infarction (MI) is classified into five distinct types based on pathophysiological mechanisms, with additional clinical classifications based on ECG findings that guide immediate treatment decisions. 1

Universal Classification of MI (Types 1-5)

Type 1: Spontaneous MI

• Caused by atherosclerotic plaque rupture, ulceration, fissuring, erosion, or dissection
• Results in intraluminal thrombus formation in coronary arteries
• Leads to decreased myocardial blood flow or distal platelet emboli
• May occur with severe CAD, but in 5-20% of cases (particularly in women), non-obstructive or no CAD is found 1

Type 2: MI Secondary to Ischemic Imbalance

• Occurs when conditions other than CAD create an imbalance between myocardial oxygen supply and demand
• Common causes include:
  • Coronary endothelial dysfunction
  • Coronary artery spasm
  • Coronary embolism
  • Tachy/brady-arrhythmias
  • Anemia
  • Respiratory failure
  • Hypotension
  • Hypertension with or without LVH 1

Type 3: MI Resulting in Death When Biomarkers Are Unavailable

• Cardiac death with symptoms suggestive of myocardial ischemia
• Presumed new ischemic ECG changes or new LBBB
• Death occurs before blood samples could be obtained or before cardiac biomarkers could rise 1

Type 4: MI Associated with Revascularization Procedures

• Type 4a: MI associated with PCI
  • Defined by elevation of cardiac troponin values >5× 99th percentile URL in patients with normal baseline values
  • Requires additional criteria such as symptoms, new ECG changes, angiographic findings, or imaging evidence 1
• Type 4b: MI related to stent thrombosis
  • Detected by coronary angiography or autopsy
  • Occurs with rise/fall of cardiac biomarkers above 99th percentile URL 1

Type 5: MI Related to CABG

• Defined by elevation of cardiac biomarker values >10× 99th percentile URL in patients with normal baseline values
• Requires additional criteria such as new Q waves/LBBB, angiographic evidence of graft/native artery occlusion, or imaging evidence of new loss of viable myocardium 1

Clinical Classification of MI (Based on ECG)

STEMI vs. NSTEMI

• STEMI (ST-Elevation MI):

  • ST elevation in two contiguous leads
  • Typically indicates complete coronary occlusion requiring immediate reperfusion 1, 2
  • Cut-points for ST elevation:
    • ≥0.1 mV in all leads except V2-V3
    • ≥0.2 mV in men ≥40 years (V2-V3)
    • ≥0.25 mV in men <40 years (V2-V3)
    • ≥0.15 mV in women (V2-V3) 1

• NSTEMI (Non-ST-Elevation MI):

  • No ST elevation at presentation
  • May still represent significant coronary occlusion in some cases 3

Q Wave vs. Non-Q Wave MI

• Q Wave MI: Development of pathological Q waves on ECG
• Non-Q Wave MI: No development of Q waves 1

Emerging Classification: OMI vs. NOMI

The American College of Cardiology has recognized a newer classification paradigm that focuses on coronary occlusion status rather than ECG findings:

• OMI (Occlusive MI):

  • Complete or near-complete coronary occlusion requiring urgent revascularization
  • Includes both STEMI and approximately 40% of NSTEMI cases with coronary occlusion
  • Characterized by TIMI 0-2 flow in culprit artery 4, 5

• NOMI (Non-Occlusive MI):

  • No complete coronary occlusion 5

Clinical Implications and Pitfalls

• The traditional STEMI/NSTEMI classification can miss approximately 40% of patients with acute coronary occlusion who don't present with classic ST elevation 4, 5
• STEMI(-) OMI patients experience significant delays to catheterization (median 437 min vs. 41 min for STEMI) despite similar adverse outcomes 5
• ECG classification correlates better with total infarct size than with transmurality of the infarction 6
• Early identification of all patients with coronary occlusion requiring immediate reperfusion, regardless of STEMI criteria, is crucial for improving outcomes 4

The classification of MI is essential for guiding appropriate treatment strategies and understanding prognosis, with immediate reperfusion being critical for patients with coronary occlusion regardless of ECG classification.