What are the types of myocardial infarction (MI)?

Types of Myocardial Infarction

According to the Universal Classification of Myocardial Infarction, there are five distinct types of MI, each with different pathophysiology, clinical presentation, and treatment approaches. 1

Universal Classification of Myocardial Infarction

• Type 1: Spontaneous Myocardial Infarction

  • Related to atherosclerotic plaque rupture, ulceration, fissuring, erosion, or dissection with resulting intraluminal thrombus in one or more coronary arteries 1
  • Leads to decreased myocardial blood flow or distal platelet emboli with ensuing myocyte necrosis 1
  • Patients typically have underlying severe coronary artery disease (CAD), but 5-20% may have non-obstructive or no CAD at angiography, particularly women 1

• Type 2: Myocardial Infarction Secondary to Ischemic Imbalance

  • Occurs when conditions other than CAD contribute to an imbalance between myocardial oxygen supply and demand 1
  • Common causes include: coronary endothelial dysfunction, coronary artery spasm, coronary embolism, tachy/brady-arrhythmias, anemia, respiratory failure, hypotension, and hypertension with or without left ventricular hypertrophy 1
  • Often seen in critically ill patients or those undergoing major non-cardiac surgery due to direct toxic effects of high circulating catecholamine levels 1

• Type 3: Myocardial Infarction Resulting in Death When Biomarker Values Are Unavailable

  • Cardiac death with symptoms suggestive of myocardial ischemia and presumed new ischemic ECG changes or new left bundle branch block 1
  • Death occurs before blood samples could be obtained, before cardiac biomarkers could rise, or in rare cases where biomarkers were not collected 1
  • These patients should be classified as having had a fatal MI even if cardiac biomarker evidence is lacking 1

• Type 4: Myocardial Infarction Associated with Revascularization Procedures

  • Type 4a: MI associated with percutaneous coronary intervention (PCI) 1

    • Defined by elevation of cardiac troponin values >5× the 99th percentile upper reference limit in patients with normal baseline values 1
    • Requires additional evidence such as symptoms of ischemia, new ECG changes, angiographic complications, or imaging evidence of new loss of viable myocardium 1

  • Type 4b: MI associated with stent thrombosis 1

    • Detected by coronary angiography or autopsy in the setting of myocardial ischemia 1
    • Accompanied by rise and/or fall of cardiac biomarker values with at least one value above the 99th percentile URL 1

• Type 5: Myocardial Infarction Associated with CABG

  • Defined by elevation of cardiac biomarker values >10× the 99th percentile URL in patients with normal baseline values 1
  • Additional criteria include new pathological Q waves, new LBBB, angiographic evidence of graft or native coronary artery occlusion, or imaging evidence of new loss of viable myocardium 1

Clinical Classification of Myocardial Infarction

In addition to the five types, MI is also clinically classified based on ECG findings and biomarker results:

• ST-Elevation MI (STEMI)

  • Characterized by ST-segment elevation in two contiguous leads on ECG 1
  • Represents complete thrombotic occlusion of an epicardial coronary vessel 2
  • Requires immediate reperfusion therapy to limit infarct size 2
  • Accounts for approximately 29-47% of all AMI patients 1

• Non-ST-Elevation MI (NSTEMI)

  • Absence of ST-segment elevation on presenting ECG 1
  • Most NSTEMI (65-90%) are Type 1 MI, though Type 2 MI can also present as NSTEMI 3
  • Requires combination of antiplatelet and anticoagulant therapy with consideration of early coronary angiography 4

• Q-wave MI vs. Non-Q-wave MI

  • Many patients with MI develop Q waves (Q-wave MI), while others do not (non-Q-wave MI) 1
  • Q waves represent transmural infarction and are associated with larger infarct size 1

Clinical Pitfalls and Considerations

• MI may present with atypical symptoms (palpitations, cardiac arrest) or even without symptoms, especially in women, elderly, diabetics, and post-operative or critically ill patients 1
• The Universal Classification primarily applies to Type 1 MI; treatment strategies for other types may differ significantly 1
• With highly sensitive troponin assays, differentiation between true MI and other causes of myocardial injury (heart failure, renal failure, myocarditis, pulmonary embolism) is crucial 1
• STEMI equivalents that require prompt recognition include 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 1