Types of Myocardial Infarction
Myocardial infarction is classified into five distinct types according to the American College of Cardiology, each with different pathophysiology, clinical presentation, and treatment approaches. 1
The Five Types of Myocardial Infarction
Type 1 MI (Spontaneous Myocardial Infarction): Results from atherosclerotic plaque rupture, ulceration, fissuring, erosion, or dissection with resulting intraluminal thrombus in one or more coronary arteries. This leads to decreased myocardial blood flow or distal platelet emboli with ensuing myocyte necrosis. 1, 2
Type 2 MI (Secondary to Ischemic Imbalance): Occurs when conditions other than coronary artery disease contribute to 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, and hypertension with or without left ventricular hypertrophy. 1, 2
Type 3 MI (Resulting in Death When Biomarker Values Are Unavailable): Occurs when patients suffer cardiac death with symptoms suggestive of myocardial ischemia, but death occurs before cardiac biomarkers can be obtained or before cardiac biomarker values would increase. 1, 2
Type 4 MI (Associated with Revascularization Procedures): Associated with percutaneous coronary intervention (PCI) and defined by elevation of cardiac troponin values >5× the 99th percentile upper reference limit in patients with normal baseline values. Additional evidence such as symptoms of ischemia, new ECG changes, angiographic complications, or imaging evidence of new loss of viable myocardium is required. 1, 2
Type 5 MI (Associated with CABG): Associated with coronary artery bypass grafting (CABG) and defined by elevation of cardiac biomarker values >10× the 99th percentile URL in patients with normal baseline values. 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, 2
Clinical Classification of Myocardial Infarction
In addition to the pathophysiological classification above, MI is also clinically classified based on ECG findings and biomarker results:
ST-Elevation MI (STEMI): Characterized by patients with chest discomfort and ST elevation in two contiguous leads. STEMI typically represents complete thrombotic occlusion of a coronary artery and requires immediate reperfusion therapy. 1, 2, 3
Non-ST-Elevation MI (NSTEMI): Characterized by patients with chest discomfort but no persistent ST-segment elevation. Management involves risk stratification and may include early invasive strategy with coronary angiography. 1, 2, 4
Q-wave MI vs. Non-Q-wave MI: Distinguished by the presence or absence of Q waves on ECG. Q waves represent transmural infarction and are associated with larger infarct size. This is an older classification that has been largely replaced by the STEMI/NSTEMI paradigm. 1, 5
Clinical Pitfalls and Considerations
MI may present with atypical symptoms or even without symptoms, especially in women, elderly, diabetics, and post-operative or critically ill patients. 1
Differentiation between true MI and other causes of myocardial injury is crucial for appropriate management. 1
Prompt recognition of STEMI equivalents is essential, 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. 1
While most NSTEMI cases (65%-90%) are type 1 MI, a significant proportion of NSTEMI patients may have acute total coronary occlusion (ATO) that would benefit from urgent reperfusion therapy. 4, 5
Some researchers advocate for revising the current STEMI/NSTEMI paradigm to an Occlusion Myocardial Infarction (OMI)/Non-Occlusion Myocardial Infarction (NOMI) paradigm to better identify patients who would benefit from urgent reperfusion regardless of traditional ECG criteria. 5