What Causes an Acute Myocardial Infarction
Acute myocardial infarction is most commonly caused by disruption (rupture or erosion) of an unstable atherosclerotic plaque in a coronary artery, leading to thrombus formation that partially or completely occludes blood flow to the myocardium, resulting in myocardial ischemia and necrosis. 1, 2
Primary Pathophysiologic Mechanism (Type 1 MI)
The dominant cause follows this sequence:
- Progressive lipid accumulation and inflammation within an atherosclerotic plaque leads to plaque instability
- Plaque disruption occurs through either:
- Rupture of the fibrous cap (more common in men)
- Superficial erosion of the endothelial surface (relatively more common in women) 3
- Exposure of thrombogenic plaque contents to circulating blood activates the coagulation cascade
- Thrombus formation develops, causing partial (NSTEMI) or complete (STEMI) coronary artery occlusion
- Myocardial ischemia and necrosis result from compromised blood flow 1, 2
Critical Plaque Characteristics
The 2008 ESC guidelines 4 emphasize that three-quarters of infarct-related thrombi develop over plaques causing only mild to moderate stenosis—not necessarily severe obstructive lesions. Vulnerable plaques typically have:
- Thin fibrous caps
- Large lipid pools
- Substantial outward remodeling ("compensatory enlargement")
- Active inflammation with macrophages and T lymphocytes 4
Less Common Causes
Beyond atherosclerotic plaque disruption, AMI can result from 1, 5, 1:
- Coronary artery spasm (Prinzmetal's angina)—intense focal vasospasm of epicardial vessels
- Coronary embolism—thromboembolic material lodging in coronary circulation
- Spontaneous coronary artery dissection—particularly in peripartum women 5
- Coronary microvascular dysfunction—affecting small intramural resistance vessels 3
Type 2 MI (Secondary Causes)
When myocardial oxygen supply-demand mismatch occurs without primary coronary plaque disruption 6:
- Increased oxygen demand: fever, tachycardia, thyrotoxicosis
- Reduced coronary flow: hypotension, severe anemia
- Reduced oxygen delivery: hypoxemia, severe anemia
- Direct toxic effects: high circulating catecholamines in critically ill patients
Sex-Specific Differences
Women present with distinct pathophysiology 3:
- Higher proportion of plaque erosion (versus rupture)
- More frequent coronary microvascular dysfunction
- Greater incidence of Takotsubo (stress-related) cardiomyopathy
- Less obstructive disease on angiography despite worse outcomes
- Higher rates of MINOCA (MI with non-obstructive coronary arteries)
Triggering Factors
The 2008 ESC guidelines 4 identify precipitants:
- Circadian variation: higher incidence in early morning hours due to β-adrenergic stimulation, hypercoagulability, and platelet hyperreactivity
- Physical or emotional stress: triggers sympathetic stimulation and vasoconstriction
- Inflammatory milieu: elevated C-reactive protein and interleukin-6 correlate with plaque instability
Timeline of Myocardial Necrosis
Once complete occlusion occurs 4:
- Myocardial necrosis begins after 15-30 minutes of severe ischemia
- Progresses from subendocardium to subepicardium in a time-dependent "wave-front phenomenon"
- Reperfusion within this window can salvage myocardium at risk
Clinical Caveat
In 25-30% of patients undergoing primary PCI, the infarct-related artery is patent on initial angiography 4, suggesting dynamic thrombosis with spontaneous lysis or intermittent flow obstruction—emphasizing the importance of rapid intervention even when initial flow appears preserved.