What is Myocardial Infarction?
Myocardial infarction (MI) is the death of heart muscle cells caused by prolonged lack of blood flow and oxygen, most commonly due to a blood clot blocking a coronary artery. 1
Pathological Definition
MI represents irreversible myocardial cell death resulting from ischemia (inadequate blood supply). The process is time-dependent:
- Cell death begins as early as 20 minutes after blood flow is cut off
- Complete necrosis of at-risk heart muscle takes 2-4 hours or longer (depending on collateral circulation and other factors)
- Full healing with scar formation requires 5-6 weeks 1
The adult mammalian heart cannot regenerate, so infarcted tissue heals by forming a permanent scar rather than new functional heart muscle 2.
Clinical Definition and Diagnosis
The diagnosis requires two key components:
- Biomarker evidence: Elevated cardiac troponin (I or T) levels showing a rise and/or fall pattern, exceeding the 99th percentile of normal values 1
- Clinical evidence of ischemia: Symptoms, ECG changes, imaging findings, or angiographic evidence 3
Important Distinction
Elevated troponin alone does NOT equal MI. Many conditions cause troponin elevation without MI (heart failure, renal failure, myocarditis, pulmonary embolism, arrhythmias) - these represent myocardial injury, not infarction 1. The rise-and-fall pattern distinguishes acute MI from chronic troponin elevations.
Types of MI
The universal classification distinguishes five types 3:
- Type 1 (most common): Spontaneous MI from atherosclerotic plaque rupture, erosion, or dissection causing coronary artery blockage
- Type 2: MI from supply-demand mismatch (e.g., severe anemia, hypotension, arrhythmias, respiratory failure) without plaque rupture
- Type 3: Sudden cardiac death with presumed MI but death occurred before biomarkers could be measured
- Type 4a: MI related to percutaneous coronary intervention (PCI)
- Type 4b: MI related to stent thrombosis
Clinical Presentation Categories
For immediate treatment decisions, MI is classified by ECG findings 3:
- STEMI (ST-elevation MI): ST-segment elevation in two contiguous ECG leads - requires immediate reperfusion therapy
- NSTEMI (non-ST-elevation MI): Elevated biomarkers without ST elevation
Common Symptoms
Typical symptoms (more common in males): Chest pain radiating to left arm, jaw, or neck 4, 5
Atypical symptoms (more common in females): Shortness of breath, nausea, vomiting, fatigue, weakness 4, 5
Critical Caveat
Females present with more atypical symptoms, have longer delays before seeking care, are older with more comorbidities, and have different underlying pathophysiology (less atherosclerotic burden, more microvascular dysfunction) compared to males 5. Males are more likely to have silent or unrecognized MI 5.
Underlying Mechanism
In most cases, MI results from thrombotic occlusion of a coronary artery caused by rupture of an atherosclerotic plaque 2. The resulting ischemia causes:
- Profound metabolic and ionic disturbances
- Rapid loss of heart muscle contraction
- A "wavefront" of cell death spreading from inner to outer heart wall layers
- Inflammatory response and eventual scar formation 2