Pathophysiology and Types of Myocardial Infarction
Myocardial infarction fundamentally results from an imbalance between myocardial oxygen supply and demand, leading to myocyte necrosis, and is classified into five distinct types based on underlying pathophysiology, with Type 1 (atherothrombotic) and Type 2 (supply-demand mismatch) being the most clinically relevant spontaneous forms. 1
Core Pathophysiology
The pathophysiologic cascade begins with myocardial ischemia—the initial step that triggers MI development. 1 This ischemia stems from inadequate oxygen delivery relative to myocardial oxygen requirements. When prolonged, ischemia activates a "wavefront" of cardiomyocyte death extending from subendocardium to subepicardium, with mitochondrial alterations prominently driving both apoptotic and necrotic cell death. 2
The dying myocardium releases cardiac troponins (the gold standard biomarker), which must demonstrate a rising and/or falling pattern with at least one value above the 99th percentile to distinguish acute MI from chronic myocardial injury. 1
Universal Classification System
The Third Universal Definition (2012) established a pathophysiology-based classification that directly influences treatment strategy 1:
Type 1 MI: Spontaneous Atherothrombotic
- Mechanism: Atherosclerotic plaque rupture, ulceration, fissuring, erosion, or dissection → intraluminal thrombus formation → decreased coronary blood flow or distal platelet emboli → myocyte necrosis 1
- Key caveat: 5-20% of patients may have non-obstructive or no CAD on angiography, particularly women 1
- Clinical significance: This is the "classic" MI requiring antiplatelet therapy, anticoagulation, and revascularization
Type 2 MI: Supply-Demand Mismatch
- Mechanism: Myocardial injury with necrosis where conditions other than CAD create oxygen supply-demand imbalance 1
- Specific triggers: Coronary endothelial dysfunction, coronary vasospasm, coronary embolism, tachyarrhythmias, bradyarrhythmias, anemia, respiratory failure, hypotension, hypertension with/without LVH 1
- Critical distinction: No atherothrombosis present; treatment targets the underlying precipitant rather than coronary intervention 3, 4
- Prognosis: Multiple comorbidities common; in-hospital mortality often non-cardiovascular 4
Type 3 MI: Sudden Cardiac Death
- Definition: Cardiac death with ischemic symptoms and presumed new ischemic ECG changes or new LBBB, but death occurs before biomarkers can be obtained or rise 1
- Diagnostic challenge: Biomarker evidence lacking; diagnosis relies on clinical and ECG findings
Type 4a MI: PCI-Related
- Criteria: Troponin elevation >5× the 99th percentile in patients with normal baseline values (or >20% rise if baseline elevated and stable) 1
- Plus one of: Ischemic symptoms, new ischemic ECG changes/LBBB, angiographic complications (loss of patency, slow/no-flow, embolization), or imaging evidence of new myocardial loss 5
Type 4b MI: Stent Thrombosis
- Detection: Confirmed by angiography or autopsy in setting of myocardial ischemia with rising/falling biomarkers above 99th percentile 5
Type 5 MI: CABG-Related
- Criteria: Biomarker elevation >10× the 99th percentile in patients with normal baseline troponin 5
- Plus one of: New pathological Q waves/LBBB, angiographic graft/native vessel occlusion, or imaging evidence of new myocardial loss
Clinical Presentation Framework
ECG-Based Classification (Treatment-Oriented)
Beyond the pathophysiologic types, MI is clinically categorized by ECG findings to guide immediate reperfusion decisions 1:
- STEMI: ST elevation in two contiguous leads → immediate reperfusion therapy indicated
- NSTEMI: No ST elevation at presentation → risk stratification and selective invasive strategy
- Q-wave vs. non-Q-wave MI: Prognostic distinction based on ECG evolution
Symptom Recognition
Typical presentation (lasting >20 minutes): Diffuse chest, upper extremity, mandibular, or epigastric discomfort—not localized, not positional, not movement-related—often with diaphoresis, nausea, or syncope 1
High-risk atypical presentations requiring heightened vigilance 1:
- Women
- Elderly patients
- Diabetics
- Post-operative patients
- Critically ill patients
These groups may present with palpitations, cardiac arrest, or no symptoms at all.
Critical Clinical Pitfalls
Misdiagnosis risk: Ischemic symptoms are non-specific and frequently attributed to gastrointestinal, neurological, pulmonary, or musculoskeletal disorders 1
Type 1 vs. Type 2 distinction: Most NSTEMI (65-90%) are Type 1, but Type 2 patients have fundamentally different treatment needs—invasive coronary intervention may not be appropriate 4
Biomarker interpretation: A single elevated troponin is insufficient; the rising/falling pattern is essential to distinguish acute MI from chronic myocardial injury 1
Non-obstructive CAD: Up to 20% of Type 1 MI patients may have minimal angiographic disease, particularly women—do not dismiss the diagnosis based on angiography alone 1