Acute Myocardial Infarction (AMI), commonly known as a heart attack, refers to myocardial necrosis due to prolonged ischemia resulting from a sudden reduction or cessation of coronary blood flow. It is part of the acute coronary syndrome (ACS) spectrum, which also includes unstable angina and NSTEMI .
Classification of AMI (Types of MI)
ST-Elevation MI (STEMI):
- Complete occlusion of a coronary artery.
- ECG: ST-segment elevation ≥ 1 mm in ≥ 2 contiguous leads.
- Cardiac markers: Elevated troponin I/T , CK-MB .
- Requires immediate reperfusion therapy (PCI or thrombolysis).
Non-ST Elevation MI (NSTEMI):
- Subendocardial infarction (partial occlusion).
- ECG: ST depression or T-wave inversion.
- Cardiac markers: Elevated troponin without ST elevation.
- Managed initially with antiplatelets, anticoagulants, and delayed PCI.
Unstable Angina (UA):
- Myocardial ischemia without necrosis.
- ECG: Normal or transient changes.
- Cardiac markers: Not elevated.
NSTEMI and UA are often grouped as Non-ST Elevation ACS (NSTE-ACS) .
Universal Classification of MI (Types 1–5):
- Type 1 : Spontaneous MI due to plaque rupture or thrombus.
- Type 2 : Supply-demand mismatch (e.g., anemia, hypoxia, hypotension, arrhythmias).
- Type 3 : Sudden cardiac death before biomarkers are obtained.
- Type 4a : MI related to PCI.
- Type 4b : MI related to stent thrombosis.
- Type 5 : MI related to CABG.
Etiology & Risk Factors
Reduced Oxygen Supply:
- Atherosclerosis (most common).
- Vasospasm (e.g., Prinzmetal angina).
- Hypoxia, anemia.
- Hypotension or decreased coronary perfusion pressure.
Increased Myocardial Demand:
- Tachyarrhythmias, hypertension.
- Fever, sepsis, hyperthyroidism.
- Drug-induced (cocaine, amphetamines).
Clinical Features of AMI
Symptoms:
- Severe, crushing retrosternal chest pain , radiating to left arm, neck, or jaw.
- Duration: ≥ 20 minutes, not relieved by rest or nitroglycerin.
- Associated: Dyspnea , nausea/vomiting , sweating , lightheadedness , sense of impending doom .
Signs:
- Pallor, diaphoresis, cool extremities.
- Hypotension or tachycardia.
- S4 gallop , murmurs (e.g., papillary muscle dysfunction).
- Jugular venous distention in RV infarction.
- Pulmonary rales or edema in LV failure.
Diagnostic Investigations
Laboratory:
- Cardiac biomarkers:
- Troponin I/T (peak at 12–24h, elevated for 7–14 days).
- CK-MB (peaks earlier, useful in reinfarction).
- Others: CBC, BMP, coagulation profile, glucose, LFTs, lipids, ABG.
Electrocardiogram (ECG):
- ST-elevation in STEMI.
- ST-depression or T-wave inversion in NSTEMI/UA.
- Serial ECGs every 15–30 minutes if initial ECG is non-diagnostic.
Imaging:
- Echocardiogram : Wall motion abnormalities.
- Chest X-ray : Rule out differential diagnoses (e.g., aortic dissection).
- Coronary angiography : Diagnostic and therapeutic in most patients.
Differential Diagnosis
- Pericarditis
- Aortic dissection
- Pulmonary embolism
- Gastroesophageal reflux
- Acute pancreatitis
- Costochondritis
Management of AMI
Initial (Prehospital or ED):
MONA-BASH-C
- M orphine (if pain persists)
- O xygen (if SpO₂ < 90%)
- N itroglycerin (sublingual or IV)
- A spirin (loading dose 160–325 mg)
- B eta-blockers (if no contraindications)
- A CE inhibitors (within 24 h)
- S tatins (high-intensity)
- H eparin (UFH or LMWH)
- C lopedogrel/ticagrelor (P2Y12 inhibitor)
Definitive Therapy:
For STEMI:
- Primary PCI : Gold standard if within 90 minutes.
- Fibrinolysis (e.g., alteplase): If PCI unavailable within 120 minutes.
- Continue DAPT (dual antiplatelet therapy) + anticoagulation.
For NSTEMI/UA:
- Risk stratify (TIMI or GRACE score).
- Early invasive strategy for high-risk patients.
- DAPT, beta-blockers, anticoagulants, statins.
Complications of AMI
- Arrhythmias : VF, VT, bradyarrhythmias.
- Heart failure , cardiogenic shock.
- Pericarditis , Dressler syndrome .
- Mechanical : Papillary muscle rupture, VSD, free wall rupture.
- Recurrent MI , mural thrombus , embolism.
- Sudden cardiac death .
Prognosis and Follow-Up
- Early revascularization improves outcomes.
- Cardiac rehab and risk factor modification (smoking cessation, weight control, BP/lipid/glucose management) are essential.
High-Yield Tip: Troponin is the most sensitive and specific biomarker for myocardial infarction. STEMI is an ECG diagnosis; do not delay treatment awaiting troponin levels.
