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ACUTE MYOCARDIAL INFARCTION (AMI) - Complete LAQ Answer

(a) DEFINITION

Acute Myocardial Infarction is defined as myocardial cell death with necrosis of the myocardium, resulting from prolonged severe ischemia due to inadequate coronary perfusion.

Per the Fourth Universal Definition of MI (ACC/ESC):

MI is present when there is acute myocardial injury (cardiac troponin above the 99th percentile upper reference limit, with a rise or fall pattern) plus at least one of:
- Symptoms of myocardial ischemia
- New ECG changes (ST/T wave changes, or pathologic Q waves)
- Imaging evidence of loss of viable myocardium or regional wall motion abnormality
- Angiographic or autopsy evidence of coronary thrombus

Classification of AMI by type:

Type 1 - Spontaneous MI from plaque rupture/erosion with thrombus (the classic ACS)
Type 2 - Secondary to increased O₂ demand or decreased supply (spasm, severe anemia, arrhythmia)
Type 3 - Sudden cardiac death before biomarker sampling
Type 4 - Post-PCI MI (troponin >3x 99th percentile URL)
Type 5 - Post-CABG MI (troponin >5x 99th percentile URL)

ECG-based classification: STEMI vs. NSTEMI (both with troponin rise; STEMI has ST elevation on ECG, NSTEMI does not).

(b) RISK FACTORS

Modifiable (Major):

Factor	Notes
Hypertension	Promotes endothelial damage
Hyperlipidemia / dyslipidemia	LDL drives plaque formation; elevated Lp(a) impairs fibrinolysis
Diabetes mellitus	Accelerates atherosclerosis
Cigarette smoking	Endothelial injury, platelet activation
Obesity	Linked to all metabolic risk factors
Sedentary lifestyle
Stress / type A personality

Non-Modifiable:

Age (men >45, women >55)
Male sex (women catch up post-menopause)
Family history of premature CAD (1st-degree relative <55M / <65F)

Emerging/Other:

Elevated CRP (inflammatory marker - positive correlation with subsequent MI)
Elevated Lp(a) - interferes with fibrinolysis by downregulating plasmin generation
Cocaine/IV drug use - coronary arterial injury and spasm
Inflammatory/autoimmune conditions (e.g., rheumatoid arthritis, psoriatic arthritis)

(c) MECHANISM (PATHOPHYSIOLOGY)

Atherosclerotic plaque formation: Chronic lipid deposition causes thickening and obstruction of coronary arterial walls. Fibrous plaques reduce luminal flow; vulnerable (unstable) fibro-lipid plaques have a lipid-rich core beneath a thin fibromuscular cap.
Plaque rupture or erosion: The vulnerable plaque ruptures or hemorrhages, exposing subendothelial collagen and lipid core to circulating blood.
Platelet activation and thrombus formation: Endothelial damage triggers platelet adhesion, activation, and aggregation. Platelet-rich thrombi form rapidly at the site of rupture. This is the key initiating event in Type 1 AMI.
Coronary occlusion: The growing thrombus occludes the coronary lumen, reducing blood flow to zero (or near-zero).
Ischemia - Injury - Infarction (temporal sequence):
- Ischemia (reversible): O₂ demand > supply; impaired contractility within seconds; ECG shows ST depression/T-wave changes
- Injury (potentially reversible): Prolonged ischemia; ECG shows ST elevation
- Infarction (irreversible necrosis): Cell death begins after ~20-40 minutes of total ischemia; progresses from subendocardium outward ("wavefront phenomenon")
Vasospasm can further reduce flow in partially occluded vessels, precipitating infarction even without complete thrombotic occlusion.

Note: Vessel stenosis usually must exceed 95% to cause ischemia at rest. However, with plaque rupture, even previously non-flow-limiting plaques can cause sudden complete occlusion.

(d) CLINICAL FEATURES

Symptoms:

Chest pain - severe, crushing/pressure/squeezing pain, classically central or retrosternal; radiation to left arm (LR+ 2.2), right arm (LR+ 7.3), both arms (LR+ 9.7), jaw, or neck
Duration >30 minutes (distinguishes from angina which lasts 2-5 min)
NOT relieved by rest or nitrates (unlike stable angina)
Nausea and vomiting
Diaphoresis (sweating)
Dyspnea (especially if LV failure develops)
Sense of impending doom (angor animi)
Palpitations (from arrhythmias)
Syncope / pre-syncope

Signs:

Anxious, pale, diaphoretic patient
Tachycardia or bradycardia (especially inferior MI)
Hypotension (LR+ 3.1 for MI) or hypertension
S3 or S4 gallop (S3 has LR+ 3.2)
Pulmonary crackles (LR+ 2.1) if LV failure
Raised JVP (right ventricular involvement)
New murmur (papillary muscle dysfunction/rupture or VSD as complications)

Atypical presentations (especially in elderly, women, diabetics, minorities):

Epigastric pain, indigestion
Jaw pain alone
Fatigue, weakness
Silent MI (no chest pain at all - common in diabetics)

(e) COMPLICATIONS

Early (hours to days):

Complication	Notes
Arrhythmias	Most common early complication - VF (leading cause of early death), VT, AF, heart block (especially inferior MI causing AV block)
Cardiogenic shock	Most serious; from extensive LV damage; hemodynamic instability
Acute LV failure / pulmonary edema	Due to impaired contractility
Rupture of free wall	Catastrophic; cardiac tamponade; usually 3-5 days post-MI
Ventricular septal defect (VSD)	Rupture of interventricular septum; new harsh pansystolic murmur
Acute Mitral Regurgitation	Papillary muscle rupture or dysfunction; new murmur + flash pulmonary edema
Right ventricular infarction	Associated with inferior MI; presents with hypotension, raised JVP, clear lungs

Late (weeks to months):

Complication	Notes
Dressler's syndrome	Post-MI pericarditis; fever + pleuritic chest pain 2-10 weeks post-MI; autoimmune mechanism
Left ventricular aneurysm	Persistent ST elevation; risk of mural thrombus and emboli
Reinfarction	Extension of infarct or new occlusion
Chronic heart failure	Loss of functional myocardium
Ventricular remodeling	Progressive LV dilation and dysfunction

(f) MANAGEMENT

Investigations

Serum Markers (★★★ HIGH PRIORITY):

Marker	Rise	Peak	Return to Normal	Notes
Troponin I / T	3-6 hrs	12-24 hrs	7-10 days (TnI), 10-14 days (TnT)	Most sensitive and specific; gold standard; serial measurement (rise and fall pattern essential for diagnosis)
CK-MB	4-8 hrs	12-24 hrs	48-72 hrs	Good specificity for myocardium; useful for detecting reinfarction (returns to baseline faster than troponin)
Myoglobin	1-2 hrs	6-8 hrs	24 hrs	Earliest marker to rise; NOT cardiac-specific; good for ruling out MI early
LDH	24-48 hrs	3-6 days	8-14 days	Historical; rarely used now
AST	8-12 hrs	24-48 hrs	4-6 days	Non-specific; historical

Serial troponin measurements at 0, 3, and 6 hours (high-sensitivity assay) are now the standard of care. A rise AND fall of troponin is required to diagnose acute MI.

ECG (MANDATORY - within 10 minutes of arrival):

STEMI: ST elevation ≥1 mm in ≥2 contiguous leads (≥2 mm in V2-V3)
NSTEMI: ST depression, T-wave inversion, or normal ECG
New LBBB is treated as STEMI equivalent
Reciprocal ST depression confirms STEMI
Q waves develop over hours-days (indicate transmural necrosis)

Other Investigations:

CBC, BMP (electrolytes, renal function), LFTs, lipid profile, fasting glucose, HbA1c
Coagulation profile (before thrombolytics/anticoagulation)
Chest X-ray (cardiomegaly, pulmonary edema, aortic widening to rule out dissection)
Echocardiogram (wall motion abnormality, EF assessment, complications)
Coronary angiography (definitive; identifies culprit vessel)

Treatment

Immediate (first 10 minutes - "TIME = MUSCLE"): Use the mnemonic MONA (modified):

M - Morphine (IV, for pain unrelieved by nitrates - use with caution)
O - Oxygen (only if SpO₂ <90% or hypoxic)
N - Nitrates (sublingual/IV nitroglycerin - vasodilates, reduces preload; avoid in RV infarct or hypotension)
A - Aspirin 300 mg oral immediately (antiplatelet - inhibits TXA₂ mediated platelet aggregation)

Antiplatelet Therapy:

Aspirin (loading 300 mg, then 75-100 mg daily lifelong)
P2Y12 inhibitor (clopidogrel 600 mg loading, or ticagrelor 180 mg, or prasugrel) - dual antiplatelet therapy (DAPT)
Glycoprotein IIb/IIIa inhibitors (e.g., abciximab) - in patients undergoing primary PCI

Anticoagulation:

Unfractionated heparin (UFH) or low-molecular-weight heparin (enoxaparin) or fondaparinux or bivalirudin (depending on reperfusion strategy)

Reperfusion (most important intervention - restore flow):

Strategy	Indication	Time
Primary PCI (preferred)	STEMI if available	Door-to-balloon time <90 min
Thrombolysis (fibrinolytics)	STEMI if PCI not available within 2 hours	Door-to-needle time <30 min; effective within 12 hours of symptom onset
CABG	Multi-vessel disease, failed PCI, mechanical complications

Thrombolytic agents: Streptokinase (1.5 MU over 60 min), Alteplase (tPA), Tenecteplase (TNK-tPA), Reteplase
Primary PCI is more efficacious than thrombolysis and is the preferred strategy

Other Medications:

Beta-blockers (metoprolol, atenolol) - reduce heart rate, O₂ demand, prevent arrhythmias; start early if no contraindication (avoid in cardiogenic shock, acute LVF, bradycardia)
ACE inhibitors / ARBs - start within 24 hours; reduce afterload, prevent remodeling (especially with reduced EF or anterior MI)
Statins (high-intensity, e.g., atorvastatin 80 mg) - start immediately; pleiotropic + lipid-lowering effects
Aldosterone antagonist (eplerenone/spironolactone) - in patients with LV dysfunction (EF <40%) or heart failure

Long-term / Secondary Prevention:

Lifelong aspirin + P2Y12 (12 months post-ACS, then aspirin alone)
Beta-blocker (minimum 1-3 years)
ACE inhibitor / ARB (lifelong if reduced EF)
High-intensity statin (lifelong)
Cardiac rehabilitation
Lifestyle modification: smoking cessation, diet, exercise, weight loss, BP and glucose control

Sources:

ROSEN's Emergency Medicine, 9th Ed. - AMI definition, pathophysiology, serum markers
Symptom to Diagnosis: An Evidence-Based Guide, 4th Ed. - Clinical features, ECG criteria, treatment
Ganong's Review of Medical Physiology, 26th Ed. - Mechanism, serum enzymes
Goldman-Cecil Medicine - Atherosclerosis and risk factors
Harrison's Principles of Internal Medicine, 22nd Ed. - Complications, management indications

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