Acute Coronary Syndrome and STEMI

Atherosclerotic Plaque Rupture and Thrombosis

• Disruption of the fibrous cap of a lipid-rich atherosclerotic plaque exposes tissue factor and von Willebrand factor to circulating blood
• Immediate platelet adhesion, activation, and aggregation occur via glycoprotein Ib and IIb/IIIa receptor interactions
• Simultaneous activation of the extrinsic coagulation cascade through tissue factor-factor VIIa complex generates thrombin
• Coronary thrombus forms, leading to acute occlusion of the coronary vessel

Myocardial Ischemia and Necrosis

• Complete or near-complete occlusion eliminates anterograde coronary blood flow, causing acute regional myocardial ischemia
• Anaerobic metabolism rapidly depletes ATP in ischemic myocardium, impairing Na+/K+-ATPase function
• Intracellular calcium accumulation via sodium-calcium exchanger reversal causes myofibril hypercontraction and cellular dysfunction
• Progressive ischemic injury develops: reversible ischemia (minutes) → irreversible necrosis begins at ~20 minutes but continues for hours
• Transmural (full-thickness) infarction occurs with prolonged total occlusion; subendocardial involvement begins first due to highest metabolic demand and poorest collateral perfusion
• Inflammatory cascade activation (cytokines, complement, neutrophil infiltration) perpetuates myocardial damage even after reperfusion

Electrical and Mechanical Consequences

• Regional ischemia causes abnormal depolarization and repolarization, producing ST-segment elevation on ECG (reflects transcontinental voltage gradient between infarcted transmural region and non-infarcted regions)
• Loss of contractility in the infarcted territory creates wall motion abnormality (hypokinesis → akinesis → dyskinesis)
• Electromechanical dissociation and ventricular arrhythmias may occur from heightened automaticity and re-entrant circuits

Inflammation and Myocardial Remodeling

• Acute inflammatory phase (hours to days): neutrophil infiltration, macrophage activation, cytokine release
• Healing phase (days to weeks): granulation tissue formation, collagen deposition
• Ventricular remodeling: progressive cavity dilation and wall thinning, particularly in large anterior wall infarcts without reperfusion

Atherosclerotic Coronary Artery Disease (95% of STEMI)

• Chronic atherosclerotic plaque development driven by endothelial dysfunction, lipid infiltration, and inflammatory cell accumulation
• Plaque rupture typically occurs at the shoulder region where fibrous cap is thinnest and hemodynamic stress is greatest
• Vulnerable plaques characterized by lipid-rich core, thin fibrous cap, and high macrophage content

Non-Atherosclerotic Etiologies (5% of STEMI)

• Coronary vasospasm (Prinzmetal angina, cocaine use): episodic coronary artery constriction without fixed obstruction
• Coronary artery dissection (spontaneous coronary artery dissection [SCAD]): intimal tear with blood tracking into media; increased risk in peripartum women, fibromuscular dysplasia
• Coronary embolism: from left atrial thrombus (atrial fibrillation), left ventricular thrombus (dilated cardiomyopathy), endocarditis, paradoxical embolism (patent foramen ovale)
• Thrombophilia: antiphospholipid syndrome, factor V Leiden, prothrombin gene mutation
• Inflammatory diseases: vasculitis (polyarteritis nodosa, Takayasu arteritis), systemic lupus erythematosus
• Aortic dissection with ostial coronary involvement
• Severe anemia, hypoxemia, tachycardia: demand ischemia exceeding supply

Traditional Cardiovascular Risk Factors

• Modifiable: hypertension, dyslipidemia, diabetes mellitus, tobacco use, physical inactivity, obesity, poor diet
• Non-modifiable: age (men >45, women >55), male sex, family history of premature CAD
• Emerging: chronic kidney disease, elevated lipoprotein(a), inflammatory markers (elevated CRP, IL-6)
• Psychosocial: depression, social isolation, chronic stress

Chest Discomfort (Typical Anginal Presentation)

• Symptom character: substernal, left precordial, or epigastric pressure, heaviness, tightness, or squeezing sensation
• Radiation pattern: to left arm/shoulder, neck, jaw, back, or right arm
• Duration: typically ≥20 minutes (distinguishes from stable angina which resolves in <10 minutes with rest or nitrates)
• Onset: often at rest or with minimal exertion, particularly in STEMI versus stable angina
• Associated symptoms: diaphoresis, nausea/vomiting, dyspnea, syncope
• Atypical presentations: isolated dyspnea, fatigue, epigastric discomfort, shoulder/back pain; more common in elderly, women, diabetics, post-transplant patients

Physical Examination Findings

• Vital signs: tachycardia (compensatory response to reduced cardiac output), hypertension (catecholamine surge) or hypotension (extensive infarction, cardiogenic shock)
• Heart sounds: diminished S1 (reduced contractility), S4 gallop (stiff ventricle from acute ischemia), S3 gallop (if acute heart failure)
• New murmur: holosystolic murmur at apex (acute mitral regurgitation from papillary muscle dysfunction), systolic murmur at left lower sternal border (acute ventricular septal defect)
• Pericardial friction rub (if pericarditis complicates infarction, usually day 2-3)
• Hypoxia and pulmonary edema signs: rales, elevated JVP, peripheral edema (indicates left or biventricular infarction)

• Anxious, diaphoretic appearance
• Cool, clammy extremities (sympathetic activation)
• Altered mental status (if cardiogenic shock or hypoxemia)

12-Lead Electrocardiogram (ECG) - First and Most Critical Test

• ST-segment elevation ≥1 mm (0.1 mV) in ≥2 anatomically contiguous leads (or ≥2 mm in V2-V3 in men >40 years; ≥1.5 mm in women or ≥2.5 mm in men <40 years for V2-V3 to increase specificity)
• PR-segment depression (especially in leads with ST elevation, indicates atrial injury or reciprocal change)
• T-wave changes: hyperacute peaked T waves (earliest sign, minutes after onset), then T-wave inversion in evolution phase

• Anterior wall STEMI (LAD occlusion): ST elevation in V1-V4 ± I, aVL; reciprocal ST depression in II, III, aVF
• Inferior wall STEMI (RCA or LCx occlusion): ST elevation in II, III, aVF; reciprocal ST depression in I, aVL
• Right ventricular (RV) STEMI (RCA): ST elevation in inferior leads + ST elevation in V4R (right-sided lead)
• Posterior wall STEMI (LCx or RCA): mirror image ST depression in V1-V3 with tall R waves (obtain posterior leads V7-V9 for confirmation)
• Left main or three-vessel disease: diffuse ST elevation with global ischemia pattern

• Left bundle branch block (LBBB): obscures ST segment analysis; use Sgarbossa criteria (see HIGH-YIELD PEARLS) to identify STEMI
• Early repolarization (normal variant in young athletes): concave ST elevation, preserved PR segment, prominent J waves
• Left ventricular hypertrophy with strain: ST-segment depression in lateral leads
• Pericarditis: diffuse ST elevation (except aVR, V1 which show ST depression), PR depression

Serial ECGs

• Obtain repeat ECGs every 10 minutes if initial ECG is non-diagnostic but suspicion remains high; ST elevation may evolve
• Dynamic ST changes (increasing elevation, new T-wave inversion) support acute coronary event

Cardiac Biomarkers

• Preferred biomarker for myocardial necrosis; specificity >99%
• Kinetics: begins rising 2-4 hours after symptom onset (conventional troponin), <1 hour with hs-cTn assays
• Interpretation: single elevated value confirms myocardial necrosis but doesn't distinguish ACS from other causes; serial troponins (baseline, 3 hours, 6 hours) establish acute rise/fall pattern confirming acute MI
• High-sensitivity troponin advantages: negative predictive value >99% at presentation, enabling earlier rule-out of ACS in low-risk patients using 0-1 hour or 0-3 hour protocols

• CK-MB: more cardiac-specific than total CK; rises 3-12 hours, peaks at 24-48 hours; less sensitive than troponin
• Myoglobin: rises earliest (1-4 hours) but non-cardiac; useful for negative value to exclude MI
• BNP/NT-proBNP: elevated in acute decompensated heart failure, prognostic for heart failure complications

Coronary Angiography

• Complete coronary artery occlusion (TIMI flow 0-1) at site of plaque rupture
• Thrombus burden: angiographic thrombus appears as filling defect within arterial lumen
• TIMI myocardial perfusion grade: assesses tissue-level perfusion independent of epicardial flow
• Grade 0: no myocardial blush
• Grade 3: normal myocardial blush (best outcome)
• Collateral circulation: pre-existing collateral vessels may limit infarct size
• Coronary anatomy variations: dominance, prior disease

• Emergent angiography is the diagnostic and therapeutic intervention in STEMI (primary PCI)
• Door-to-balloon time goal: <90 minutes (or <120 minutes if transfer from non-PCI hospital)

Cardiac Imaging

• Purpose: assess left ventricular function, regional wall motion abnormality, mechanical complications
• Findings: hypokinesis, akinesis, or dyskinesis in territory of occluded coronary artery; reduced ejection fraction
• Mechanical complications: acute mitral regurgitation (papillary muscle rupture), ventricular septal defect (septal rupture), free wall rupture/pseudoaneurysm, left ventricular thrombus
• Timing: obtain early in hospitalization; serial echo if complications suspected

• Superior myocardial tissue characterization: late gadolinium enhancement (LGE) shows infarct distribution and size
• Useful for: detecting myocarditis/takotsubo mimics, assessing viability, detecting complications (VSD, pseudoaneurysm)
• Timing: typically delayed after acute phase due to hemodynamic instability

• Role: limited in acute STEMI (angiography is standard); useful in NSTEMI if angiography deferred
• Advantages: evaluates coronary anatomy, aortic dissection, alternative diagnoses (PE, aortic dissection)

Diagnostic Criteria for STEMI

1. 1.Clinical symptoms consistent with myocardial ischemia AND
2. 2.Rise and/or fall of cardiac biomarker (preferably hs-cTn) with ≥1 measurement >99th percentile AND
3. 3.Evidence of acute myocardial injury demonstrated by:

• ST-segment elevation ≥1 mm in ≥2 contiguous leads OR
• New LBBB with ischemic symptoms (not a stand-alone criterion; apply Sgarbossa criteria, see ECG Pitfalls) OR
• Angiographic findings of acute thrombotic coronary occlusion OR
• Cardiac imaging evidence of new myocardial wall motion abnormality consistent with ischemic etiology

Differential Diagnosis to Exclude

• NSTEMI: elevated troponin WITHOUT ST elevation
• Unstable angina: chest pain/ischemic symptoms WITHOUT elevated troponin or ST elevation
• Myocarditis: diffuse ST elevation (sparing aVR, V1), PR depression, normal coronary angiography, elevated troponin
• Takotsubo cardiomyopathy: apical ballooning, ST elevation, elevated troponin, normal coronaries, recent emotional/physical stress
• Pulmonary embolism: ST elevation in V1-V3 (right heart strain pattern), sinus tachycardia, hypoxia
• Aortic dissection: ST elevation if dissection extends into RCA, severe hypertension, pulse differential, widened mediastinum on imaging
• Pericarditis: diffuse ST elevation, PR depression, positional chest pain, pericardial friction rub

Immediate Management (Prehospital and Emergency Department)

• Aspirin 325 mg PO (non-enteric coated, for rapid buccal absorption) or 500 mg IV if unable to take PO; inhibits platelet cyclooxygenase to prevent thromboxane A2 production
• Oxygen supplementation if SpO2 <90% (avoid hyperoxia; target SpO2 94-98%)
• Establish IV access, continuous cardiac monitoring, supplemental oxygen
• 12-lead ECG within 10 minutes of symptom onset
• Obtain prehospital notification to receiving facility for STEMI alert activation (enables cardiac catheterization lab preparation)
• Sublingual nitroglycerin 0.3-0.6 mg if systolic BP >90 mmHg and RV infarction excluded (contraindicated in RV STEMI due to afterload dependence)

Reperfusion Therapy - Time-Dependent Decision

Primary Percutaneous Coronary Intervention (PCI) - Preferred Reperfusion Strategy

• Indication: STEMI presenting within 12 hours of symptom onset
• Technique: emergent coronary angiography with mechanical revascularization via balloon angioplasty and stent deployment
• Advantages: superior to fibrinolysis in reducing mortality, reinfarction, and stroke; enables culprit vessel identification and treatment of additional lesions
• Timing goals:

Arrhythmic (peak risk in the first 48 hours)

• Ventricular fibrillation / pulseless VT: ischemic re-entry around the border zone of necrosis; emergency — immediate defibrillation per AHA ACLS, not antiarrhythmic loading first. Primary VF in the first 48 hours does not by itself indicate an ICD.
• AV block/bradycardia: inferior (RCA) infarcts ischemize the AV node and trigger the Bezold–Jarisch reflex — usually narrow-complex, atropine-responsive, transient. Anterior MI with Mobitz II or complete block means extensive septal necrosis of the His–Purkinje system: wide complex, atropine-refractory, poor prognosis, pacing.
• Accelerated idioventricular rhythm: a benign reperfusion marker, not a reason to abort therapy.

Mechanical (all emergencies, classically days 3–5 after a non-reperfused MI)

• Papillary muscle rupture: the posteromedial papillary muscle has a single blood supply (posterior descending artery), so inferior MI is the culprit — new holosystolic apical murmur, flash pulmonary edema, shock; emergent surgical repair.
• Interventricular septal rupture: harsh murmur with thrill at the lower sternal border, left-to-right shunt with an oxygen step-up on right heart catheterization; emergent surgery.
• Free wall rupture: macrophage collagenolysis leaves the wall weakest late in the first week — sudden tamponade, PEA arrest, or contained rupture as a pseudoaneurysm (no myocardium in the wall).
• Cardiogenic shock / RV infarction: pump failure or preload-dependent RV failure with elevated JVP and clear lungs.

Late structural

• True aneurysm: thinned fibrotic scar with persistent ST elevation weeks later, mural thrombus, and embolic stroke risk.
• Post-infarction remodeling and HFrEF: per ACC/AHA heart failure guidance, EF ≤40% warrants all four classes — ARNI (or ACEI/ARB), beta blocker, MRA, and SGLT2 inhibitor.
• Pericarditis: early peri-infarction (day 2–4) versus autoimmune Dressler syndrome weeks later; aspirin/colchicine preferred, since other NSAIDs and glucocorticoids impair scar formation.

Treatment-related

• Stent thrombosis: usually premature DAPT interruption; presents as recurrent STEMI — emergency angiography.
• Fibrinolytic bleeding: intracranial hemorrhage is the feared event; any new neurologic deficit stops the infusion and triggers emergent head CT.
• Post-catheterization: contrast-associated AKI, access-site or retroperitoneal hematoma, cholesterol embolization.

• ECG within 10 minutes is the single best next step in any adult with suspected ischemic chest pain — before troponin, before imaging. Once STEMI criteria are met, do not wait for biomarkers to activate reperfusion (ACC/AHA).
• Inferior ST elevation + hypotension + clear lungs + elevated JVP = RV infarction. The RV is preload-dependent, so the answer is IV fluid bolus; nitrates, morphine, and diuretics are the classic distractors that precipitate profound hypotension.
• New murmur + abrupt hemodynamic collapse days after MI: apical holosystolic murmur with pulmonary edema → papillary muscle rupture (inferior MI, posteromedial muscle's single blood supply); lower sternal murmur with thrill and an oxygen step-up in the RV → septal rupture. Both are surgical emergencies; echocardiography is the confirming test.
• Sgarbossa criteria salvage the ECG in LBBB or paced rhythm: any concordant ST elevation, concordant ST depression in V1–V3, or excessively discordant ST elevation relative to the S wave suggests occlusion MI.
• Troponin is specific for myocardial injury, not for ACS. Chronic kidney disease, myocarditis, sepsis, pulmonary embolism, and takotsubo all elevate it; the rise-and-fall pattern plus clinical context makes the diagnosis (Fourth Universal Definition of MI).
• Do not give nitrates within 24–48 hours of a PDE5 inhibitor (sildenafil/tadalafil) — the association examiners love, because of catastrophic vasodilatory hypotension. Similarly, hold beta blockers in acute heart failure, hypotension, bradycardia, or cocaine-associated ischemia.
• Discharge regimen is a reliable question stem: dual antiplatelet therapy (aspirin plus a P2Y12 inhibitor), high-intensity statin regardless of baseline LDL (ACC/AHA cholesterol guideline), beta blocker, and ACEI/ARB — with an MRA and SGLT2 inhibitor added when EF is reduced. Cardiac rehabilitation and smoking cessation are guideline-recommended, not optional extras.
• ICD timing: primary-prevention ICD is considered only when EF remains ≤35% at least 40 days after MI (and ≥90 days after revascularization) on optimal therapy — early post-MI VF alone does not qualify.