Myocardial infarction Study Guide

📖 Core Concepts Myocardial Infarction (MI) – acute death of heart muscle due to prolonged loss of blood flow (ischemia) → necrosis. STEMI vs. NSTEMI – STEMI shows persistent ST‑segment elevation on ECG (full‑thickness infarct); NSTEMI lacks ST elevation, has troponin rise with subendocardial injury. Acute Coronary Syndrome (ACS) – umbrella term for unstable angina, NSTEMI, and STEMI; all involve sudden reduction in coronary perfusion. Biomarker cornerstone – high‑sensitivity cardiac troponin (hs‑cTn) is the most sensitive/specific test; rises 2–3 h after injury, peaks 1–2 days. Re‑vascularization goal – restore coronary flow quickly to limit infarct size and prevent death/complications. Primary PCI – preferred reperfusion for STEMI if performed ≤120 min from first medical contact; otherwise fibrinolysis. Secondary prevention – lifelong antiplatelet therapy, β‑blocker, ACE‑I/ARB, high‑intensity statin, lifestyle change. --- 📌 Must Remember Aspirin dose: 162–325 mg chewed immediately → ↓ mortality ≥50 %. ST‑elevation criteria: ≥1 mm (≥2 mm in V2‑V3 men <40 y) in contiguous leads. Troponin rise: ≥20 % change within 3 h suggests acute MI. PCI time window: ≤90–120 min from first contact → best outcomes. Fibrinolysis timing: ≤30 min from hospital arrival when PCI unavailable. Dual antiplatelet therapy (DAPT): aspirin + P2Y12 inhibitor (clopidogrel, prasugrel, ticagrelor) for ≥12 mo post‑MI; extended >12 mo in high‑risk patients. Key risk factors: age, male sex, family history, smoking (36 % of CAD), hypertension, diabetes, dyslipidemia, obesity, sedentary lifestyle. Complication timeline: 0–24 h: arrhythmias, cardiogenic shock. 3–7 d: free‑wall rupture, ventricular aneurysm. 1–3 wks: Dressler’s pericarditis. --- 🔄 Key Processes Plaque Rupture → Thrombosis Thin‑cap atherosclerotic plaque → rupture → exposure of subendothelial collagen & tissue factor → platelet aggregation → occlusive thrombus. Ischemic Cell Death 15–30 min: subendocardial necrosis (subendocardial MI). 3–4 h: wavefront expands → transmural infarct (STEMI). Re‑perfusion Injury Sudden blood flow → calcium overload + ROS → additional myocyte death despite vessel patency. Diagnostic Algorithm Step 1: ECG → assess ST changes. Step 2: hs‑cTn (initial & 3‑h repeat if needed). Step 3: If criteria met → initiate antiplatelet + anticoagulation + reperfusion strategy. Risk‑Stratification (HEART/GRACE/TIMI) Assign points → determine need for early invasive strategy vs. conservative management. --- 🔍 Key Comparisons STEMI vs. NSTEMI STEMI: persistent ST elevation, usually full‑thickness necrosis, requires emergent PCI. NSTEMI: no ST elevation, subendocardial necrosis, managed with anticoagulation ± early PCI. Aspirin vs. P2Y12 Inhibitor Aspirin: irreversible COX‑1 inhibition → ↓ thromboxane A₂. P2Y12 (clopidogrel, prasugrel, ticagrelor): block ADP receptor → stronger, more consistent platelet inhibition; prasugrel/ticagrelor preferred over clopidogrel. PCI vs. Fibrinolysis PCI: mechanical reperfusion, lower mortality, lower intracranial bleed. Fibrinolysis: pharmacologic clot dissolution; used only if PCI >120 min delay; higher bleeding risk. --- ⚠️ Common Misunderstandings “Oxygen for all MI patients” – Routine high‑flow O₂ offers no benefit unless SaO₂ < 90 % or severe dyspnea. “Nitroglycerin reduces mortality” – Improves symptoms only; no survival advantage. “Normal initial troponin rules out MI regardless of ECG” – A single normal hs‑cTn with a normal ECG effectively rules out MI; abnormal ECG still mandates repeat testing. “All MI patients need fibrinolysis” – Only when PCI unavailable within guideline time. --- 🧠 Mental Models / Intuition “Ischemia‑Infarction Timeline” – Think of the heart like a lawn: loss of water (blood) → wilting (ischemia) within minutes; irreversible brown (necrosis) after 30 min → scar (collagen) later. “STEMI = Open Door” – ST elevation signals the “door” is wide open for blood flow; you must shut it quickly (PCI). “Troponin Trail” – Troponin behaves like a delayed smoke alarm: it takes a few hours to sound, stays on for days, indicating the fire (injury) occurred earlier. --- 🚩 Exceptions & Edge Cases Silent MI – No symptoms; diagnosis via ECG changes, troponin rise, or autopsy. Coronary spasm‑induced MI (e.g., cocaine, Takotsubo) – May present with ST elevation but without atherosclerotic occlusion; treat with calcium channel blockers, nitrates. Contraindications to fibrinolysis – Active internal bleeding, recent intracranial hemorrhage, severe uncontrolled hypertension. Prasugrel contraindicated – Prior stroke/TIA, age ≥ 75, weight < 60 kg. --- 📍 When to Use Which Reperfusion PCI: STEMI ≤120 min from first contact, or high‑risk NSTEMI with hemodynamic instability. Fibrinolysis: STEMI when PCI >120 min; aim ≤30 min from arrival. Antiplatelet Loading Aspirin: all suspected MI (162–325 mg chewed). P2Y12: prasugrel/ticagrelor for STEMI & high‑risk NSTEMI; clopidogrel if contraindications to newer agents. Beta‑Blocker Start within 24 h unless signs of shock, bradycardia, or acute heart failure. ACE‑I/ARB Initiate within 24 h for all MI patients; continue indefinitely, especially with LV dysfunction. Statin High‑intensity statin (e.g., atorvastatin 80 mg) for every MI patient, regardless of baseline LDL. --- 👀 Patterns to Recognize Chest pain >20 min, not positional, radiation to left arm/jaw → classic MI. Atypical symptoms in women/elderly – dyspnea, fatigue, nausea → maintain high suspicion. ECG “reciprocal changes” (ST depression opposite the area of ST elevation) → supports STEMI localization. Rapid troponin rise + ST elevation → STEMI; rise without ST changes → NSTEMI. Post‑reperfusion ST‑segment resolution >50 % within 90 min → successful PCI or fibrinolysis. --- 🗂️ Exam Traps “All patients with MI need routine oxygen” – Wrong; only hypoxic patients. “Morphine improves survival” – Incorrect; provides analgesia but no mortality benefit and may delay oral antiplatelet absorption. “CK‑MB is the preferred biomarker” – Outdated; troponin is superior. “NSTEMI always gets PCI within 24 h” – Only high‑risk NSTEMI (GRACE/TIMI high); low‑risk may be managed conservatively. “A single normal troponin rules out MI regardless of timing” – Misleading; must consider timing of symptom onset; early presenters may need repeat testing. ---