Pathophysiology
1) discrete area of necrosis in cardiac muscle 2) can be transmural (usually owing to coronary artery occlusion) or subendocardial (owing to low flow states, hypotension, or shock) 3) right coronary occlusion – causes “inferior” infarcts in posterior basal left ventricle and posterior intraventricular septum 4) left anterior descending coronary occlusion – causes infarcts in apical, anterior, and anterior-septal left ventricle 5) left circumflex occlusion – causes infarcts in lateral wall of left ventricle
Signs and Symptoms
1) chest pain – usually sudden onset, substernal, “crushing” quality, may radiate to chin or down left arm
2) sweating 3) nausea and vomiting 4) feelings of impending doom 5) hypotension
Characteristic Test Findings
EKG – 1) Q waves 2) ST segment elevation 3) increased T waves 4) arrhythmias (e.g., ventricular fibrillation, heart block) Laboratory – 5) increased serum troponin 6) increased seum CK-MB (peaks at 20 hours) and LDH1 (peaks at 8 hours)
Histology/Gross Pathology
1) area of necrosis is pale and patchy at 24 hours, soft and gelatinous at 2 weeks, and firm and contracted at 4 weeks 2) fibrinous epicarditis (transmural infarcts) 3) greatly swollen mitochondria 4) disrupted sarcolemma
Associated Conditions
1) increased incidence of arrhythmias, cardiogenic shock, cardiac rupture (usually within first 4 days post-MI), left ventricular aneurysms, mural thrombus and embolism, pericarditis (Dressler’s syndrome 2) increased incidence with smoking, hypertension, diabetes, obesity, increased cholesterol, decreased folic acid
Treatment
1) symptomatic – nitrates, morphine IV, oxygen, anticoagulation (heparin, aspirin, Plavix) 2) to increase coronary blood flow (and limit area of infarct) – tissue plasminogen activator PTCA (with or without stent placement), coronary bypass grafting
Tips for USMLE
1) 25-50% of MIs are asymptomatic (especially in diabetics) 2) troponin (if not a fractionated value reported by the lab) is elevated in necrosis of smooth muscle; can also be increased in dead bowel 3) if question mentions q waves, think MI 4) if question mentions elevated ST segments, think MI 5) inferior infarcts show up in leads II, III, avF 6) left circumflex infarcts show up in V4-6 7) Dressler’s syndrome is post-MI pericarditis which occurs at 2-10 weeks post MI
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