Myocardial Infarction for USMLE Step 2
Myocardial Infarction for the USMLE Step 2 Exam
Myocardial infarction (MI), commonly known as a heart attack, results from a significant reduction in coronary blood flow, leading to myocardial ischemia and cell death. It is commonly caused by atherosclerotic plaque rupture with subsequent thrombus formation, partially or fully blocking a coronary artery. MI is classified into ST-segment elevation MI (STEMI) and non-ST-segment elevation MI (NSTEMI).
Pathophysiology
- Atherosclerosis: Plaque buildup in the coronary arteries reduces blood flow. Plaques consist of a lipid core and fibrous cap. Risk factors include hyperlipidemia, hypertension, smoking, and diabetes.
- Plaque Rupture and Thrombosis: When a plaque ruptures, it exposes the lipid core, activating platelets and the coagulation cascade. This results in thrombus formation, which may partially or fully occlude the coronary artery.
- Ischemia and Infarction: Prolonged ischemia leads to myocardial cell death. In STEMI, there is complete occlusion with transmural ischemia; NSTEMI involves partial occlusion, causing subendocardial ischemia.
Risk Factors
- Modifiable: Smoking, hypertension, hyperlipidemia, diabetes, obesity, and sedentary lifestyle.
- Non-Modifiable: Age, race, sex, family history of early CAD. Risk is higher in African Americans than in White Americans. Women have higher mortality than men.
Clinical Presentation
- Chest Pain: Described as a crushing, squeezing pain in the chest, often radiating to the left arm, neck, jaw, or back. It typically lasts more than 20 minutes and does not resolve with rest.
- Associated Symptoms: Diaphoresis, nausea, vomiting, dyspnea, and a sense of impending doom.
- Atypical Symptoms: Common in older adults, women, and diabetics, and may include fatigue, abdominal pain, or syncope.
Types of Myocardial Infarction
ST-Segment Elevation Myocardial Infarction (STEMI)
- Definition: Full-thickness myocardial infarction associated with ST-segment elevation on ECG.
- Diagnosis:
- ECG: ST-segment elevation in two or more contiguous leads, suggesting transmural ischemia.
- Biomarkers: Elevated troponins confirm myocardial injury.
- Management:
- Reperfusion Therapy:
- Primary PCI: Preferred within 90 minutes of first medical contact.
- Fibrinolysis: Given within 30 minutes if PCI is unavailable within 120 minutes.
- Medications: Aspirin, P2Y12 inhibitors (e.g., clopidogrel), heparin, beta-blockers, ACE inhibitors, and high-intensity statins.
Non-ST Segment Elevation Myocardial Infarction (NSTEMI)
- Definition: Subendocardial infarction without ST-segment elevation, diagnosed by elevated troponins and ischemic symptoms.
- Diagnosis:
- ECG: ST depression, T-wave inversion, or nonspecific changes.
- Biomarkers: Elevated troponins indicate myocardial necrosis.
- Management:
- Medications: Dual antiplatelet therapy (aspirin and P2Y12 inhibitor), anticoagulation, beta-blockers, ACE inhibitors, and high-intensity statins.
- Risk Stratification: TIMI or GRACE scores to assess the need for early invasive management, such as PCI.
Complications of Myocardial Infarction
- Arrhythmias: Ventricular fibrillation, tachycardia, atrial fibrillation, and sinus bradycardia are common, especially in the first 24 hours.
- Heart Failure: Left ventricular dysfunction due to infarcted tissue may lead to heart failure.
- Cardiogenic Shock: Severe left ventricular impairment causes hypotension and poor perfusion.
- Mechanical Complications: Papillary muscle rupture (causing mitral regurgitation), ventricular septal rupture, and free wall rupture.
- Pericarditis: Inflammation of the pericardium, often occurring within days post-MI.
Key Points
- Pathophysiology: MI is primarily due to atherosclerotic plaque rupture and thrombus formation, resulting in coronary artery occlusion.
- Types of MI:
- STEMI: Complete occlusion with ST-segment elevation; requires immediate reperfusion (PCI or fibrinolysis).
- NSTEMI: Partial occlusion; treated with antithrombotic therapy and risk-based approach for PCI.
- Diagnosis:
- ECG: ST elevation in STEMI; ST depression or T-wave inversion in NSTEMI.
- Biomarkers: Elevated troponins confirm myocardial injury in both STEMI and NSTEMI.
- Management:
- STEMI: Immediate reperfusion therapy with PCI or fibrinolysis.
- NSTEMI: Dual antiplatelet therapy, anticoagulation, and beta-blockers; PCI in high-risk cases.
- Complications:
- Arrhythmias: Ventricular arrhythmias are common early complications.
- Heart Failure and Cardiogenic Shock: Result from significant myocardial damage.
- Mechanical Complications: Papillary muscle rupture, ventricular septal rupture, and pericarditis are serious post-MI issues.