Myocardial Infarction for USMLE Step 1
Myocardial Infarction for the USMLE Step 1 Exam
Overview of Myocardial Infarction
Myocardial infarction (MI), commonly known as a heart attack, occurs when there is insufficient blood flow to the myocardium, causing cell death due to prolonged ischemia. MI is usually caused by coronary artery occlusion from atherosclerotic plaque rupture and thrombus formation. It is classified into ST-segment elevation MI (STEMI) and non-ST-segment elevation MI (NSTEMI) based on ECG findings.
Pathophysiology
- Atherosclerosis: Plaque buildup in coronary arteries narrows the vessel lumen, reducing blood flow. Plaques consist of a lipid core with a fibrous cap.
- Plaque Rupture and Thrombosis: When a plaque ruptures, the lipid core is exposed, activating platelets and the coagulation cascade, which results in thrombus formation. This can partially or completely occlude a coronary artery, reducing oxygen supply to the myocardium.
- Ischemia and Infarction: Prolonged ischemia leads to myocardial necrosis. In STEMI, there is complete coronary occlusion and transmural infarction, while NSTEMI involves partial occlusion, leading to subendocardial infarction.
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: Commonly described as a crushing or squeezing pain in the chest, often radiating to the left arm, neck, jaw, or back. Pain usually lasts over 20 minutes and may not resolve with rest.
- Associated Symptoms: Diaphoresis, nausea, vomiting, shortness of breath, and a sense of impending doom.
- Atypical Presentations: More common in older adults, women, and diabetics; symptoms may include fatigue, abdominal pain, or syncope.
Types of Myocardial Infarction
ST-Segment Elevation Myocardial Infarction (STEMI)
- Definition: Full-thickness infarction associated with ST-segment elevation on ECG.
- Diagnosis:
- ECG: ST elevation in two or more contiguous leads, indicating transmural ischemia.
- Biomarkers: Elevated troponins confirm myocardial injury.
- Management:
- Reperfusion Therapy:
- Primary PCI: Preferred within 90 minutes of first medical contact.
- Fibrinolysis: Used if PCI is unavailable within 120 minutes, ideally within 30 minutes of STEMI diagnosis.
- Medications: Aspirin, P2Y12 inhibitors (e.g., clopidogrel), heparin, beta-blockers, ACE inhibitors, and statins.
Non-ST Segment Elevation Myocardial Infarction (NSTEMI)
- Definition: Subendocardial infarction with ischemic symptoms but without ST elevation.
- Diagnosis:
- ECG: ST depression, T-wave inversion, or nonspecific changes.
- Biomarkers: Elevated troponins indicate myocardial injury.
- Management:
- Medications: Dual antiplatelet therapy (aspirin and P2Y12 inhibitor), anticoagulation, beta-blockers, ACE inhibitors, and statins.
- Risk Assessment: TIMI or GRACE scores to guide invasive management, such as PCI.
Complications of Myocardial Infarction
- Arrhythmias: Ventricular fibrillation, tachycardia, atrial fibrillation, and bradycardia are common, especially in the first 24 hours.
- Heart Failure: Left ventricular dysfunction from infarcted tissue can lead to heart failure.
- Cardiogenic Shock: Severe left ventricular dysfunction can cause low cardiac output and hypotension.
- Mechanical Complications: Papillary muscle rupture (leading to mitral regurgitation), ventricular septal rupture, and free wall rupture.
- Pericarditis: Inflammation of the pericardium, presenting as chest pain, often occurring within days post-MI.
Key Points
- Pathophysiology: MI is primarily due to atherosclerotic plaque rupture and thrombus formation, causing coronary artery occlusion.
- Types of MI:
- STEMI: Complete occlusion with ST-segment elevation on ECG; 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 Issues: Papillary muscle rupture, ventricular septal rupture, and pericarditis are serious post-MI complications.