USMLE Guide: Cardiology Of Myocardial Infarction

Introduction

This guide aims to provide a comprehensive overview of the cardiology of myocardial infarction (MI) for the United States Medical Licensing Examination (USMLE). MI, commonly known as a heart attack, is a critical condition that requires prompt diagnosis and management. This article will cover the epidemiology, etiology, pathophysiology, clinical presentation, diagnostic modalities, treatment, and complications of MI.

Epidemiology

• MI is one of the leading causes of death worldwide, particularly in developed countries.
• The incidence of MI increases with age, and men are more commonly affected than women until menopause when the risk becomes equal.
• Several risk factors contribute to the development of MI, including hypertension, smoking, diabetes, obesity, family history, and high cholesterol levels.

Etiology

• Most MIs are caused by the rupture of an atherosclerotic plaque in a coronary artery, leading to the formation of a thrombus and subsequent myocardial ischemia.
• Other less common causes include coronary artery vasospasm, coronary artery embolism, coronary artery dissection, and coronary artery spasm in the setting of cocaine use.

Pathophysiology

1. Plaque Rupture:
   • Atherosclerotic plaques, composed of lipids, cholesterol, and inflammatory cells, develop in the coronary arteries.
   • Plaque rupture exposes the underlying thrombogenic material, leading to platelet activation and subsequent thrombus formation.
2. Coronary Artery Occlusion:
   • The formed thrombus obstructs the coronary artery, causing partial or complete occlusion.
   • This leads to an imbalance between myocardial oxygen supply and demand, resulting in ischemia and subsequent necrosis.
3. Myocardial Injury:
   • The affected myocardial tissue undergoes irreversible injury and necrosis due to the lack of oxygen and nutrients.
   • The extent and location of the myocardial damage determine the clinical presentation and severity of the MI.

Clinical Presentation

• The classic presentation of MI includes severe, crushing chest pain that may radiate to the left arm, neck, or jaw. The pain is typically prolonged and not relieved by rest or nitroglycerin.
• Other common symptoms include shortness of breath, diaphoresis, nausea, vomiting, and lightheadedness.
• Atypical presentations can occur, especially in older adults, diabetics, and women, and may include fatigue, weakness, epigastric pain, or even no chest pain at all.

Diagnostic Modalities

1. Electrocardiogram (ECG):
   • ST-segment elevation or new-onset left bundle branch block (LBBB) is indicative of an acute MI.
   • Non-ST-segment elevation MI (NSTEMI) may show ST-segment depression, T-wave inversion, or no significant ECG changes.
2. Cardiac Biomarkers:
   • Troponin I or T are highly specific and sensitive markers of myocardial injury.
   • Serial measurements are necessary for diagnosis, as they may initially be negative and rise within a few hours.
3. Echocardiography:
   • Assesses regional wall motion abnormalities, left ventricular function, and identifies any structural abnormalities.
4. Coronary Angiography:
   • Invasive procedure used to visualize the coronary arteries and identify the site and severity of coronary artery disease.

Treatment

1. Reperfusion Therapy:
   • Percutaneous Coronary Intervention (PCI) is the preferred method if available within 12 hours of symptom onset.
   • Thrombolytic therapy is an alternative if PCI cannot be performed within the appropriate timeframe.
2. Pharmacotherapy:
   • Antiplatelet agents, such as aspirin and P2Y12 inhibitors, are administered to prevent further thrombus formation.
   • Beta-blockers, ACE inhibitors, and statins are commonly prescribed for secondary prevention.
3. Lifestyle Modifications:
   • Smoking cessation, regular exercise, healthy diet, weight management, and blood pressure control are crucial for long-term management.

Complications

• Complications of MI may include arrhythmias (ventricular fibrillation, heart block), heart failure, cardiogenic shock, mechanical complications (ventricular septal rupture, papillary muscle rupture), and pericarditis.
• Long-term complications may include recurrent MI, post-MI angina, and ventricular remodeling.

Remember to consult the appropriate guidelines and resources for more in-depth information and updates on the management and specific recommendations for myocardial infarction.

Note: This guide is for educational purposes and should not replace professional medical advice.