Myocardial Infarction Pathophysiology, Clinical Presentation, Diagnosis, and Management

Introduction

Myocardial infarction (MI), commonly known as a heart attack, is a leading cause of morbidity and mortality worldwide. It occurs when blood flow to a part of the heart muscle (myocardium) is abruptly reduced or blocked, causing tissue ischemia and necrosis. Early recognition and timely treatment are critical to improve patient outcomes. This article provides an overview of the pathophysiology, clinical features, diagnostic approaches, and current management strategies of myocardial infarction.

Pathophysiology

MI usually results from atherosclerosis, a chronic inflammatory disease of the arterial walls characterized by plaque formation. Atherosclerotic plaques can rupture or erode, exposing thrombogenic material to circulating blood, leading to thrombus formation and subsequent occlusion of the coronary artery. The resulting ischemia causes myocardial cell injury and death.

There are two main types of MI based on the mechanism:

• ST-segment elevation myocardial infarction (STEMI): Complete occlusion of a coronary artery causing transmural infarction.
• Non-ST-segment elevation myocardial infarction (NSTEMI): Partial occlusion leading to subendocardial infarction.

Risk factors contributing to atherosclerosis and MI include hypertension, hyperlipidemia, diabetes mellitus, smoking, obesity, sedentary lifestyle, and family history.

Clinical Presentation

The classic symptom of MI is chest pain described as crushing or pressure-like, often radiating to the left arm, neck, jaw, or back. Patients may also experience shortness of breath, diaphoresis (sweating), nausea, vomiting, and lightheadedness. However, presentations can vary, especially among women, elderly patients, and diabetics, who may have atypical symptoms like fatigue, indigestion, or silent ischemia.

Physical examination findings may include:

• Tachycardia or bradycardia
• Hypotension or hypertension
• Signs of heart failure (jugular venous distension, pulmonary rales)
• New murmurs due to mechanical complications such as mitral regurgitation

Diagnosis

Diagnosis of MI is based on a combination of clinical history, electrocardiogram (ECG) changes, and cardiac biomarkers.

• Electrocardiogram (ECG): STEMI is characterized by persistent ST-segment elevation in two or more contiguous leads. NSTEMI may show ST depression or T-wave inversion. ECG also helps identify arrhythmias or conduction abnormalities.
• Cardiac biomarkers: Troponins I and T are highly sensitive and specific markers for myocardial injury and rise within hours after infarction. Creatine kinase-MB (CK-MB) can also be used but is less specific.
• Imaging: Echocardiography evaluates wall motion abnormalities and complications such as ventricular aneurysm or pericardial effusion.
• Coronary angiography: This invasive procedure identifies the location and severity of coronary artery occlusion and guides interventional treatment.

Management

Management of MI involves rapid restoration of coronary blood flow, prevention of complications, and secondary prevention.

Acute Management

1. Reperfusion therapy:
   • Primary percutaneous coronary intervention (PCI): The preferred method for STEMI if performed within 90-120 minutes of first medical contact.
   • Thrombolytic therapy: Used when PCI is not available timely; agents include alteplase, reteplase, and tenecteplase.
2. Medical treatment:
   • Antiplatelet therapy: Aspirin and P2Y12 inhibitors (clopidogrel, ticagrelor) reduce thrombus formation.
   • Anticoagulation: Heparin or low-molecular-weight heparin prevents further clot propagation.
   • Nitrates: Relieve ischemic chest pain and reduce preload.
   • Beta-blockers: Decrease myocardial oxygen demand and reduce arrhythmias.
   • ACE inhibitors/ARBs: Improve remodeling and reduce mortality, especially in patients with left ventricular dysfunction.
   • Statins: Lower cholesterol and stabilize plaques.
3. Supportive care: Oxygen for hypoxia, analgesics for pain relief, and management of arrhythmias or heart failure.
   

Secondary Prevention

• Lifestyle modification: Smoking cessation, diet, exercise, weight control.
• Control of risk factors: Hypertension, diabetes, and hyperlipidemia.
• Cardiac rehabilitation programs.

Complications

Potential complications following MI include:

• Arrhythmias (ventricular tachycardia, fibrillation)
• Heart failure
• Cardiogenic shock
• Mechanical complications (ventricular septal rupture, papillary muscle rupture)
• Pericarditis
• Recurrent ischemia

Early recognition and management of these complications are vital.

Prognosis

The prognosis after MI depends on the size and location of the infarct, time to reperfusion, comorbidities, and development of complications. Advances in acute care and secondary prevention have significantly reduced mortality rates over recent decades.

Conclusion

Myocardial infarction remains a major global health issue. Prompt diagnosis and timely reperfusion are the cornerstones of effective treatment. Multidisciplinary care involving lifestyle modification, pharmacotherapy, and cardiac rehabilitation can improve long-term outcomes. Continued research and public health measures are necessary to reduce the burden of MI worldwide.

References

1. Thygesen K, Alpert JS, Jaffe AS, et al. Fourth Universal Definition of Myocardial Infarction (2018). Journal of the American College of Cardiology. 2018;72(18):2231-2264. doi:10.1016/j.jacc.2018.08.1038
2. Ibanez B, James S, Agewall S, et al. 2017 ESC Guidelines for the management of acute myocardial infarction in patients presenting with ST-segment elevation. European Heart Journal. 2018;39(2):119-177. doi:10.1093/eurheartj/ehx393
3. Amsterdam EA, Wenger NK, Brindis RG, et al. 2014 AHA/ACC Guideline for the Management of Patients With Non–ST-Elevation Acute Coronary Syndromes. Circulation. 2014;130(25):e344-e426. doi:10.1161/CIR.0000000000000134
4. Braunwald E. Heart Disease: A Textbook of Cardiovascular Medicine. 11th Edition. Elsevier; 2018.
5. O’Gara PT, Kushner FG, Ascheim DD, et al. 2013 ACCF/AHA Guideline for the Management of ST-Elevation Myocardial Infarction. Circulation. 2013;127(4):e362-e425. doi:10.1161/CIR.0b013e3182742c84
6. Antman EM, Anbe DT, Armstrong PW, et al. ACC/AHA Guidelines for the Management of Patients With ST-Elevation Myocardial Infarction. Journal of the American College of Cardiology. 2004;44(3):671-719.
7. Libby P, Ridker PM, Hansson GK. Inflammation in Atherosclerosis: From Pathophysiology to Practice. Journal of the American College of Cardiology. 2009;54(23):2129-2138. doi:10.1016/j.jacc.2009.09.009