Is Acute Coronary Syndrome (ACS) different from Myocardial Infarction (MI)?

Acute Coronary Syndrome vs. Myocardial Infarction: Understanding the Difference

Acute Coronary Syndrome (ACS) is a broader umbrella term that includes Myocardial Infarction (MI) as one of its subtypes, along with unstable angina. 1

Relationship Between ACS and MI

• ACS encompasses three related clinical conditions that exist along a continuum of severity: unstable angina, non-ST-segment elevation myocardial infarction (NSTEMI), and ST-segment elevation myocardial infarction (STEMI) 1
• MI specifically refers to myocardial necrosis (cell death) in a clinical setting consistent with acute myocardial ischemia, requiring elevated cardiac biomarkers (troponin) 1
• Unstable angina, which is part of ACS but not MI, is defined by myocardial ischemia without detectable myocardial necrosis (normal troponin levels) 1

Pathophysiology Differences

• ACS is typically caused by disruption (rupture or erosion) of an unstable coronary artery atherosclerotic plaque with associated partial or complete coronary thrombosis, resulting in diminished blood flow to the myocardium 1
• In MI, the ischemia is severe or prolonged enough to cause myocardial cell death, detectable by elevated cardiac biomarkers 1
• In unstable angina (a form of ACS), there is transient myocardial ischemia without significant myonecrosis 1

Diagnostic Classification

• ACS is initially classified based on ECG findings into ST-elevation ACS and non-ST-elevation ACS 1
• Further classification into unstable angina or MI depends on cardiac biomarkers (troponin) 1
• With increasingly sensitive troponin assays, biomarker-negative ACS (unstable angina) is becoming rarer 1

Clinical Presentation Spectrum

• ACS presents with a broad spectrum of symptoms, from cardiac arrest and cardiogenic shock to patients who are pain-free at presentation 1
• The hallmark symptom of ACS is acute chest discomfort described as pain, pressure, tightness, or burning 1
• Chest pain-equivalent symptoms may include dyspnea, epigastric pain, and pain in the left arm 1

Types of Myocardial Infarction

• Type 1 MI is characterized by atherosclerotic plaque rupture, ulceration, fissure, or erosion with resulting intraluminal thrombus 1
• Type 2 MI is myocardial necrosis caused by conditions other than coronary plaque instability (e.g., hypotension, severe anemia, coronary spasm) 1
• Under the Universal Definition of MI, ACS-related MI events would be classified as type 1 MI events 1

Treatment Implications

• STEMI requires immediate reperfusion therapy (primary PCI within 120 minutes or fibrinolytic therapy) 2
• High-risk NSTE-ACS patients benefit from an early invasive strategy with cardiac catheterization and prompt revascularization 2, 3
• Pharmacological management includes antiplatelet agents like clopidogrel for all ACS subtypes 4

Common Pitfalls and Caveats

• Not all chest pain represents ACS, and not all ACS presents with chest pain 2, 3
• With increasingly sensitive troponin assays, the distinction between unstable angina and NSTEMI has become more challenging 1
• The pathophysiology of ACS can be dynamic, with patients potentially progressing from one clinical condition to another during presentation and initial evaluation 1
• Non-coronary causes of myocardial ischemia (e.g., coronary spasm, embolism, dissection) can also present with similar symptoms but require different management approaches 1

Emerging Nomenclature

• Recent guidelines propose a new binary classification of "acute myocardial ischemic syndromes" (AMIS) and "non-acute myocardial ischemic syndromes" (NAMIS) to better encompass both obstructive and non-obstructive causes of angina and myocardial ischemia 1
• This new terminology aims to harmonize different pathophysiologic causes of myocardial ischemia and refine diagnostic and therapeutic approaches 1