Non-ST Elevation Acute Coronary Syndrome (NSTE-ACS)

Myocardial Infarction (MI)

Myocardial infarction is classified into five types based on the underlying etiology and clinical circumstances:

• Type 1 MI: Spontaneous myocardial infarction resulting from ischemia due to a primary coronary event, such as plaque rupture, erosion, fissuring, or coronary artery dissection.

• Type 2 MI: Myocardial ischemia secondary to an imbalance between oxygen supply and demand. This may occur in the setting of increased demand (e.g., severe hypertension) or decreased supply (e.g., coronary artery spasm, embolism, arrhythmias, or hypotension).

• Type 3 MI: Sudden unexpected cardiac death, including cardiac arrest, often with symptoms suggestive of myocardial ischemia and presumed new ECG changes or ventricular fibrillation before biomarkers can be obtained.

• Type 4a MI: Myocardial infarction associated with percutaneous coronary intervention (PCI), defined by an elevation in cardiac troponin (cTn) values >5 times the 99th percentile upper reference limit (URL), along with evidence of ischemia.

• Type 4b MI: Myocardial infarction associated with documented stent thrombosis.

• Type 5 MI: Myocardial infarction occurring in the context of coronary artery bypass grafting (CABG), with cTn elevation >10 times the 99th percentile URL and supporting clinical or imaging findings.

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Infarct Location and Clinical Implications

• Right Ventricular (RV) Infarction: Most commonly results from obstruction of the right coronary artery or a dominant left circumflex artery. It is characterized by elevated right ventricular filling pressures, which may be accompanied by severe tricuspid regurgitation and reduced cardiac output. RV infarction can significantly impair hemodynamics.

• Inferoposterior Infarction: Often leads to some degree of RV dysfunction in approximately 50% of cases and hemodynamic compromise in about 10–15%. In patients with inferoposterior infarction, elevated jugular venous pressure in conjunction with hypotension or shock should prompt consideration of RV involvement. RV infarction in the setting of left ventricular infarction markedly increases mortality risk.

• Anterior Infarction: These infarcts are typically larger and are associated with worse outcomes compared to inferoposterior infarctions. They usually result from obstruction of the left coronary artery, particularly the left anterior descending artery. In contrast, inferoposterior infarctions are commonly due to right coronary artery or dominant left circumflex artery occlusion.