• Acute myocardial infarction (MI): damage to the myocardium defined by an elevated cardiac troponin above the 99^th percentile upper reference limit with a dynamic rise and/or fall over time AND clinical evidence of myocardial ischemia (oxygen supply unable to meet the metabolic demands of the body) based on at least one of the following:
  • Anginal symptoms.
  • New ischemic ECG changes or the development of pathological Q waves.
  • Imaging evidence of new regional wall motion abnormality or loss of viable myocardium in an ischemic distribution.
  • Angiographic evidence of coronary thrombus.
• Chronic myocardial injury: damage to myocardium defined by an elevated cardiac troponin above the 99^th percentile upper reference limit with persistently elevated values without a dynamic rise or fall of troponin levels.
• It is important to note that myocardial infarction is a specific diagnosis but does not describe a specific etiology.

After an acute MI has been diagnosed, the next step is to determine the etiology/type of MI, which can guide appropriate next steps in management.

It is important to note that the level of troponin elevation does NOT distinguish between types of MI.

We define acute coronary syndrome (ACS) or type 1 MI as an MI due to atherothrombotic coronary artery occlusion. This definition excludes unstable angina from the definition of ACS (see “Unstable Angina” below).

• ST-elevation acute coronary syndrome (STE-ACS) or ST-elevation myocardial infarction (STEMI): type 1 MI due to complete coronary artery occlusion from an atherothrombotic lesion leading to new, persistent ST-segment elevations that meet electrocardiographic criteria (see section Diagnosis of Suspected ACS).
• Non-ST-elevation acute coronary syndrome (NSTE-ACS) or Non-ST-elevation myocardial infarction (NSTEMI): type 1 MI due to partial atherothrombotic coronary artery occlusion with specific ECG changes (see section Diagnosis of Suspected ACS).
• STE-ACS (STEMI) and NSTE-ACS (NSTEMI) are electrocardiographic descriptors of the severity of coronary artery occlusion due to atherothrombotic disease. Therefore, the terminology of STE-ACS (STEMI) and NSTE-ACS (NSTEMI) should only be used in the context of ACS (type 1 MI).

• Unstable angina (UA): rest, new, or worsening angina in the absence of evidence of myocardial ischemia as detected by troponin.
• Traditionally, unstable angina has been included as a subtype of NSTE-ACS, where there is myocardial ischemia due to coronary artery thrombosis that is not severe enough to cause evidence of myocardial ischemia resulting in elevated serum troponin. However, with the high sensitivity of modern troponin assays, significant myocardial ischemia in the absence of elevated cardiac troponin is extremely rare.
• Therefore, while unstable angina could be a symptom of progressive high-grade, obstructive atherosclerotic coronary artery disease and should receive appropriate diagnosis and treatment, it is not an ACS and should not be managed as such.

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