The non-ST elevation acute coronary syndromes (ACS) — unstable angina (UA) and non-ST elevation (non-Q wave) myocardial infarction (NSTEMI) — occur in a variety of clinical settings. The definitions of and classification systems for UA and NSTEMI and their use to assess risk will be reviewed here. Issues related to the evaluation, treatment, and risk stratification of patients with non-ST elevation ACS are discussed separately. (See "Overview of the acute management of unstable angina and non-ST elevation myocardial infarction" and "Risk stratification after non-ST elevation acute coronary syndrome".)
Both UA and NSTEMI, as well as ST elevation MI, are associated with unstable coronary lesions and plaque rupture. (See "The role of the vulnerable plaque in acute coronary syndromes".)
Unstable angina (UA) and non-ST elevation (non-Q wave) myocardial infarction (NSTEMI) differ primarily in whether the ischemia is severe enough to cause sufficient myocardial damage to release detectable quantities of a marker of myocardial injury. Unstable angina is considered to be present in patients with ischemic symptoms suggestive of an ACS and no elevation in troponin, with or without ECG changes indicative of ischemia (eg, ST segment depression or transient elevation or new T wave inversion). Since an elevation in troponin may not be detectable for up to 12 hours after presentation, UA and NSTEMI are frequently indistinguishable at initial evaluation. ST segment and/or T wave changes are often persistent in NSTEMI while, if they occur in UA, they are usually transient. (See "Troponins and creatine kinase as biomarkers of cardiac injury".)
It should be kept in mind that the sensitivity of the biomarker test (usually troponin) used to evaluate patients with ACS determines whether a patient is labeled as having UA or NSTEMI. With the development of increasingly more sensitive troponins, it can be anticipated that the percent of patient with UA will decrease and the percent with NSTEMI will increase .
The 2012 joint task force of the European Society of Cardiology (ESC), American College of Cardiology Foundation (ACCF), the American Heart Association (AHA), and the World Health Federation (WHF) defined MI (in pathologic terms) as myocardial cell death due to prolonged ischemia . (See "Criteria for the diagnosis of acute myocardial infarction", section on 'Third Universal Definition of MI'.)