Unstable angina is a type of acute coronary syndrome that is defined as one or more of the following in patients whose cardiac marker levels do not meet criteria for acute myocardial infarction (MI): Rest angina that is prolonged (usually > 20 minutes) New-onset angina of at least class 3 severity in the Canadian Cardiovascular Society (CCS) classification (see table) Increasing angina, ie, previously diagnosed angina that has become distinctly more frequent, more severe, longer in duration, or lower in threshold (eg, increased by ≥ 1 CCS class or to at least CCS class 3) Unstable angina is clinically unstable and often a prelude to myocardial infarction or arrhythmias or, less commonly, to sudden death.