Acute coronary syndrome (ACS) refers to a spectrum of acute myocardial ischaemia and/or infarction. Unstable angina and non-ST-elevation MI (NSTEMI) represent a continuum of pathology, differing mainly by the presence of markers of myocardial damage in NSTEMI.  Therefore some guidelines have grouped unstable angina and NSTEMI as 'non-ST-elevation acute coronary syndromes'. 
New ST-segment depression or T-wave inversion in the presence of ischaemic symptoms suggests UA or NSTEMI. If there is no elevation in CK-MB or troponin (cardiac biomarkers) the patient has UA; elevated cardiac biomarkers are consistent with NSTEMI. However, with the availability of increasingly sensitive markers, a diagnosis of UA has become less common.  UA may present with angina at rest, new-onset severe angina, or increasing angina. The initial therapeutic steps for patients presenting with findings consistent with UA focus on initial interventions and triage according to the most likely presumptive diagnosis.
New ST-segment depression or T-wave inversion with elevated CK-MB or troponin suggests NSTEMI. The distinction from UA is based on cardiac biomarkers, which in NSTEMI may be raised several hours after presentation. Treatment is directed towards relief of ischaemia, prevention of further thrombosis or embolism, and stabilisation of haemodynamic status, followed by early risk stratification for further treatment.
STEMI is characterised by ST-segment elevation or new left bundle branch block (LBBB). Cardiac biomarkers will be elevated. Immediate and prompt revascularisation with percutaneous coronary intervention within 90 minutes of first presentation, or thrombolysis within 12 hours of symptom onset, can prevent or decrease myocardial damage and decrease morbidity and mortality by preventing acute complications. [ ]
BMJ Publishing Group
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