ACS is a clinical syndrome that includes MI and unstable angina. An MI can present as either a STEMI or NSTEMI. A STEMI typically occurs as a result of plaque rupture and subsequent thrombosis of a coronary artery that leads to infarction and subsequent ECG changes. An NSTEMI does not have the ECG changes seen in STEMI but can also occur secondary to plaque rupture (Type 1 NSTEMI) or to inadequate blood flow to meet the metabolic demands of the myocardium in conditions like sepsis, pulmonary embolism, or coronary vasospasm (Type 2 NSTEMI). In either case, both STEMI and NSTEMI will result in an elevated troponin. Unstable angina is characterized by new onset chest pain, pain lasting greater than 20 minutes or pain at rest or with minimal exertion. While unstable angina can occur with ECG changes not diagnostic of STEMI, it does not have elevated troponins.10 It is essential to differentiate between STEMI and NSTEMI since those with STEMI require emergent coronary intervention.9 NSTEMI is more common than STEMI, representing 60-70% of myocardial infarctions.14 NSTEMI with coronary occlusion is present in 25% of cases.15

ACS pain is classically described as exertional, pressure-like chest pain with radiation to the jaw, neck, one or both arms/shoulders, or back. Associated symptoms include nausea/vomiting, shortness of breath, and diaphoresis.10 Atypical symptoms (dyspnea alone, fatigue, weakness, epigastric pain, palpitations, syncope, nausea alone, and cardiac arrest) are more common in the elderly, women, and diabetics. Atypical symptoms are presented symptoms in one-third of confirmed myocardial infarctions.16

Troponin is a protein released from myocardial tissue and is indicative of cardiac cell death. Serum troponin levels begin to rise measurably as early as 2 to 3 hours after onset and can remain elevated for up to 7 days.10 When available, cardiac troponin assays (cardiac specific I or T) should be used in the diagnosis of MI. A troponin value above the 99th percentile upper reference limit (determined by the assay type) is considered positive. Troponin values may continue to rise, or they may decline depending on the timing of the clinical event, and both situations should raise concern for ACS.10,17 Due to the time elapsed between cardiac cell death and laboratory detectable rise in troponin levels, the troponin level may be checked serially, usually every 3 to 6 hours.18

The following are clinical findings in combination with a rise and/or fall in troponin values that constitute the diagnosis of acute MI:19

- Symptoms of ischemia (See above paragraph.)
- New ischemic ECG changes
- Development of pathological Q waves
- Imaging evidence of new loss of viable myocardium or new regional wall motion abnormality
- Identification of an intra-coronary thrombus on angiography or autopsy

Other clinical conditions may present with elevated troponin. Some of these conditions may manifest symptoms similar to ACS (myocarditis, pulmonary embolism, and aortic dissection), while other conditions may present without features of ACS (such as rhabdomyolysis, sepsis and renal failure) and their management is distinct from that of ACS.19 It is therefore important to consider the clinical manifestations and additional studies (ECG, echocardiography) in addition to troponin in the diagnosis of ACS. Keep in mind that in forward deployed locations, laboratory testing for troponin may not be available, and a high index of suspicion must be maintained in the presence of appropriate symptoms.