Injury to myocardial tissue results in the release of unique

cardiac enzymes into the vascular space, which can be

readily measured via serum analysis. Keep in mind that

patients with ECG findings consistent with STEMI do not

require confirmatory testing with serum markers but

rather warrant immediate reperfusion therapy. That said,

serum markers are very useful in patients with nondiag ­

nostic ECGs to diagnose the presence of a NSTEMl. Of

note, there is no single cardiac marker analysis that has

sufficient accuracy to reliably identify or exclude AMI

within the first 6 hours of symptoms onset. Furthermore,

elevations can and do occur secondary to non-ACS-related

conditions, including myocarditis, decompensated CHF,

and acute pulmonary embolism.

The usual laboratory studies used for the diagnosis of

AMI are the troponins (both T and I subtypes). Troponin

(Tn) levels are the most specific marker for myocardial

necrosis and have become the gold standard for diagnosis.

Elevated levels can be detected within 3 hours of injury,

peak at 12 hours, and remain elevated for a period of 3-10

days. The degree of myocardial damage and mortality is

correlated with the degree of troponin elevation.

Creatinine kinase is found in all forms of muscle tissue,

but the MB subunit is far more specific for myocardial

injury. CK-MB elevations can usually be detected within

4-6 hours after symptom onset, peak at 24 hours, and

typically return to normal within 2-3 days. Myoglobin

assays are also in common use for the evaluation of AMI.

Although attractive in theory as significant elevations

can be detected within 1-2 hours of symptom onset, a

poor specificity limits the clinical utility of serum myoglo ­

bin analysis.

..... Imaging

Obtain an emergent chest x-ray in all patients who present with a chief complaint of chest pain or shortness of

breath. That said, there are no radiographic findings

specific for the diagnosis of ACS, and its role in this setting is primarily for excluding alternative diagnoses.

Acute CHF secondary to ACS may present with classic

radiographic fmdings.

MEDICAL DECISION MAKING

Order an ECG immediately on presentation to identify

patients with STEMI, as they require immediate and

aggressive reperfusion. Patients with cardiogenic shock,

acutely decompensated CHF, ventricular dysrhythmias,

and severe symptoms refractive to aggressive medical

therapy also typically warrant emergent percutaneous

coronary intervention (PCI). In patients with nondiagnostic ECGs, proceed with cardiac marker testing. Patients

with elevated cardiac markers should be treated as having

a NSTEMI. Those whose initial set of cardiac markers are

negative require serial ECG and biomarker testing. These

patients should be stratified to identify those who are at

high risk for adverse cardiovascular outcomes. Concerning