Wallach's Interpretation of Diagnostic Tests: Pathways to Arriving at a Clinical Diagnosis (33 page)

   T-wave changes are the most sensitive.

   ST-segment elevation of >1 mm in two contiguous precordial or two adjacent limb leads that is persistent and accompanied by symptoms consistent with ACS (>30 minutes) should be considered for immediate mechanical or pharmacologic reperfusion due to the poor short-term prognosis of STEMI. This category also includes ECG changes of hyperacute T waves, new LBBB, or posterior MI (may require posterior leads for diagnosis).

   If STEMI (or equivalent) is excluded, the presence of ST-segment depressions and T-wave abnormalities should be assessed.

   Horizontal or downsloping depressions of ≥0.05 mV are important indicators of ongoing ischemia.

   T-wave inversions or “pseudonormalizations” may aid diagnosis, particularly with symptoms, but are less sensitive for ischemia.

   As ACS is highly dynamic, serial ECGs (every 20–30 minutes) and clinical reassessment should be performed if the initial ECG is nondiagnostic and the patient remains symptomatic.

   Continuous ECG monitoring should be performed in all UA/NSTEMI patients admitted to the hospital for surveillance of arrhythmias and ongoing ischemia.

   
Cardiac biomarkers
, along with the ECG, remain a cornerstone for the diagnosis of MI. Cardiac troponin T and I are preferred markers given the myocardial specificity. CK-MB is the next favored biomarker and is released more rapidly with ischemia than troponin, although it lacks the former’s absolute tissue specificity (see Chapter
16
, Troponin limitations).

   Most NSTEMI patients have troponin elevation within 4–6 hours after symptom onset. Initially negative biomarkers should be remeasured within 8–12 hours after symptom onset.

   New “high-sensitivity” troponin assays increase sensitivity with an associated loss of specificity, particularly in low-risk patients and must be interpreted in the clinical context.

   Even without ACS as an etiology, however, an elevation in troponin >99th percentile portends a worse prognosis when compared to patients without elevation.

   
Cardiac imaging
is emphasized in the definition of acute MI and can aid in clinically indeterminate cases. Because of its widespread availability and mobility, echocardiography is often used to differentiate myocardial ischemia from nonischemic etiologies of chest pain. Regional wall motion abnormalities can help distinguish ischemia from perimyocarditis, valvular heart disease, cardiomyopathy, pulmonary embolism, or ascending aortic dissection. Wall thickness (or lack thereof) may aid in determining if MI is acute or subacute/old. While MRI is validated for these purposes as well, its availability, cost, and time make it less efficient for acute chest pain evaluation.