Myocardial Infarction w/ ST-Segment Elevation Diagnostics

Electrocardiogram (ECG)  

Obtain and interpret an ECG as soon as possible, preferably within 10 minutes of emergency department (ED) arrival, since even at the early stage, ECG is rarely normal. If the ECG shows ST-segment elevations or new or presumed new LBBB, then the patient should be immediately evaluated for primary PCI or if this cannot be performed in a timely manner by experienced operators, for fibrinolytic therapy. If the resting ECG is without ST elevation (eg ST-segment depressions or deep T-wave inversions without Q waves) or is non-interpretable (eg new RBBB, paced rhythm), consider NSTE-ACS (eg NSTEMI or unstable angina).

ECG changes of AMI are evolutionary. Hyperacute changes of a tall peaked T wave or ST-segment elevation followed by development of a Q wave. In resource-limited settings, development of pathological Q waves can confirm the diagnosis of MI only if the patient’s clinical history and previous ECG or cardiac biomarker results are available. Then, there is a return of ST-segment to isoelectric and T-wave inversion.  

ST segment elevation is a new or presumed new ST-segment elevation at the J point in ≥2 contiguous leads that should be considered at ≥0.2 mV for men ≥40 years, ≥0.25 mV for men <40 years, or ≥0.15 mV for women regardless of age in leads V2-V3, and/or ≥0.1 mV in other contiguous leads. ST-segment depression and T-wave changes are new horizontal or downsloping ST-segment depression ≥0.05 mV in two contiguous leads or T-wave inversion ≥0.1 mV in two contiguous leads with prominent R wave or R/S ratio >1.  

The ECG may be vague in the early hours and may not show ST-segment elevation or new Q waves. A repeat or serial ECG every 10 to 15 minutes should be taken to compare with previous records and detect evolving infarction. Additional chest leads (V7-9) and right ventricular leads may be helpful. Posterior leads (V7-V9) should be obtained in patients with suspected left circumflex occlusion, particularly in the setting of isolated ST-segment depression ≥0.5 mm in leads V1-V3, to assess for ST-segment elevation in the posterior leads, which could indicate a posterior STEMI.

Confirmatory Tests for the Diagnosis of Myocardial Infarction 

Elevated biochemical markers of myocardial necrosis show an increase and/or decrease of cTn values with at least one value >99th percentile upper reference limit (URL). An acute myocardial injury has an increase and/or decrease in cTn values over time without evidence of ischemia, while chronic myocardial injury has stable or persistently elevated cTn values. These can confirm diagnosis in the absence of ECG changes. 2D echocardiography and perfusion scintigraphy, if available, can be used to look for (presumed) new regional wall motion abnormalities or loss of viable myocardium consistent with an ischemic cause. Intracoronary thrombus may be identified by angiography or autopsy.  

Biochemical Indicators for Detecting Myocardial Necrosis  

Blood samples should be taken on first assessment and repeated in 1 to 3 hours for high-sensitivity troponin and 3 to 6 hours later for conventional troponin assays if the initial result was non-diagnostic. In the acute phase of STEMI, a routine blood sampling for serum markers is indicated as soon as possible, but it should not delay initiation of reperfusion therapy. The preferred biomarker for myocardial damage is cTn (T or I). High-sensitivity cTn (hs-cTn) assays have higher negative predictive value for AMI than standard assays and hs-cTn levels >5-fold the URL have high positive predictive value for acute type 1 MI. The hs-cTn assays are associated with a two-fold increase in identifying type 2 MI, eg tachyarrhythmias, and HF. 







If cTn assays are not available, CK-MB assay is the preferred alternative. Maximal CK-MB exceeding the 99th percentile of the values for a reference control group on two consecutive samples or maximal value over two times the upper limit of normal for the specific institution at least once during the first hour following the index clinical event. The values for CK-MB should rise and fall. If the values remain elevated without change, this is typically not due to MI. A diagnosis of reinfarction is supported by a value increase of ≥20% between two samples of troponins or CK-MB collected 3 to 6 hours apart.

Imaging is helpful in detecting wall motion abnormalities or loss of viable myocardium in the presence of elevated cardiac biomarker values. Without any delay to therapy, obtain a chest X-ray. Perform chest X-ray, transthoracic and/or transesophageal echocardiography, and contrast computed tomographic (CT) scan to differentiate STEMI from aortic dissection, pulmonary embolism, pulmonary edema, pneumothorax or pneumonia in a doubtful presentation.  

Perform echocardiography to allow risk stratification of patients with chest pain upon emergency department (ED) arrival and in those whose diagnosis is unclear and ECG is not diagnostic, especially in patients with LBBB or pacing and suspicion of posterior STEMI with anterior ST-segment depressions. A transthoracic echocardiography may provide evidence of focal wall motion abnormalities and facilitate triage in patients with ECG findings that are difficult to interpret.