Myocardial Infarction w/ ST-Segment Elevation Công cụ chẩn đoán

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; compare with previous ECGs if available. If the ECG shows ST-segment elevations or new or presumed new LBBB, then the patient should be immediately evaluated for primary percutaneous coronary intervention 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 in two contiguous leads in a patient with ischemic symptoms is the cardinal feature of STEMI. It 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.  

A completely normal ECG does not exclude the possibility of ACS. 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 or transient changes. Additional chest leads (V7-V9) 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 MI

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 in combination with other clinical criteria rule in MI. 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  

In patients with STEMI, cTn is not recommended for diagnostic evaluation as clinical history and ECG are generally sufficient, with cardiac biomarkers serving to support diagnosis. 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 and clinical suspicion of MI and/or the pretest probability of MI is high. 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. If a single cTn measurement is used, sampling should be performed at least 3 hours after symptom onset for high-sensitivity cTn (hs-cTn), and at least 6 hours after symptom onset for conventional cTn. The preferred biomarker for myocardial injury and necrosis is cTn (T or I). It must be noted that hs-cTn assays can detect lower levels of cTns and allow earlier diagnosis of ACS. These 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, HF).  







If cTn assays are not available at the time of first assessment, CK-MB assay is the preferred alternative. It may be measured in patients presenting within 6 hours of symptom onset as an early biomarker of MI. The values for CK-MB should rise and fall. If the values remain elevated without change, this is typically not due to MI. In STEMI with clear ECG changes, a single CK-MB or cTn measurement obtained >8 hours after symptom onset may be sufficient to support the diagnosis. 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.  

Other Diagnostic Tests  

The following laboratory investigations should be performed, as clinically indicated, in patients with suspected STEMI: Complete blood count, renal profile, blood glucose, and lipid profile.

Imaging is helpful in detecting new wall motion abnormalities or loss of viable myocardium consistent with MI in the presence of elevated cardiac biomarker values. Without any delay to therapy, obtain a chest X-ray and in doubtful presentations, perform transthoracic and/or transesophageal echocardiography, computed tomography coronary angiography (CTCA), magnetic resonance imaging (MRI), and radionucleotide techniques, as appropriate, to differentiate STEMI from aortic dissection, pulmonary embolism, pulmonary edema, pneumothorax or pneumonia.

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. In the presence of normal or uninterpretable ECG and ongoing chest pains suggestive of myocardial ischemia, echocardiogram may be helpful to look for new/presumed new wall motion abnormalities. A transthoracic echocardiography may provide evidence of focal wall motion abnormalities and facilitate triage in patients with ECG findings that are difficult to interpret.