For patients with suspected ACS, an ECG should be obtained within 10 minutes of arrival.2 For patients arriving by ambulance, compare the prehospital ECG to the initial ECG obtained in the ED. A small prospective study showed that 12.5% of prehospital ECGs had clinically significant abnormalities (ST elevation or depression, T-wave inversion, or arrhythmia) that were not seen on the initial ED ECG, leading to a change in physician management nearly two-thirds of the time.35
In the United States, 29% to 38% of patients with ACS present with STEMI.36 STEMI is defined as new ST elevation at the J point of ≥ 1 mm (0.1 mV) in ≥ 2 contiguous leads. (Exception: some degree of ST elevation is considered normal in leads V2-V3; up to 1.5 mm in women, 2 mm in men aged ≥ 40 years, and 2.5 mm in men aged < 40 years.)4 Significant ST elevation typically signifies transmural ischemia from acute coronary artery occlusion (type 1 myocardial infarction).37 New horizontal or downsloping ST depression ≥ 0.5 mm (0.05 mV) and T-wave inversion ≥ 1 mm (0.1 mV) in ≥ 2 contiguous leads can also indicate myocardial ischemia, though this typically signifies subendocardial ischemia. A large retrospective review found these abnormalities in 22.9% and 14% of patients with NSTEMI, respectively.38 T waves and other ECG features can vary from minute to minute in an ischemic event. (See Figure 1.) Serial ECGs at 5- to 10-minute intervals are recommended if the initial ECG is nondiagnostic but the patient still has concerning symptoms.
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