The recent STEMI guideline has made the EKG criteria very vague. Here is the 2013 guideline text:
Diagnostic ST elevation in the absence of left ventricular (LV) hypertrophy or left bundle-branch block (LBBB) is defined by the European Society of Cardiology/ACCF/AHA/World Heart Federation Task Force for the Universal Definition of Myocardial Infarction as new ST elevation at the J point in at least 2 contiguous leads of ≥2 mm (0.2 mV) in men or ≥1.5 mm (0.15 mV) in women in leads V2–V3 and/or of ≥1 mm (0.1 mV) in other contiguous chest leads or the limb leads (7). The majority of patients will evolve ECG evidence of Q-wave infarction. New or presumably new LBBB has been considered a STEMI equivalent. Most cases of LBBB at time of presentation, however, are “not known to be old” because of prior electrocardiogram (ECG) is not available for comparison. New or presumably new LBBB at presentation occurs infrequently, may interfere with ST-elevation analysis, and should not be considered diagnostic of acute myocardial infarction (MI) in isolation (8). Criteria for ECG diagnosis of acute STEMI in the setting of LBBB have been proposed (see Online Data Supplement 1). Baseline ECG abnormalities other than LBBB (e.g., paced rhythm, LV hypertrophy, Brugada syndrome) may obscure interpretation. In addition, ST depression in ≥2 precordial leads (V1–V4) may indicate transmural posterior injury; multilead ST depression with coexistent ST elevation in lead aVR has been described in patients with left main or proximal left anterior descending artery occlusion (9). Rarely, hyperacute T-wave changes may be observed in the very early phase of STEMI, before the development of ST elevation. Transthoracic echocardiography may provide evidence of focal wall motion abnormalities and facilitate triage in patients with ECG findings that are difficult to interpret. If doubt persists, immediate referral for invasive angiography may be necessary to guide therapy in the appropriate clinical context (10,11). Cardiac troponin is the preferred biomarker for diagnosis of MI.
I encourage the reader to go to Dr. Steve Smith’s blog with his examples of EKGs. His interview at EMCRIT.org is very helpful; there is an outline of the talk on the website. His book is free, and is available here: link. The book is slightly out of date (as it does not discuss the Smith modification of the Sgarbossa Criteria), but should be still useful.
My own experience is that: the more STEMI calls you take, the more you will be able to discern which EKGs are real STEMI and which are not. In addition, clinical presentation, troponin, and handheld echo are all very helpful; they should be used in combination of EKG to make the decision. EKG alone should not be the sole criteria.