Posterior myocardial infarction is commonly confused with anterior subendocardial ischemia; however, the T waves are expected to be negatively deflected with anterior subendocardial ischemia (unless early on when they can mirror posterior hyperacute T waves). 5-10% of all myocardial infarctions are isolated posterior and not associated with inferior ST elevation. Isolated posterior myocardial infarctions are associated with longer door-to-balloon times and worse outcomes because they are frequently missed. ST elevation of at least 0.5 mm in posterior leads can help secure the diagnosis.
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Posterior leads placed under the left scapula and designated as V7, V8, and V9.