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The traditional teaching has always been to use supplemental high-flow oxygen routinely for patients with acute MI. I recall specifically being taught in residency by EM, IM, and cardiology attendings that every acute MI patient should receive a minimum of 6 liters of supplemental oxygen via nasal canula, if not 100% oxygen, regardless of the initial pulse oximetry.
Mounting evidence, however, is demonstrating that the use of supplemental oxygen in patients that are "normoxic" (i.e. the production of "hyperoxia") is detrimental. Studies are demonstrating that there is no improvement in mortality or prevention of dysrhythmias; and in fact a trend towards increased mortality when patients are hyperoxic. This detrimental effect is likely the result of coronary vasoconstriction which occurs through several different mechanisms, all induced by hyperoxia. Oxygen, it turns out, is a vasoactive substance.
The takeaway point is very simple: if an AMI patient is not hypoxic, don't go overboard with the supplemental oxygen!
[Moradkhan R, Sinoway LI. Revisiting the role of oxygen therapy in cardiac patients. J Am Coll Cardiol 2010;56:1013-1016.]