Sub endocardial ischemia produces ST segment depression while transmural or subepicardial ischemia produces ST segment elevation.
It should be possible to record an ECG within 10 minutes of reporting to the emergency department in a case of suspected acute myocardial infarction.

Initial peaking and broadening of T wave is one of the earliest change in acute ST elevation myocardial infarction. But this alone is not specific enough to recommend thrombolytic therapy. Nothing is lost by waiting a few minutes to look for ST elevation to be more specific in diagnosing acute myocardial infarction. Specifically because there is a small but definite risk of cerebral hemorrhage for thrombolytic therapy. But waiting too much is also not good. If it is delayed till the appearance of deep Q waves and T waves get inverted, the role of thrombolysis becomes limited.