Well.. first of all an EMT does not diagnosis, they only treat symtoms and signs. In a case where an EMT is faced with a patient complaining of chest pains, there has to be an assumtion of cardiac until proven otherwise. A good secondary evaluation would be called for and then the adminstering of O2 with the level being dependent upon other observations (ie. Pulse Ox low = higher flow).
There are protocol differentiation from department to department. In my old department we were a long way away from the local hospital (sometimes 30 minutes) so once the patient was given O2 the next step was to begin transportation (code 1 – speed limit only). In route, our AED would be placed on the patient. Part of the secondary evaluation would be a glucose scan. Then, depending upon the patient’s condition an IV would be started with normal saline or dextrose if the gluco-scan called for it. The IV of course, only done if the EMT is level B or above.
After this was done, it was just transportation and patient monitoring. Most of the time I would also run EKG strips at various intervals to give to the docs for evaluation once we arrived at the hospital.
Hope this answers the question. Just keep in mind which department’s medical director should have very specific process and procedures for most cases as you described.

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