An EMT may administer aspirin to a patient if?

Aspirin is given to people by EMS providers when there is a suspicion of a heart attack.
In most heart attacks, blockage of one or more of the coronary arteries initially causes pain and, as the blockage worsens, can lead to cardiac arrest. One of the most common forms of blockage is by a clot on the sides of the blood vessels.
Aspirin acts by making the platelets in your blood less "sticky" so that they won't attach to existing clots, thereby preventing the clot in the heart from getting worse. This gives the person more time to get to a definitive care facility and get the clot treated.