Nitroglycerin is to knock your blood pressure down, I believe it dilates yuor blood vessels. Reason you need to call EMS, is one pill may not be enough. Also some get a huge headache, like someone hit them in the top of the head with a frying pan. I get Chest wall pain from an injury, my pain is either a 1 or a 10, no in between. Been to the ER with Chest pains, BP through the roof, and given the max of Nitro, to get it down. Never had a headache from it. Doc said I got one Hell of a tough heart, but lets not test it like that again.
When you go to the Doctor take someone with you to take notes, as a patient, you only grasp about fifth of what the Doctor tells you.