When the heart goes out of rhythm enough to cause a heart attack (your pulse feels irregular, too fast, too slow, or pounding) then it MUST be brought back into rhythm. You also need oxygen because your heart isn't pumping well enough to meet your body's need for blood-carrying-oxygen, especially to your brain. A doctor may prescribe medication, or you may need defibrillation.
You may not think clearly.
A quick assessment for stroke:
S -- Smile. If lopsided, call 9-1-1.
T -- Talk. Any difficulty with speech, call 9-1-1.
R -- Raise arms -- if unevenly raised, 9-1-1.
Call 9-1-1 immediately if you suspect a heart attack or stroke. It's better to be embarrassed than dead, or risk having your lifestyle diminished because you delayed.
If you are driving a car, the best thing to do is stop at the very first place where a phone can be found -- a house, a store, some motorist with a cell phone, wherever -- and call 9-1-1.
If you're 5 miles or LESS from the nearest hospital stopping by the roadside and flagging someone to drive you there could be a better option, because of the time it takes for an ambulance to reach you.
However, ambulances and fire department Quick Response units carry oxygen, which vastly improves your chances. So if you know an ambulance or fire station is close, that's your best choice.