Go through these in order and stop at the first one that applies. If you are reading this for someone else, answer for them.
Call your local emergency number immediately if you develop possible heart attack symptoms, especially chest pressure, squeezing, or heaviness that lasts more than a few minutes or returns.
Call even if the discomfort is mild or you are unsure.
Emergency medical teams can assess you, begin treatment, and respond if your condition changes.
If you have prescribed nitroglycerin, use it as you were instructed, but do not let this delay the emergency call.
An emergency dispatcher may advise you about aspirin.
If someone collapses, is unresponsive, and is not breathing normally, call emergency services.
Begin CPR if you can and use an automated external defibrillator, or AED, if one is available.
Call emergency services rather than waiting for a routine appointment.
And for anyone, someone who:
After a heart attack, recurrent chest discomfort, sudden breathlessness, fainting, or a new fast or irregular heartbeat also need urgent assessment.
And for anyone, someone who:
The article also says
Do not drive yourself to hospital.
For non-emergency concerns, contact your usual healthcare team.
Examples include gradually increasing ankle swelling, ongoing tiredness, low mood, possible medicine side effects, or questions about returning to work and activity.
Every sign above is quoted from the sofidex article on Heart attack. If you are not sure, err upward — it is always reasonable to be checked.