Go through these in order and stop at the first one that applies. If you are reading this for someone else, answer for them.
Cardiac arrest is always an emergency.
Call emergency services immediately if someone becomes unresponsive and is not breathing normally or is only gasping.
Begin CPR and use an AED if available.
After surviving cardiac arrest, seek emergency help for another collapse, fainting, severe breathing difficulty, new weakness, or chest pressure that may represent a new heart problem.
Contact your healthcare team promptly for worsening chest-wall pain, fever, increasing shortness of breath, wound or device concerns, repeated palpitations, or problems taking prescribed medicines.
Routine follow-up is appropriate for ongoing tiredness, sleep problems, memory changes, anxiety, low mood, questions about activity, or planning a return to driving and work.
Every sign above is quoted from the sofidex article on CPR. If you are not sure, err upward — it is always reasonable to be checked.