Go through these in order and stop at the first one that applies. If you are reading this for someone else, answer for them.
1
Is any of this happening now?
Call 911
Call emergency services if a rapid heartbeat occurs with:
Fainting or near-fainting
Chest pain or heavy pressure
Severe difficulty breathing
Confusion or reduced alertness
Marked weakness, cold clammy skin, or signs of collapse
Do not drive yourself for emergency care.
If another person becomes unresponsive and is not breathing normally, call emergency services and begin cardiopulmonary resuscitation if you are able.
Other emergency signs, for anyone
Someone who is struggling to breathe, gasping, choking, or cannot get words out
Someone who has chest pain, tightness or pressure lasting more than a few minutes
Someone who has sudden weakness or drooping on one side of the face or body, slurred speech, or sudden confusion
Someone who is unconscious, or cannot be woken
Someone who is bleeding heavily and it will not stop with firm pressure
Someone who has had a seizure that will not stop, or does not wake up afterwards
Someone who has lips or skin that look pale, blue, grey or blotchy
Someone who you believe has taken an overdose or swallowed something poisonous
2
Is any of this happening?
Be seen today
Seek urgent medical advice for a first sustained episode of a very fast heartbeat, an episode that does not stop as expected, or symptoms that are clearly different from your usual SVT.
Follow any action plan you have already been given.
3
Does any of this apply?
Book an appointment
Arrange a routine appointment if you have repeated brief palpitations, even if you feel well between episodes.
Assessment is also appropriate if a known SVT is becoming more frequent, lasting longer, or affecting daily life.
Every sign above is quoted from the sofidex article on Supraventricular tachycardia. If you are not sure, err upward — it is always reasonable to be checked.