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 or go to an emergency department now if stridor occurs with:
Severe or rapidly worsening trouble breathing
Blue, grey, or unusually pale lips or skin
Pauses in breathing, collapse, confusion, or unusual sleepiness
Drooling or inability to swallow
Sudden onset after choking or possible object inhalation
Swelling of the lips, tongue, face, or throat
Skin pulling deeply around the ribs or neck with each breath
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 same-day medical advice for new stridor even if breathing seems comfortable.
This is particularly important for a baby, after airway surgery or removal of a breathing tube, or when there is fever, neck swelling, feeding difficulty, or a changed voice.
3
Does any of this apply?
Book an appointment
Book a routine appointment if an episode has fully stopped but keeps returning, or if there is ongoing mild noisy breathing, hoarseness, poor feeding, or disturbed sleep.
A previously diagnosed condition may also need follow-up if its usual pattern changes.
The article also says
Keep the person upright and as calm as possible while waiting for help.
Do not put fingers or objects into the throat.
If breathing is severely affected, do not drive yourself.
Every sign above is quoted from the sofidex article on Stridor. If you are not sure, err upward — it is always reasonable to be checked.