Two features on modern watches borrow their names from a hospital.
One is SpO2 and the other is ECG.
Seeing them on a listing makes it sound like your wrist is now a clinic.
It is not.
Both are worth understanding before you lean on them.
What is SpO2 and how does a wrist read it?
SpO2 is the percentage of oxygen your blood is carrying.
The watch shines red and infrared light into your wrist.
Blood with plenty of oxygen reflects those two colours differently.
From that difference the watch estimates a percentage.
The clip a nurse puts on your fingertip works the same way.
But it shines light straight through a thin fingertip.
Your wrist is thicker, hairier and moving.
So a wrist reading is disturbed far more easily.
What does the ECG feature actually check?
This is a different sensor, and it uses no light at all.
An ECG, or electrocardiogram, records the electrical signal your heart makes.
You place a finger on the crown or the side of the watch.
That closes a circuit across your body and records a short trace.
It is looking for one thing in particular.
It asks whether your beat is regular, or shows a pattern that could be atrial fibrillation.
That is one narrow question, not a check of your heart.
Is this the same as a test at a hospital?
No, and the gap is not a small one.
A hospital ECG reads your heart from many angles at once, through electrodes placed across your chest and limbs.
Your watch records a single view, between the wrist it sits on and a finger of your other hand.
There is a legal difference as well.
Some models are sold as wellness features.
Others have been formally cleared by a regulator for that specific use.
Look in the app or the manual for wording like "not intended for medical use".
What is approved also differs from country to country.
What should an odd reading make you do?
Take it again, still and warm, with the strap snug.
If a strange result repeats over days, save it and book an appointment.
Chest pain, breathlessness, dizziness or fainting mean help immediately.
That holds whatever the watch is showing you.
If you feel unwell and the watch says normal, believe your body.
A wearable can raise a question.
It cannot answer one.
Things to Remember
- Take SpO2 readings sitting still, arm resting, strap snug.
- Check whether your ECG is a wellness feature or a cleared one.
- Never treat a wrist reading as a diagnosis or an all-clear.
- Repeat any odd reading before you act on it.
- Symptoms outrank the watch, so see a doctor.