Is it in normal resting condition or after activity you are talking about
We need to monitor closely and maintain a diary first because HR upto 120 is normal in a 6year old
But if it persists till 140 in resting condition , needs to be evaluated
Can be due to lots of conditions
Next Steps
1. Monitor and maintain a diary of HR
2. See a doctor physically so that he can palpate it because pulse oximeter readings are faulty sometimes
3. If everything is ruled out , we can do some blood investigation for
TSH , FT3, TBG etc
Health Tips
You can consult me at Practo or at 900201+3551 if you wish