Hi Did you ever find out what was up with your 3 year old, my four year old has the same thing I noticed becuase I was checking his pulse when he was sleeping, he has such a strong pulse compared to my 10 year old, seems to be normal then speed up then normal again? so wierd, the dr did say he has a mild heart murmur a few years ago when he was sick with bronchitis, but then other drs say the cant hear it, some say they can ?!? wondering if I should take hime to see the dr and get and ECG, as I am freaked out.
I'm curious how you noticed the heart rate change? Is he complaining of feeling strange or chest pain? If his heart rate increases a bit with an inhaled breath and slows when he exhales, that's pretty normal. Our normal sinus rhythm will speed up and slow down in relation to what we're doing - sleeping, walking, moving our arms around.
If his heart rate is jumping from slow to very fast, it would be good to mention it to his doctor.