Disclaimer : I am obviously not a doctor.
But something you could try, Marb......
Breathe in through the nose with mouth closed for a count of 4
You breathe in such a way that the diaphragm raises (ie it feels like your stomach area is expanding, rather than the lungs. So if you put a hand on your stomach, you would feel it pushing outwards)
Hold the breath for a count of 4
Breathe out slowly through the mouth for a count of 8 (it feels like you are gently blowing air out through the lips)
When you get used to it, you can make the breath out a longer one so it feels like you are emptying the stomach of air and bringing it inwards towards the spine (this has other reportedly beneficial physiological effects).
Among other (positive) things this is doing to the Oxygen/Carbon Dioxide levels in the blood, it acts to activate the parasympathetic nervous system (the part of the nervous system responsible for controlling the organs that you can't control through direct physical action - heart, liver etc)
You can read more about it here....
https://www.cbc.ca/life/wellness/fr...our-nervous-system-with-the-breath-1.4485695#
Obviously, this doesn't mean a lot (because we're all different) but I occasionally used to get small flutters (not quite the sort of palpitations you seem to be experiencing) and this type of breathing seems to have stopped it.
Once you are used to it, then twice a day - for 8 repetitions each time - make it 4 in, hold 7, out 8. This has other reportedly beneficial physiological effects, over time.
NOTE Nasal breathing is believed to be highly important for wellbeing in general. It takes conscious thought to do it, many of us just naturally breathe in through the mouth a lot of the time.