What can someone do to control his or her breathing when they have heart palpitations?

Relax. Seek help.: Although some palpitations can compromise cardiac function enough to cause shortness of breath, most breathing issues associated with palpitations are caused by the accompanied panic. Breathing control will not change the palpitations per se; getting to the cause of the palpitations and treating the cause to control or eliminate the reason for the palpitations is the goal. See your doctor!

Yes but: It may or may not alleviate the palpitations which may require further evaluation.