Clonidine is an old blood pressure medication that works by lowering the epinephrine level in your blood. My psychiatrist prescribed this to me for awhile to decrease my agitation and anxiety. This medication also slows your heart beat rate, which would in turn, slow the speed your blood flows through your arteries and veins. I believe it did help to lower my agitation which might have indirectly helped to lessen my psychotic symptoms of schizophrenia.
I heard that people can use bio-feedback to help teach themselves to lower their heartbeat rate.