Most of what I've read throughout the years, regarding breathing and target shooting, says to fully exhale and shoot or shoot while you are exhaling. For whatever reason I have always held my breath while pulling the trigger, just the habit I got into at an early age and that's how I shoot today. If you hold your breath, your heart rate will decrease a little because of vasoconstriction of your peripheral vascular system, increased blood pressure, yada, yada, so that might be a technique to try. You can experiment with it by checking your pulse while your breathing normal, then check it again while holding your breath for about 15-20 seconds, or more if you can tolerate it without getting too air hungry.