Beat rate can be slowed down by taking a deep breath, submerging your mouth under water for a few secs and coming up at top while blowing bubbles out (Exhalling). This works damn good, its a swimmer's move to gain stamina. Hope it helps :)
Breath in as deeply as comfortable through your nose, then take twice as long exhaling through pursed lips. I have COPD and this is the method we use to improve breathing regularity and slow down a racing heartbeat. It really works.