I was prescribed Propranolol for a fast heart beat! I called to have it renewed and then forgot to get it and take it! Had no sysmptoms until today while walking up stairs from a beach walk could not catch my breath had to sit down several times and almost passed out, I did call the Pharmacist and got back on med but I am feeling very shaky and week after the episode, What exactly does a Beta Blocker do to prevent this from happening?