Are you sure her heart rate is too fast? Rat's heart rates are normally pretty fast, and the baby ones are slightly faster. Could something have spooked her? They will often act like you described, and it can be for something seemingly so minor. Especially for babies. Do they have plenty of places in their cage to hide, and play? Where is their cage located? It's best to have it in a busy part of the house, so they get used to all the sights, sounds, and smells faster. Also, it's not a bad idea to take them to a vet and get a health check, so you have a baseline to compare to for when they do get sick. Also, I would get kitten revolution, and keep it on hand, or go ahead and treat for mites. Most vets don't know what to look for in rats when it comes to mites, and they won't show up on a skin scrape test. Lilspaz can help you with dosing.