I remember always getting the rapid pulse rate for at least 30 min into the high when I started out smoking. That is normal. If you're worried, only be if you know you have any heart conditions. Remember it has never been documented of any one dieing directly from cannabis.

You have to think all them methhead's and crackhead's on cops running their asses off while their high on speed. Couldn't imagine how fast their heart rates were going. The human body can take a lot. Remember this is normal and safe.

EDIT: I should say yes the more you smoke the more you get use to it. A lot more.