I never noticed a racing heart on pot until one time I took DXM and smoked a little pot on top of it. For some reason even though I'd done this dozens of times before that night I had a panic attack with a heart rate around 200bpm and felt like I was on the verge of death for 2 hours until it wore off.
After that occasion, which was about 2 years ago, I noticed my heart would race and beat pretty hard after smoking pot, and sometimes this would give me mild panic attacks or at least an unenjoyable high. Obviously this sucked when I was tryin' to chill with my buddies when I was having paranoid thoughts about my physical condition racing through my head.
Now that it is 2 years later and this has failed to deter me from smoking pot on a daily basis (even though many of those highs were completely unenjoyable...) I can say I don't really have this problem anymore.
Yes my heart rate does still get elevated after smoking pot, sometimes more than others and it seems to depend on the kind of ganja. The way I got over this was by slowly training myself to basically ignore my racing heart, and I found that if I don't pay attention it will decrease, maybe not back to normal but certainly less than it would be if you are thinking about it every second.
Also:
Don't get super ripped on the first bowl / joint / bong of the day. If you smoke slowly rather than taking a few bong rips back to back your body has more time to adjust to it, therefore your heart won't freak out as much.
If this is a real problem for you now you might want to limit your smoking to just a couple hits per session (depends how good your weed is, if a couple hits won't do much then smoke more I guess). During this period that you are paranoid about your heart rate you might find your psychological tolerance for weed has decreased and you don't need as much to get high. Also you will probably find a mild high more enjoyable than getting really ripped because your heart won't race and you won't be paranoid about your body.
Once you start to overcome this, which is essentially a psychological overreaction to a normal physical response, you can start to smoke more if you want to.
Of course during this period it is essential that you avoid mixing marijuana with amphetamines and psychedelics because your chance of freaking out would be very high.
hope that helps