My 10 yo has had several episodes of near fainting: pale, sweating, trouble breathing, rapid heartbeat. blood tests fine. could it be heart related?

Yes: Yes it can. What blood tests were done? I would definitely get an ekg, but it also might be anxiety, asthma, and few other problems, needs some workup and good physical.

Absolutely: Your child needs a comprehensive medical evaluation. These symptoms could be related to a rhythm problem or other structural heart problem. They could also be related to a problem with the autonomic nervous system.