Where in the US are you traveling to?
Could you fly to a city where heat is not such a problem, and then drive to your destination - even if it means driving a looooooong way?
Most important, avoid flights where you'd have to transfer, only fly direct. The danger is that the animals might get left on the hot tarmac during the transfer.
Were it me, I'd use a professional pet shipping agent. A good shipping agent will have experience with this problem, and should be able to find a safe solution for your pet.
I have used a very good one in the Chicago area - if that's your destination, I can give you his details. Or, contact this Zürich based shipper:
http://www.acepetmoving.com/en/about-us.html While I have not used them yet, the correspondence I've had with this company left a good impression.
If all else fails, could you either postpone your return until the fall, or could you find temporary accomodation for your dog until the heat embargo is lifted?