Don't be temped to give your cat any sedatives before the flight, even if this is recommended by a vet.
A short while ago, I also took a cat on a flight (on one leg of the journey she was cabin bagage, and in the hold for the 2nd leg).
I had given the cat something to calm her down (as recomended by my vet), but when the airline staff saw that she was a bit dozy, they would not allow her to go on the planned flight and was delayed a few hours until the medication had worn off a bit.
This was due to the possible side effects of the drugs, and the enhanced effects due to the reduced pressure, and the airline not wanting t held liable if there a problem. ( I couldn't blame them in the least)