I agree that a week off should not make a big difference.
But even with my very fit dressage horse, if he has a week off for reasons other than injury, I still only walk him the first day, add a little trot the second day, more trot and a little canter the third day, and see how that goes. This is to avoid over-stressing his joints, tendons and ligaments, not just his respiratory system. I feel it's better to be safe than sorry.
You don't say how old your horse is, or whether he's a bit arthritic, etc. A week off can make a bigger difference to a horse like that, and he'll benefit from the gradual working-in above to get his joints moving again without undue strain.
If your horse has been off because of injury, you'll need to bring him back into work more slowly, which your vet will explain, depending on the nature of the injury.