N/N- Does not carry and is not effected with HYPP
N/H- Half the foals will be N/H
H/H- All foals will be N/H or H/H
Just make sure the horse your buying with Impressive on his papers/bloodlines be tested for HYPP, and will be tested N/N.
I would personaly never buy a horse that is N/H or H/H as there symtoms can come back at any time, even if they never shown them before.