it is considered medical supplies and is NOT counted.
i've flown with my daughter and I and brought: laptop (personal item), backpack with snacks and books for me, smaller backpack with activites for my daughter, her diaper bag (personal item), and the cpap on board.
Just know on screening they want you to take the cpap out of the bag for it to be scanned through, and they will bring you aside for extra test on the cpap. was very quick.