I've flown with a CPAP several times, including in and out of Italy.
I agree that you should confirm CPAP travel rules, and everything I'm about to write, with the airlines and the transportation security of any country you'll be passing through. But in general, in the USA a CPAP is considered a medical device, which means if you have to carry it in its own bag, it does not count against your carry-on limit.
Although I haven't done either of the following and have never had a problem traveling with my CPAP, in case there are any questions at a checkpoint, some choose to attach a baggage tag that clearly marks that bag as carrying an exempt medical device (some CPAP supply stores sell these), or include a prescription or letter from your doctor.
I am normally a two-bag traveler (carry-on and small personal item bag). I only traveled with my full size CPAP once, and I hated having to carry the third bag for it through airports, trains, subways, etc. so I decided to bite the bullet and pay for one of those tiny travel CPAPs (AirMini). That is small enough to be carried in my personal item bag with my other items that must never be checked (medications ,camera gear…). With the right cable (separate purchase) it can be powered from my multi-port USB AC adapter so now I do not need to even carry a separate power adapter for it. It was not cheap, and it's not as quiet as a full size CPAP, but I love being able to stick to two bags.
This probably will not affect you if you're a one-bag carry-on traveler, but a CPAP machine should never be in checked luggage because it could easily be damaged or lost that way.
Have an adapter for plug & voltage
Are you sure you need the voltage transformer? Look at the label on the power supply for the CPAP, and if the voltage rating on it says something like 100-240 volts, then it does not need a voltage adapter because it can work fine with the 230 volts of European AC electricity. If it does not need a voltage transformer, then you only need to bring the plug adapter. It seems like that 100-240 volt capable power supplies have gotten so common that every device I travel with today can do that, so I can't remember the last time I brought a transformer to Europe. But always check the voltage rating on all of the adapters you pack, to make sure.
Of course if you're packing any item where the power supply label doesn’t say it covers 230 volts, then OK, you pack a voltage transformer to avoid frying that device.