On Italian menus you get an antipasti course, which is starters and some of them might be lower carb. Then you get primi piatti, which is where all the carby pastas will be. Then comes the secondi piatti, which are usually the meat or fish courses. They could well be your best bet for lchf food, but they often do not have vegetables, so you might have to order contorno - the veggie side dishes. I love Italy and Venice is fab. I couldn't resist the ice-cream though - so ended up indulging in some carbs and having to walk them off sight-seeing!
Have a great time!