Some cancer patients have a stoma. This means they don't breathe through their mouth or nose but through a tube cut into their throat. It is covered by a filter which has to be changed daily. Water getting into the tube would go straight to the lung which would be fatal. It is not really a tube but the original airway cut into the throat. Some do have a tube fitted to keep it open but it is not always necessary.
How can such patients immerse into a mikva? I suppose it is possible to straight away pump the water out, but it still would be very dangerous. Although this is not necessarily a halacha question but more likely a practical question it is still very relevant.