My mother has Lung Cancer. They say to watch for hallow breathing at the end. What is hollow breathing?

I realized after writing that I was very abrupt... and I'm not even a doctor. Lung cancer is an awful disease to watch, so my heart goes out to you and your mother. You will know as she is slipping away, both in your spirit and in the color of the skin on her face. I hope she passes peacefully and that you have some pleasant times together before she goes. Hugs to you and your mother.

They are probably referring to cavernous breathing, which sounds hollow. The breathing is usually fast and sounds hollow like a rush of air over a bottle. It is difficult to describe how something sounds. To me it sounds like the throat is an open hollow tube. You will know the sound when you hear it because it does sound hollow or cavernous.