The lungs lose their capacity to increase vital capacity primarily in five ways. My order of priority. The ribcage either shrinks, becomes stiff and inflexible, or freezes in hyper-open position. People with large frozen chests may well be heart attack or lung disease prone. A sustained bear hug from big uncle Ben , or a belt or girdle, will make it harder to breathe.
Pythons slowly squeeze their prey and it suffocates to death or dies of fright.
Underbreathing is also a lot like boiling a frog. I am told that if you place a live frog in a pot of cold water that you can raise the temperature so slowly that it will not notice the rising temperature. It will allow itself to be boiled to death. With food in your stomach you can not breathe as easily. Food also lowers your blood oxygen.