I heard about someone having life threatening problems from using a high elevation training mask. This male had an unknown lung condition which predisposed him to severe pain that was not being properly diagnosed; because, the doctor does not know this device is being used so they do not know to look extra carefully at the tests and images. Please be sure to tell the medical staff every little detail of training and lifestyle to get the best care possible that just may save your life and get you pain free quicker.
There are some good analyses in this thread as well.
Some other studies on the efficacy of the masks.
If I was going to implement it’s use, I would definitely make sure the client is clear to use such a tool by a doctor.