Lung function tests

[Da Boys]

My husband is having lung function tests next week. He has limb onset ALS, is able to work but now using a cane. He is currently wearing a stretchy band around his upper stomach area to give him extra support because he has weakness in his intercostal muscles.
My question is: at what point is it necessary for some form of intervention to support my husband's breathing? ie what do the test results need to show.
Also, what form is the intervention likely to take? ie medication? Bipap?
Thanks for your help.