Paywall, https://journals.lww.com/acsm-msse/Abstract/9000/Exercise_Intolerance_in_Cystic_Fibrosis_.96154.aspx Sci hub, unable to access.
Presumably they are monitoring blood oxygen levels as well, which tend to be lower i Cf due to lung pathology - "sticky mucus".
Posts about a study on use of sildenafil in chronic fatigue syndrome have been moved to a new thread here: Use of sildenafil (viagra) to alter fatigue, functional status and impaired cerebral blood flow in ... CFS, 2020, Pfizer