This has been noticed a number of times in me since about 2006. I had to have an exercise stress test abandoned due to my o2 sats dropping to 80%. Also when ive been admitted to hospital it has been noted that after walking my oxygen levels drop. I was curious as to what my o2 levels were doing on the days when i feel really bad so invested in an o2 monitor that has a 72 hour memory and the software to analyse the data. The results have been a real eye opener and so i discussed them with my gp who felt they needed further investigating and wrote to my hospital (cardiology) to ask them to see m