I have an early case of COPD. I've been running for nearly 30 years (I'm 83), and today I did 9 miles at a moderate pace (about 19 minute miles) on a mountain trail at around 5-7000 ft elevation. After I stopped, I checked my oxygen with the little finger-clip tester that I just got, and it came up as 54. I'm normally in the mid-90s. My heart rate was at 60, and had averaged about 120 during the run. I felt comfortable and wasn't breathing hard. I checked it again after I got home and it was back at 97. Should I be worried about this?