My resting is around 90 and goes up to around 175 every time I run. But, if I were to ease up on my running, I wouldn't feel like I was working as hard, so I think its different for everyone.
I think the rule of thumb for a max heart rate is to take 220 and minus your age and that is the number you NEVER want to go above. For example, 220-28=192 for my MAX HR. I usually take another 10 beats or so off this to account for having CF, so if I go above 180, I slow down.
Honestly, I don't test my O2 when running cause my finger is never steady to get a good reading, but this would be awesome info. to have.