heartrate

hey guys, i'm looking for the explanation of relationship between arterial oxygen saturation and heart rate, i've been searching on google but got stuck in the situation where i have to pay an account for reading journal with the same topic. could anyone explain to me please? thank you.

So I got a Kaplan question saying that Ach administration INCREASES heart rate? I now get the mechanism they're going for. Ach --> peripheral vasodilation --> reflex tachycardia. But...like...what about the M2 muscarinic receptor on the heart?? Wouldn't Ach directly cause bradycardia by acting...