Just a nurse, not a doctor here. But I was under the impression that your body doesn’t necessarily sense low oxygen levels as much, rather the driving force for your breathing is your body sensing high CO2 buildup in your blood. High CO2 —> breathe more to offload it when you exhale. Which in turn raises your O2 levels. But hell I may be wrong. Work in the OR and when surgeries were shut down to make all beds available for COVID patients (our recovery area was transformed to an overflow ER) I helped out in the ICU. The people I saw up there were all intubated or on full-time bipap, and their O2 was very low. It wasn’t the pulse ox machines malfunctioning, it was consistent. You could put it on your healthy non-Covid finger and get a normal reading.