My experience with a pulse oxymeter in an altitude chamber was that
blood oxygen levels responds very quickly to changes in available
oxygen. This was reinforced by the instructor also. Just breathing
once or twice from the mouth while using a cannula will reduce blood
oxygen, but it recovers quickly when breathing through the nose
resumes.

The altitude chamber instructor cited loss of visual acuity and night
vision over 5,000 ft. and also emphasized that prior exposure to even
very small amounts of carbon monoxide prior to or during flight
significantly reduces blood oxygen due to COs much higher affinity for
hemoglobin.

Best regards,
Craig