A capnography system (100, 400), comprising: a controller (110, 410) configured to obtain a sample gas flow from a physical interface (107) for a patient (101)； determine a change in a characteristic of the sample gas flow during a sampling time interval； determine whether the change in the characteristic of the sample gas flow during the sampling time interval is equal to or greater than a corresponding threshold value； determine that supplemental oxygen is provided when it is determined that the change in the characteristic of the sample gas flow is equal to or greater than the threshold value； and determine that supplemental oxygen is not provided when it is determined that the change in the characteristic of the sample gas flow is less than the threshold value.
