In certain example embodiments, an air delivery system includes a controllable flow generator operable to generate a supply of pressurized breathable gas to be provided to a patient for treatment and a pulse oximeter. In certain example embodiments, the pulse oximeter is configured to determine, for example, a measure of patient effort during a treatment period and provide a patient effort signal for input to control operation of the flow generator. Oximeter plethysmogram data may be used, for example, to determine estimated breath phase； sleep structure information； autonomic improvement in response to therapy； information relating to relative breathing effort, breathing frequency, and/or breathing phase； vasoconstrictive response, etc. Such data may be useful in diagnostic systems.
