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.