A respiratory event determination system can have a controller that determines the presence of a respiratory event. The respiratory event can be a mouth puff event. The controller determines the presence as a function of a sub-window of an expiratory window of the breath. The expiratory window extends between a first time ti and a second time t2. The sub-window is limited to a portion of the expiratory window. The sub-window can extend between a third time t3 and a fourth time I4. The fourth time U can be before the second time t2.
