A pressurized flow of breathable gas is delivered to the airway of a subject in accordance with a therapy regimen. One or more fluid parameters of the pressurized flow of breathable gas are adjusted based on the therapy regimen. The therapy regimen dictates that such adjustments be made based on the respiratory state of the subject. Transitions in respiratory state are identified without relying on measurement or estimation of flow at or near the airway of subject. Transitions in respiratory state are identified based on changes in the first time derivative of flow at or near the airway of the subject. In one embodiment, an effort parameter is determined that approximates the second time derivative of flow. Based on comparisons of the effort parameter to a dynamic threshold, transitions in respiratory state are identified.