1. A method for identifying obstructed breathing by a patient undergoing continuous positive airway pressure therapy, including at least one of an apnea condition and a hypopnea condition, the method comprising: receiving a signal representative of a flow rate of therapeutic gas being delivered to the patient for a predetermined time window over a plurality of respiration cycles; deriving a peak flow value of each of the respiration cycles within the predetermined time window; deriving a minimum flow value of each of the respiration cycles within the predetermined time window; generating a polynomial difference function from the peak flow values of a plurality of the respiration cycles in the predetermined window and the minimum flow values of a plurality of the respiration cycles in the predetermined window; deriving a respiration index from a minimum of the polynomial difference function; and indicating the at least one of the apnea condition and the hypopnea condition based upon a comparison of the respiration index to a predefined obstruction threshold over a predefined obstruction time period.