A controller or processor for a respiratory pressure treatment device determines an estimate of patient respiratory flow based on a measure of pressure associated with flow generated by a flow generator and a measure of frequency of the flow generator, such as a rotational velocity of a servo-controlled blower motor. The estimate can be made without data from a flow sensor. The measure of frequency may be converted to an expected pressure based on characteristics of the flow generator and subtracted from the measured pressure to determine the flow estimate. The flow estimate can be implemented in the provision of respiratory pressure treatment with the flow generator. For example, the flow estimate may be utilized to trigger expiratory pressure relief during a patient's expiration as detected with the estimated flow signal.
