An apparatus for generating respiratory flow comprising: a patient interface to carry a flow of breathable gas to a patient; a flow generator coupled with the patient interface to generate a flow of the breathable gas through the patient interface; a pressure transducer to provide a pressure signal indicative of pressure associated with the flow generator; a tachometer to provide a velocity signal indicative of a speed of the flow generator; and a processor to control the flow generator, the processor coupled with the pressure transducer to process the pressure signal and coupled with the tachometer to process the velocity signal, the processor being configured to control: determining a measure of pressure with the pressure signal; determining a measure of frequency that is a rotational velocity of the flow generator with the velocity signal; deriving an estimate of patient respiratory flow as a function of a difference of the measure of pressure and an expected pressure derived from the measure of frequency; and comparing the estimate of the patient respiratory flow to a trigger threshold and triggering a change of a pressure generated with the flow generator based on the comparison, the trigger threshold being a function of a peak value of an estimate of patient respiratory flow of a prior respiratory cycle. a patient interface to carry a flow of breathable gas to a patient; a flow generator coupled with the patient interface to generate a flow of the breathable gas through the patient interface; a pressure transducer to provide a pressure signal indicative of pressure associated with the flow generator; a tachometer to provide a velocity signal indicative of a speed of the flow generator; and a processor to control the flow generator, the processor coupled with the pressure transducer to process the pressure signal and coupled with the tachometer to process the velocity signal, the processor being configured to control: determining a measure of pressure with the pressure signal; determining a measure of frequency that is a rotational velocity of the flow generator with the velocity signal; deriving an estimate of patient respiratory flow as a function of a difference of the measure of pressure and an expected pressure derived from the measure of frequency; and comparing the estimate of the patient respiratory flow to a trigger threshold and triggering a change of a pressure generated with the flow generator based on the comparison, the trigger threshold being a function of a peak value of an estimate of patient respiratory flow of a prior respiratory cycle. determining a measure of pressure with the pressure signal; determining a measure of frequency that is a rotational velocity of the flow generator with the velocity signal; deriving an estimate of patient respiratory flow as a function of a difference of the measure of pressure and an expected pressure derived from the measure of frequency; and comparing the estimate of the patient respiratory flow to a trigger threshold and triggering a change of a pressure generated with the flow generator based on the comparison, the trigger threshold being a function of a peak value of an estimate of patient respiratory flow of a prior respiratory cycle.