A respiratory treatment apparatus generates an indication of inspiration or expiration based on a measure of motor current of a flow generator. In an example, first and second current signals are derived from the measurement. The first derived current signal may be a long term measure or average of current and the second derived current signal may be a short term measure or average of current. A processor (120) determines an indication of inspiration or expiration as a function of the first and second derived current signals. The function of the first derived current signal and the second derived current signal may be a comparison of the derived current signals. The derived signals may be determined by filtering. The indication of inspiration or expiration may serve as a trigger or cycle control for changing treatment pressure in synchrony with patient respiration without measured signals from pressure, flow or speed sensors.
