A system for delivering a flow of breathing gas to a patients airway that includes a gas flow generator and a patient circuit for communicating the flow of gas to the patients airway. A sensor measures a characteristic associated with the flow of gas, such as flow rate, maximum average inspiratory flow, tidal volume or minute ventilation. A controller determines a first characteristic based on the measured characteristic and provides positive pressure support to the patient if the first characteristic is below a first target and provides a negative support if the first characteristic is above a second target. When the positive pressure support is provided, the pressure provided during inspiration is higher than the pressure provided during expiration. When the negative pressure support is provided, the pressure provided during inspiration is lower than the pressure provided during expiration.