A pressure-assisted breathing system is provided that comprises: a pressure-generating circuit for maintaining a positive pressure within the system; a patient interface device coupled to a patient's respiratory system; a respiratory circuit for providing gas communication between the pressure-generating circuit and the patient interface device; means for introducing aerosol particles into the gas flow in the respiratory circuit; and means for discontinuing the introduction of aerosol particles into said respiratory circuit gas flow when the patient exhales. In one embodiment, a flow sensor is disposed in an auxiliary circuit in fluid communication with the respiratory circuit and electronically coupled with a nebulizer. The flow sensor is adapted to detect changes in the volumetric flow rate of gas in the auxiliary circuit when the patient exhales and stops exhaling and sends corresponding electronic signals to the nebulizer to turn off and turn on, respectively.