A ventilatory assist system and method are disclosed. The system comprises a tube for connection to a patient's airway, inspiratory and expiratory tube lumens connected to the tube, an inspiratory air source connected to the inspiration tube lumen, and a controller of air pressure in the expiratory tube lumen. The pressure controller is responsive to a physiological breathing signal representative of patient's inspiratory effort to allow air flow through the expiratory tube lumen during a patient's expiration phase, partially restricting the air flow through the expiratory tube lumen to a so minimum air flow during a patient's inspiration phase. During both respiratory phases, a unidirectional air flow is produced through the inspiratory and expiratory tube lumens to prevent air expired by the patient from being breathed again. The physiological breathing signal allows synchronization of the ventilatory assist with breathing efforts of the patient.