A high frequency positive pressure ventilation (HFPPV) support system, comprising: a pressure generating system (4,16); a patient circuit operatively coupled to the pressure generating system (4,16) and a patient; and a controller operatively coupled to the pressure generating system (4,16), the controller being adapted to: control the pressure generating system to deliver a flow of breathing gas to the patient through the patient circuit, the flow of breathing gas having a first positive pressure level and a second positive pressure level, the first and second positive pressure levels alternating with one another in a plurality of cycles in the flow of breathing gas to have a frequency and an amplitude, the flow of breathing gas having a mean airway pressure; and determine whether the patient is breathing spontaneously or is trying to breath spontaneously based on data from at least one of a flow sensor and pressure sensor of the patient circuit, or whether there is change in user settings for high frequency ventilation based on a user input, and characterized in that the controller is further adapted to, via simultaneous control, (i) adjust an active exhalation valve of the patient circuit to adjust the mean airway pressure, (ii) control the pressure generating system to modulate the frequency and duty cycle of the flow of breathing gas, and (iii) control the pressure generating system to modulate the amplitude of the breathing gas, to implement an HFPPV mode with positive pressure pulses at frequencies from 2 Hz to 25 Hz, in response to determination that the patient is breathing spontaneously or is trying to breath spontaneously, or that the user wants to deliver certain sub-physiologic or physiologic volume to the patient. a pressure generating system (4,16); a patient circuit operatively coupled to the pressure generating system (4,16) and a patient; and a controller operatively coupled to the pressure generating system (4,16), the controller being adapted to: control the pressure generating system to deliver a flow of breathing gas to the patient through the patient circuit, the flow of breathing gas having a first positive pressure level and a second positive pressure level, the first and second positive pressure levels alternating with one another in a plurality of cycles in the flow of breathing gas to have a frequency and an amplitude, the flow of breathing gas having a mean airway pressure; and determine whether the patient is breathing spontaneously or is trying to breath spontaneously based on data from at least one of a flow sensor and pressure sensor of the patient circuit, or whether there is change in user settings for high frequency ventilation based on a user input, and control the pressure generating system to deliver a flow of breathing gas to the patient through the patient circuit, the flow of breathing gas having a first positive pressure level and a second positive pressure level, the first and second positive pressure levels alternating with one another in a plurality of cycles in the flow of breathing gas to have a frequency and an amplitude, the flow of breathing gas having a mean airway pressure; and determine whether the patient is breathing spontaneously or is trying to breath spontaneously based on data from at least one of a flow sensor and pressure sensor of the patient circuit, or whether there is change in user settings for high frequency ventilation based on a user input, and characterized in that the controller is further adapted to, via simultaneous control, (i) adjust an active exhalation valve of the patient circuit to adjust the mean airway pressure, (ii) control the pressure generating system to modulate the frequency and duty cycle of the flow of breathing gas, and (iii) control the pressure generating system to modulate the amplitude of the breathing gas, to implement an HFPPV mode with positive pressure pulses at frequencies from 2 Hz to 25 Hz, in response to determination that the patient is breathing spontaneously or is trying to breath spontaneously, or that the user wants to deliver certain sub-physiologic or physiologic volume to the patient.