A patient interface system for delivering a gas to a patient includes a patient interface device that includes at least one inhalation valve and at least one exhalation valve. The system also includes a venturi device that has at least one port for connection to a gas source. The venturi device has at least one primary air entrainment window and at least one secondary air entrainment window which is downstream of the at least one primary air entrainment window. The inhalation valve is disposed between: (1) the main body and (2) the primary and secondary air entrainment windows of the venturi device. At least one of the primary air entrainment window and secondary air entrainment window includes a means for closing the respective window, thereby changing a degree at which the respective window is open and changing a flow rate of the air flowing through the respective window.