Apparatus, such as a respiratory therapy device 4000, may generate a signal representing an estimate of a patient breathing pattern when using a flow therapy device with an un-sealed patient interface. The device may include a blower and may generate a flow of air at a controlled flow rate. The apparatus may include a controller. The generation may involve computing an estimate of the breathing pattern as a function of a device flow rate signal; a device pressure signal; a device conductance; a patient conductance; and a coupling parameter that characterizes a degree of coupling between the interface and the patients nares. An estimate of an exit pressure representative of a pressure of a flow of air generated by the therapy device just outside an orifice of the unsealed interface may also be computed. An estimate of a flushing flow rate from the estimated exit pressure may also be computed.