An Expiratory breathing simulator device is provided and configured to simulate a real patient's ETCO2. The device is configured to be used with a vital signs simulator to simulate a patient's behaviour on a medical monitoring apparatus, for example for training medical professionals.The device comprises a housing comprising:a) a CO2 inlet configured to receive CO2 from a CO2 source；b) a CO2 outlet from which CO2 exits the interface housing, the CO2 outlet being configured to be connected to a medical monitoring apparatus；c) a CO2 flow path through the housing from the a CO2 inlet to the CO2 outlet；d) a CO2 pressure adjuster means configured to control the pressure of CO2 in the flow path,e) at least one sensor configured to measure a property of the CO2 in the housing；f) an air pump configured to deliver a controlled flow of air into the flow of CO2 in the flow pathg) a controller configured to control the pressure adjuster to control the flow of CO2 output from the a CO2 outlet to generate a flow of CO2 from the a CO2 outlet having a desired or predetermined waveform；h) the controller being further configured to control the air pump to deliver air into the CO2 flow to prevent a value of ETCO2 being significantly greater than zero during inspiration.
