An anesthetic breathing apparatus and system having a patient circle system (7) for re-breathing exhaled gases by a patient is disclosed having a volume reflector (26), a fresh gas delivery line (18), and a gas sensor unit (640) arranged to measure a gas stream upstream a fresh gas connection (19) and downstream said reflector unit (26) and provides a signal for detection of a reflector driving gas (RDG) crossing over said volume reflector (26) during inspiration based on said at least one property of said gas stream measured by said gas sensor unit (640). Appropriate action may be taken based on this measurement, for instance in disclosed methods.
