Methods and apparatus provide automated circuit disconnection monitoring such as for a respiratory apparatus or system. Disconnection of a patient circuit, including a patient interface and air delivery circuit, may be detected and a message or alarm activated. In some versions, detecting occurrences of circuit disconnection event(s), such as by a processor, may be based on an instantaneous disconnection parameter as a function of a disconnection setting. The disconnection setting may be determined based on patient circuit type. The instantaneous disconnection parameter may be determined from detected pressure and flow rate, and may be, for example, a conductance value or an impedance value. Disconnection events may be qualified by one or more detected respiratory indicators. In some cases, instantaneous impedance or conductance may be used to assess re-connection of a patient circuit, detection of flow starvation, determine breath shape for triggering and cycling and to detect patient or circuit obstructions.