1. A method for detecting an interruption in the supply of breathing gas to a patient, comprising the steps of: during a predetermined time period within at least one breathing cycle of a patient initiated solely by an operator and significant of a reference time period during which a normal breathing cycle of the patient occurs, sensing the pressure in the patient's breathing gas; deriving from the pressure sensed during the reference time period, reference breathing information including a reference breathing cycle waveform parameter, storing, in memory apparatus, the reference breathing information including said reference breathing cycle waveform parameter derived from the pressure sensed during said reference time period; after said reference time period, sensing the pressure in the patient's breathing gas and deriving from the pressure sensed after said reference time period, active breathing information including an active breathing cycle waveform parameter; comparing the active breathing information including said active breathing cycle waveform parameter derived from the pressure sensed after said reference time period with said reference breathing information including said reference breathing cycle waveform parameter; and, producing an alarm signal upon detection, during said comparing step, of a predetermined variation between the breathing cycle waveform parameters of said reference breathing information and said active breathing information, respectively.