A respirator for breathing gas supply , comprising a breathing gas source , a control unit , a memory , a pressure sensor apparatus and/or a flow sensor apparatus , a replaceable breathing gas tube , at least one connection nozzle for the breathing gas tube, and a patient interface , wherein the control unit is configured and designed to actuate the breathing gas source in order to specify a substantially constant CPAP pressure that is maintained regardless of the breathing phase of the patient and wherein the control unit is configured and designed, based on the signals of the pressure sensor apparatus and/or flow sensor apparatus , • to ascertain the breathing phase - inspiration and expiration - of the patient, • to ascertain the current tidal volume of the patient during successive inspirations and expirations, • to compare at least one first, specified volume threshold for the tidal volume with the current tidal volume, • to determine whether the current tidal volume is lower than the first volume threshold, • in response to said current tidal volume being lower, ∘ to actuate the breathing gas source in order to specify a second pressure IPAP for the breathing gas for the inspiration, wherein the second pressure IPAP is greater than the CPAP pressure and the second pressure IPAP is increased stepwise until the specified volume threshold for the tidal volume is reached and/or in that the control unit increases the second pressure IPAP from one inspiration to the immediately subsequent inspiration, ∘ to actuate the breathing gas source in order to specify the CPAP pressure for the breathing gas for the expiration, wherein the respirator comprises at least one valve which is arranged in the breathing gas tube or in the respirator, characterized in that the control unit is configured and designed to specify the CPAP pressure to pressure values below 4 hPa, such that CO2 is reliably washed out of the exhalatory air by means of the valve even at low pressures. • to ascertain the breathing phase - inspiration and expiration - of the patient, • to ascertain the current tidal volume of the patient during successive inspirations and expirations, • to compare at least one first, specified volume threshold for the tidal volume with the current tidal volume, • to determine whether the current tidal volume is lower than the first volume threshold, • in response to said current tidal volume being lower, ∘ to actuate the breathing gas source in order to specify a second pressure IPAP for the breathing gas for the inspiration, wherein the second pressure IPAP is greater than the CPAP pressure and the second pressure IPAP is increased stepwise until the specified volume threshold for the tidal volume is reached and/or in that the control unit increases the second pressure IPAP from one inspiration to the immediately subsequent inspiration, ∘ to actuate the breathing gas source in order to specify the CPAP pressure for the breathing gas for the expiration, ∘ to actuate the breathing gas source in order to specify a second pressure IPAP for the breathing gas for the inspiration, wherein the second pressure IPAP is greater than the CPAP pressure and the second pressure IPAP is increased stepwise until the specified volume threshold for the tidal volume is reached and/or in that the control unit increases the second pressure IPAP from one inspiration to the immediately subsequent inspiration, ∘ to actuate the breathing gas source in order to specify the CPAP pressure for the breathing gas for the expiration, <EOS>