Apparatus for changing a concentration of a target gas at a blood compartment of a patient's lung from an actual target gas concentration to a desired target gas concentration during artificial ventilation with an inspiratory gas composition, the apparatus including a respirator for ventilating the inspiratory gas composition to a patient's lung, comprising target gas varying means for varying the fraction of the target gas supplied to the inspiratory gas composition, re-breathed gas varying means for varying the fraction of the re-breathed gas supplied to the inspiratory gas composition, parameter varying means for varying a set of ventilation parameters being responsible for a ventilated lung volume, and controlling means for controlling the target gas varying means, the re-breathed gas varying means and the parameter varying means, characterized in that a) the target gas is an anaesthetic agent, b) the lung is ventilated in a first ventilation stage by setting a fraction of target gas and a fraction of re-breathed gas, wherein said setting results in the actual target gas concentration, and wherein said set of ventilation parameters comprises at least a first peak inspiratory pressure and a first positive end-expiratory pressure, c) the lung is ventilated in a second ventilation stage, - wherein the set of ventilation parameters is based on a time-varying second peak inspiratory pressure above the first peak inspiratory pressure and a time-varying second positive end-expiratory pressure above the first positive end-expiratory pressure for yielding an increased ventilated lung volume compared to the first ventilation stage, and - wherein on the basis of the increased ventilated lung volume the fraction of target gas, or the fraction of re-breathed gas, or a combination thereof, is varied such that the target gas concentration is changed towards the desired target gas concentration. a) the target gas is an anaesthetic agent, b) the lung is ventilated in a first ventilation stage by setting a fraction of target gas and a fraction of re-breathed gas, wherein said setting results in the actual target gas concentration, and wherein said set of ventilation parameters comprises at least a first peak inspiratory pressure and a first positive end-expiratory pressure, c) the lung is ventilated in a second ventilation stage, - wherein the set of ventilation parameters is based on a time-varying second peak inspiratory pressure above the first peak inspiratory pressure and a time-varying second positive end-expiratory pressure above the first positive end-expiratory pressure for yielding an increased ventilated lung volume compared to the first ventilation stage, and - wherein on the basis of the increased ventilated lung volume the fraction of target gas, or the fraction of re-breathed gas, or a combination thereof, is varied such that the target gas concentration is changed towards the desired target gas concentration. - wherein the set of ventilation parameters is based on a time-varying second peak inspiratory pressure above the first peak inspiratory pressure and a time-varying second positive end-expiratory pressure above the first positive end-expiratory pressure for yielding an increased ventilated lung volume compared to the first ventilation stage, and - wherein on the basis of the increased ventilated lung volume the fraction of target gas, or the fraction of re-breathed gas, or a combination thereof, is varied such that the target gas concentration is changed towards the desired target gas concentration.