A flexible patient cassette that can be optimized for different types of breathing circuits, different drive circuits and different patient categories has a first inlet arranged to be connected to a drive circuit, and at least a second inlet arranged to be connected to a patient connector, the first and second inlets being pneumatically connected with each other through a gas conducting passage constituting a patient circuit having a certain volume. The patient cassette has a volume-varying arrangement for varying the patient circuit volume to allow that volume to be varied in dependence of at least one of the type of breathing circuit in which the patient cassette is used; the type of drive circuit to which the patient cassette is connected; the tidal volume of a patient connected to the patient connector.