A respiratory anesthetic aggregate devised for coupling to a breathing apparatus has an anesthetic gasifier unit that has a breathing gas input port connected to a first point in an inspiratory patient gas delivery path of a breathing apparatus, and a fresh gas output port connected to a second point in the patient gas delivery path downstream the first point. The inspiratory patient gas delivery path is connected via a mainstream connection between the first and second point. Thereby the anesthetic gasifier unit is connected in a sidestream configuration to the patient gas delivery path for adding the gasified anesthetic agent in a desired concentration to the patient gas delivery path at the second point. When aggregate is coupled to a breathing apparatus, a substantially volume neutral delivery of anesthetic agent is provided to the breathing apparatus.