The disclosure provides a way to supplement the tidal volume delivered to the patient by a leaking re-breather when the delivered volume becomes less than that set by the ventilator (in either pressure-regulated or volume modes). This may be accomplished with a shunt—；a gas conduit joining the non-patient side of the re-breather to the patient side. A low-resistance, plenum or a draw-over vaporizer may also be incorporated into the gas pathway. Such a device may include a housing with a movable partition separating an actuating side from a patient side. The housing includes a ventilator orifice for pneumatic communication between a ventilator and the actuating side and a patient orifice for pneumatic communication between the patient side and a patient. A shunt defines a bypass flow path from the actuating side and to the patient side when the moveable partition is at a maximal displacement towards the patient side.
