An adapter couples a patient's breathing circuit to a ventilator. In the preferred embodiment, the adapter comprises an inhalation conduit having a first end for coupling the adapter to the ventilator and a second end for coupling the adapter to the patient breathing circuit; an exhalation conduit having a first end for coupling the adapter to the patient breathing circuit and a discharge conduit, the discharge conduit transversely oriented to the inhalation conduit; an exhaust port communicating with the discharge conduit for releasing breathed gas from same; a valve selectively opening and closing the discharge conduit to release gas from same; and a base comprising means for releasably coupling the adapter to the ventilator.