A medical mouthpiece may include a generally U-shaped member forming a lateral interocclusal passageway extending from an anterior aperture to a posterior aperture configured to provide a conduit for respiratory gas exchange. A medical mouthpiece may include a sampling port for fluid communication with an analyzing apparatus configured to analyze exhaled respiratory gases. A medical mouthpiece may include a supplemental gas conduit for fluid communication between a source of supplemental gas and a supply orifice. A method for delivering a gas to a patient may include delivering a gas to the patient through a mouthpiece having a lateral interocclusal passageway such that the gas is delivered through a posterior aperture to a space adjacent the patient's posterior oropharynx.