A naso-oral device (10) for use in medical procedures such as endoscopy includes a bite block (12) and a gas manifold or nasal cannula (14) which are releasably engageable. When engaged, gas supplied to the gas manifold passes, in use, (a) towards a patient's nostrils, through openings (58) in the gas manifold; and (b) into the patient's mouth via a passageway (28) through the bite block. Exhaled air is sampled for CO