Abstract: A device for providing a gas-air mixture to a patient in response to suction applied by the patient, comprising means defining an air inlet, a backward-facing step, an air passage for directing air past said backward-facing step, a non-return valve for controlling flow of air from said inlet to said air passage and a gas passage communicating with said air passage through at least one orifice adjacent to said backward-facing step.

Abstract: Air flow control means comprising a substantially annular member having an outer surface and an inner surface, a port formed through said member and connecting said surfaces, a flexible, resilient band having at least one passage therethrough, said band surrounding and slidable on said outer surface and the shape of said outer surface being such that said band follows a path that is spaced from said outer surface in at least two spaced-apart regions thereof and contacts said outer surface in at least two further regions thereof, one of said further regions being the region of said port, the distortion of said band being such that said band is pulled into sealing engagement with said outer surface in the region of said port.