A gas flow control structure for a respiratory therapy system

A gas flow control structure for a respiratory therapy system configured to function as both an anti-asphyxia valve (AAV) and a vent, wherein the gas flow control structure separates a first volume that is pressurised during use and a second volume that is the surrounding ambient air, the gas flow control structure comprising:

• a base member;
• a vent member comprising at least one vent opening through which a flow of gas can pass from the first volume to the second volume during use;
• an AAV opening formed between the base member and the vent member;
• and
• an AAV member mounted to the base member, wherein the AAV member is a flexible, resilient membrane which is substantially annular in shape when projected on to a plane, the annular shape of the AAV member forming a central hole,
• wherein the AAV member is movable between:
• an activated configuration in which gas can flow between the first volume and the second volume through the AAV opening;
• and a deactivated configuration in which the AAV member forms a seal with a portion of the vent member so gas is blocked from flowing through the AAV opening,
• wherein the AAV member is biased towards the activated configuration such that, when the first volume contains gas having a pressure that is below an activation pressure, the AAV member is in the activated configuration, and, when the first volume contains gas having a pressure that is above a deactivation pressure, the AAV member is in the deactivated configuration;
• wherein the AAV member is arranged such that gas from the first volume flows through the central hole in the AAV member when passing to the second volume through the at least one vent opening.