Patent ID: 6986352

Claim:
A mask for delivering breathable gas to a user, comprising: a mask shell having a portion adapted to receive a supply of pressurized breathable gas and a user side; a gusset portion having a first side attached to the user side of the shell and having a second side; a cushion having a first portion constructed and arranged to attach to the second side of the gusset portion and a second portion constructed and arranged to contact a user's face in use and provide a seal between the mask and the user's face; and a headgear constructed and arranged to attach the mask shell to the user, said headgear including at least one headgear strap; wherein the gusset portion is constructed and arranged such that it can be selectively moved within a range of displacement to be set at a distance between the mask shell and the cushion, the gusset portion defining a gusset area exposed to the supply of pressurized breathable gas in use such that the supply of pressurized breathable gas acting on the gusset area provides a component of a contact force F c of the cushion on the user's face, and wherein the mask shell, gusset portion, cushion and headgear are structured and arranged with respect to one another in use so that the force F c is maintained in approximately constant proportion to the pressure of the supply of pressurized breathable gas, and a total force of the mask on the face F m is maintained within a range of about 35-108 grams per gf/cm 2 pressure of the supply of pressurized breathable gas to thereby maintain the seal between the mask and the user's face over an operating pressure range of the mask, including a minimum pressure of the operating pressure range, without adjusting the headgear strap.