opening on the order of the unexpanded state of the variable orifice with a fixed orifice device much of the exhaled gas backs up into the source tube and may be re inhaled by the patient during subsequent inhalation advantageously the variable orifice cap may be used with any form of positive nasal airway pressure therapy e g ncpap or bipap u2122 therapy of course numerous modifications and alterations to the variable orifice will occur to those skilled in the art by way of example only the stretchable orifice defining surface could be mounted to the mask in a variety of ways such as by mounting the stretchable membrane directly to a surface of the device across an opening such as by gluing a latex rubber membrane defining the variable orifice to an inside surface of the mask over an opening similarly a substantially flat variable orifice defining member could be placed over aperture 34 with a substantially rigid open centered cap placed over the orifice defining member to capture the orifice defining member under the cap in addition it will be understood that the variable orifice member can be positioned at other locations than shown in the figs as long as the variable orifice member is placed reasonably close to the patient 39 s nose along the path of the gas supply these and other modifications will occur to those skilled in the.