A patient interface assembly for use with gas monitoring. The patient interface includes a mask shell having a portion that seals against the face of a patient and a mask attachment coupled to an opening of the shell. The mask attachment has a mask connection section coupled to the opening of the shell, an airway adapter section that serves as a gas measurement site for use in measuring a respiratory variable, and a breathing system connection section adapted to be coupled to a to a patient circuit. The gas measurement site, which includes a window, may be disposed on the conduit, the shell, or both. An access port, defined in the mask, the mask attachment, or both permits a sampling tube to be placed in fluid communication with an airway of a patient.