Sidestream sampling of gas to determine information related to the composition of gas at or near the airway of a subject is implemented. From such information one or more breathing parameters of subject 12 (e.g., respiratory rate, end-tidal CO2, etc.) are determined, respiratory events (e.g., obstructions, apneas, etc.) are identified, equipment malfunction and/or misuse is identified, and/or functions are performed. To improve the accuracy of one or more of these determinations, information related to pressure at or near the airway of subject is implemented. This information may include detection of pressure at or near a sidestream sampling cell.