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Respiratory monitoring is standard after anaesthesia and surgery. Abnormal respiratory rate is a sensitive indicator of respiratory problems, even in patients receiving supplemental oxygen, but the best method for its continuous measurement in spontaneously breathing patients is unclear. This study compared respiratory rate assessment by capnometry using a new oxygen mask with a carbon dioxide sampling port (Capnomask®) and thoracic impedance pneumography with clinical measurement (used as a reference method) in extubated patients receiving supplemental oxygen.Adult males admitted to the post-anaesthesia care unit after general anaesthesia were studied. Immediately after extubation, a Capnomask® connected to a capnometer was positioned appropriately. Respiratory rate was measured by visual inspection of chest movement for 1 min, by capnometry, and thoracic impedance pneumography. One set of measurements was obtained for every patient receiving supplemental oxygen at different flow rates.Twenty men, mean (inter-quartile range) age 54 (23–66) yr and BMI 25 (21–31) kg m−2, were studied. Compared with visual inspection, the bias and limits of agreement were 0.0 (1.0 to −1.0) bpm for the Capnomask® and −2.2 (2.0 to −6.5) bpm for the impedance pneumography. The accuracy of respiratory rate assessment using Capnomask® was not influenced by the supplemental oxygen flow rate.In extubated patients, continuous assessment of respiratory rate with the Capnomask® is more accurate than by thoracic impedance pneumography even when supplemental oxygen is delivered at a high flow rate.