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BACKGROUND: Capnometry detects hypoventilation earlier than pulse oximetry while supplemental oxygen is being administered. We compared the end-tidal CO2 (...) measured using a newly developed oxygen nasal cannula with a CO2-sampling port and the ... in extubated subjects after abdominal surgery. We also investigated whether the difference between ... and ... is affected by resting, by spontaneous breathing with the mouth consciously closed, and by deep breathing with the mouth closed. METHODS: Adult post-abdominal surgery subjects admitted to the ICU were enrolled. After extubation, oxygen was supplied at 4 L/min using a capnometry-type oxygen cannula. The breathing frequency, ... and ... were measured after 30 min of oxygen supplementation. PETro was continuously measured during rest, during breathing with the mouth consciously closed, and during deep breathing with the mouth closed. The difference between ... and ... during various breathing patterns was analyzed using the Bland-Altman method. RESULTS: Twenty subjects were included. The bias ± SD (limits of agreement) for breathing frequency measured by capnometry compared with those obtained by direct measurement was 0.4 ± 3.6 (-6.7 to 7.4). In ... compared with ..., the biases (limits of agreement) were 14.8 ± 8.2 (-1.3 to 30.9) at rest, 10.2 ± 6.4 (-2.3 to 22.7) with the mouth closed, and 7.7 ± 5.6 (-3.2 to 18.6) for deep breathing with the mouth closed. ... determined using the capnometry device yielded unreliable and widely ranging values under various breathing patterns. However, deep breathing with the mouth closed decreased the difference between ... and ..., as compared with other breathing patterns. CONCLUSIONS: ... measurements under deep breathing with mouth closed with a capnometry-type oxygen cannula improved the prediction of the absolute value of ... in extubated post-abdominal surgical subjects without respiratory dysfunction. [ABSTRACT FROM AUTHOR]