Accuracy of postoperative end-tidal Pco2 measurements with mainstream and sidestream capnography in non-obese patients and in obese patients with and without obstructive sleep apnea.

BACKGROUND Obtaining accurate end-tidal carbon dioxide pressure measurements via nasal cannula poses difficulties in postanesthesia patients who are mouth breathers, including those who are obese and those with obstructive sleep apnea (OSA); a nasal cannula with an oral guide may improve measurement accuracy in these patients. The authors evaluated the accuracy of a mainstream capnometer with an oral guide nasal cannula and a sidestream capnometer with a nasal cannula that did or did not incorporate an oral guide in spontaneously breathing non-obese patients and obese patients with and without OSA during recovery from general anesthesia. METHODS The study enrolled 20 non-obese patients (body mass index less than 30 kg/m) without OSA, 20 obese patients (body mass index greater than 35 kg/m) without OSA, and 20 obese patients with OSA. End-tidal carbon dioxide pressure was measured by using three capnometer/cannula combinations (oxygen at 4 l/min): (1) a mainstream capnometer with oral guide nasal cannula, (2) a sidestream capnometer with a nasal cannula that included an oral guide, and (3) a sidestream capnometer with a standard nasal cannula. Arterial carbon dioxide partial pressure was determined simultaneously. The major outcome was the arterial-to-end-tidal partial pressure difference with each combination. RESULTS In non-obese patients, arterial-to-end-tidal pressure difference was 3.0 +/- 2.6 (mean +/- SD) mmHg with the mainstream capnometer, 4.9 +/- 2.3 mmHg with the sidestream capnometer and oral guide cannula, and 7.1 +/- 3.5 mmHg with the sidestream capnometer and a standard cannula (P < 0.05). In obese non-OSA patients, it was 3.9 +/- 2.6 mmHg, 6.4 +/- 3.1 mmHg, and 8.1 +/- 5.0 mmHg, respectively (P < 0.05). In obese OSA patients, it was 4.0 +/- 3.1 mmHg, 6.3 +/- 3.2 mmHg, and 8.3 +/- 4.6 mmHg, respectively (P < 0.05). CONCLUSIONS Mainstream capnometry performed best, and an oral guide improved the performance of sidestream capnometry. Accuracy in non-obese and obese patients, with and without OSA, was similar.