An evaluation of a transcutaneous and an end-tidal capnometer for noninvasive monitoring of spontaneously breathing patients

BACKGROUND: Since there is a growing use of analgesia and sedation in spontaneously breathing patients undergoing diagnostic or therapeutic interventions, recommendations by national societies of anesthesiologists call for the application of capnometry during all anesthetic procedures. METHODS: We compared readings from a transcutaneous capnometer (Tosca) and an end-tidal capnometer (Microcap Plus) to P_aCO2 measurements made via arterial-blood-gas analysis. We studied 30 spontaneously breathing patients who were recovering from general anesthesia, and we used Bland Altman analysis to compare the capnometry readings to the arterial-blood-gas values. Expiratory gas samples for end-tidal capnometry were taken either from a conventional face mask or an oral/nasal cannula. RESULTS: The Tosca significantly overestimates P_aCO2 (mean ± SD difference 5.6 ± 3.4 mm Hg). The Microcap Plus significantly underestimates P_aCO2 (mean ± SD difference -14.1 ± 7.4 mm Hg). There was no significant difference between the face mask and oral/nasal cannula with regard to collecting end-tidal samples. CONCLUSION: Both the Tosca and Microcap Plus provide just an approximate estimation of P_aCO2. Clinical use of these monitors can not be proposed under actual conditions but will be advantageous after correction of the limiting errors.