Evaluation of transcutaneous and end-tidal carbon dioxide levels during inhalation sedation in volunteers

Measurement of end-tidal carbon dioxide (P_ETCO₂) is useful because of its noninvasiveness, continuity, and response time when sudden changes in ventilation occur during inhalation sedation. We compared the accuracy of P_ETCO₂ using a nasal mask and nasal cannula with the accuracy of transcutaneous carbon dioxide (TC-CO₂) and determined which method is more useful during inhalation sedation in volunteers. We used a modified nasal mask (MNM) and modified nasal cannula (MNC) for measurement of P_ETCO₂. The capnometer measured P_ETCO₂ in the gas expired from the nasal cavity by means of two devices. The volunteers received supplemental O₂ by means of each device at a flow rate of 6 L/min. After the volunteers lay quietly for 5 min with a supply of 100 % O₂, they received supplemental N₂O by means of each device at concentrations of 10, 20, and 25 % for 5 min and 30 % for 25 min. The correlation coefficient was poorer in the MNM than in the MNC, and the mean difference between TC-CO₂ and P_ETCO₂ in the MNM was greater than that in the MNC. The difference between the TC-CO₂ and P_ETCO₂ ranged from 3 to 6 mmHg in the MNM and from 2 to 5 mmHg in the MNC. The difference between two variables against the TC-CO₂ and the CO₂ waveforms obtained by means of the two devices were within the clinically acceptable range. Our two devices can provide continuous monitoring of P_ETCO₂ with a supply of N₂O/O₂ in patients undergoing inhalation sedation.