Detection of hypoventilation during deep sedation in patients undergoing ambulatory gynaecological hysteroscopy: A comparison between transcutaneous and nasal end-tidal carbon dioxide measurements

Background. Transcutaneous measurement of carbon dioxide (Tc_co2) provides a non-invasive estimation of arterial carbon dioxide (Paco2). Nasal capnography (Pe'co2) is used to assess ventilation during monitored anaesthesia care (MAC) with sedation since it can readily detect apnoea. We compared the agreement between Tc_co2 and Pe'_co2 with Pa_co2 and the ability to detect hypercarbia in patients under deep sedation. Methods. Forty healthy female subjects receiving deep sedation for hysteroscopy were studied. A Tc_co2 (TOSCA 500, Radiometer, Inc., Westlake, OH, USA) electrode was applied to the earlobe and Pe'_co2 capnography was monitored using nasal side-stream sampling. All subjects received oxygen (3 litre min^-1). Subjects were evaluated at intervals using a modified Ramsay sedation score until they reached a score ≥5. Arterial blood gas values were compared with Tc_co2 and Pe'_co2 values. Bland-Altman, linear regression, and receiver operator characteristics analysis were performed.Results. The mean (sd) absolute difference between the Tc _co2, Pe'_co2, and the Pa_co2 were 0.43 (0.35) and 1.06 (0.8) kPa, respectively (P=0.002). Tc_co2demonstrated a mean bias (2 sd) of 0.23 (0.07-0.4) kPa with Paco2 compared with -0.93 (-1.24 to -0.63) kPa for Pe'_co2. One minute before blood sampling, the sensitivity of the Tc_co2 monitor for detecting Pa_co2 >6.65 kPa was greater than for Pe'_co2 (66.7 vs 33.3, P<0.01).Conclusions. Tc_co2 demonstrated better agreement with Pa_co2 than Pe'_co2 for patients under MAC with deep sedation. Tc_co2 monitoring was more sensitive for detection of Pa_co2 >6.65 kPa than Pe'_co2.