Accuracy Of End-tidal Carbon Dioxide Measurements Via The Nasopharyngeal Airway During Monitored Anesthesia Care

Objective To evaluate the accuracy of end-tidal carbon dioxide(PETCO2) measurements compared with the arterial carbon dioxide(PaCO2) via the nasopharyngeal airway in spontaneous breathing patients during monitored anesthesia care(MAC), and to analyze the influence of the fresh gas flow(FGF).Methods Twenty patients ASAI ~ II status aged≥20, undergoing elective CSEA and intravenous anesthesia were enrolled in this study, maxillofacial deformities, head fractures, nasal obstruction, coagulation dysfunction, cardio-pulmonary dysfunction and full stomach were excluded. Right foot dorsal artery was catheterized and sealed with heparin cap after local anesthesia, then midazolam 0.04mg/kg, sufentanil 0.2ug/kg was applied, followed by propofol 50mg intravenous bolus and 3 mg.kg-1.h-1 TCI to maintain sedation, the nasopharyngeal airway was set and connected to a mainstream PETCO2 measuring instrument as well as the screwed pipe(picture 1). All patients were divided into three groups according to the difference of the FGF:1L/min, 3L/min, 6L/min, 10 minutes after the stability of PETCO2 wave and value, PETCO2 was recorded and arterial blood samples was taken for gas analysis. it's need to calibrate the end-tidal carbon dioxide monitor after adjusting the oxygen flow every time. To analyze the correlation of PETCO2 and PaCO2,to calculate the difference between the PETCO2 and PaCO2(Pa-ETCO2),and to further analyze the difference of the FGF.Results PETCO2 well correlated with PaCO2 via the nasopharyngeal airway. According to the difference of FGF:1L/min, 3L/min, 6L/min, the correlation coefficient r was 0.950, 0.904, 0.852 (P <0.01), respectively; Pa-ETCO2 (mean±SD) was (4.3±1.1)mmHg, (8.5±1.3) mmHg, (13.5±1.5)mmHg, respectively; the difference of Pa-ETCO2 between every two groups were statistically significant (P <0.01).Conclusion The way of the nasopharyngeal airway insertion was reasonable to continuously monitor PETCO2, which can be well correlated with PaCO2. The measure not only provided a solution to deal with respiratory obstruction but also provided a good way to strengthen the monitoring of respiratory tract for the MAC. The value of Pa-ETCO2 was significant difference among the three groups, when FGF was 1L/min, the Pa-ETCO2 was minimum. We think that, on the condition of well-oxysenation for patients, reduction of the FGF is benefit for saving oxygen, as well as monitoring of PETCO2.