Cuffed oropharyngeal airway and capnometry: Comparison of end-tidal and arterial carbon dioxide pressures

Purpose. The aim of this study was to investigate the reliability of end-tidal CO2 tension (PETCO2) as a predictor of PaCO2 during anesthesia in patients breathing spontaneously via a cuffed oropharyngeal airway (COPA). Methods. Twenty adult patients scheduled for minor sur gery were included in this study. After propofol injection: an appropriate size of COPA was inserted. Anesthesia was maintained with 60% nitrous oxide in oxygen (total flow rate of 51.min-1) supplemented with propofol infusion. The patients were allowed to breathe spontaneously throughout the procedure. PaCO2 and PETCO2 were simultaneously measured when a steady state of anesthesia was reached. Results. PaCO2 (48.8 ± 5.4 mmHg, range 36.2-58.0 mmHg) was higher than PETCO2 (43.1 ± 4.2 mmHg, range 32-51 mmHg) in all patients. The difference between end-tidal and arterial CO2 tension was 5.7 ± 3.2 mmHg (range 0.5-13.0 mmHg), and was significantly correlated with PaCO2 (P < 0.01). Conclusion. The results of this study suggest that PETCO2 in anesthetized patients breathing spontaneously through a COPA is sometimes unreliable as an indicator of PaCO2 level, and there is some possibility of unexpected hypercapnia.