Background and Goal of Study
Sedation and general anesthesia especially at remote locations are hazardous because of potential upper airway obstruction. For avoidance a Guedel airway is in use. Additionally, monitoring of breathing by a CO2/O2 nasal cannula and pulse oximetry has become common standard1. In contrast to a nasal cannula, the use of a Guedel airway facilitates airway patency. However it lacks a standardized recording of CO2 and oxygen supply up to now. Moreover, a CO2 sampling
port in closer proximity to the alveolar space results in a smaller difference between the alveolar and the measured end- tidal CO22. 

The aim of this study was to compare the capnography of a modified CO2/O2 Guedel airway with a CO2/O2 nasal cannula without and with oxygen supply.

Hypothesis: Use of a modified CO2/O2 Guedel airway results in a smaller device to tracheal CO2 difference compared to a CO2/O2 nasal cannula.

Methods
A manikin of a 6-month-old infant (TruCorp, IRL) was connected to a lung model3 (Soloplex, SWE) and ventilated with respiratory rates (RR) between 20-60 min-1 and tidal volumes between 20-80 ml. Tracheal CO2 was adjusted to 51.7±1.7 mmHg (Mean±SD) and device capnography (CO2/O2 Guedel Airway or CO2/O2 nasal cannula (Salter Labs, USA)) with different O2 flows (0-2 l*min-1) was recorded. The difference of the maximal CO2 between the trachea and the devices was compared using linear regression analysis (JMP® 12, USA).

Results and Discussion
A significant smaller device to tracheal CO2 difference with a modified CO2/O2 Guedel airway compared to a CO2/O2 nasal cannula was observed. Tidal volume, device, O2 flow and RR had a statistically significant effect (Log Worth: 41.69; 35.19; 6.0 respectively 2.58; P-value < 0.002).

Conclusion
A realistic capnography could be established within the measured range of respiratory settings. Using a modified Guedel Airway in a model resulted in a significantly smaller device to tracheal CO2 difference compared to a nasal cannula over the examined O2 supply range (0-2 l*min-1).

References
1 Pediatrics 2006; 118(6):2587-602.
2 Can J Anesth 1993; 40(11):1038-43. 3 PLoS ONE 2012; 7(4): e36083-8.

Comparison of CO2 measurement using a modified CO2/O2 Guedel Airway with a CO2/O2 nasal cannula without and with oxygen supply in an infant manikin