The Science Journal of the American Association for Respiratory Care

2008 OPEN FORUM Abstracts

COMPARISON OF MEAN EXPIRED CO2 MEASUREMENTS CALCULATED USING THE DRAGER XL VENTILATOR VOLUMETRIC CAPNOGRAPHY VS THE RESPIRONICS NICO2 MONITOR

Mark Siobal1, Julin Tang1, Hannah Ong2, Wesley Toy3



Background: Calculation of VD/VT requires a PeCO2 measurement. The Drager XL Ventilator is equipped with integrated volumetric CO2 monitoring and calculates VCO2. This enables calculation of PeCO2 using ventilator measurements of VCO2 and VE, which is corrected for circuit compression volume. By dividing the VCO2 by the VE, the fraction of exhaled CO2 (FeCO2) can be calculated (FeCO2 = VCO2 / VE). PeCO2 can then be calculated by multiplying FeCO2 and the barometric pressure minus water vapor pressure (PeCO2 = FeCO2 (760 - 47). PeCO2 can then be used to calculate VD/VT using the Enghoff modification of the Bohr equation: (VD/VT = ( PaCO2-PeCO2) / PaCO2). We calculated PeCO2 from the Drager XL Ventilator measurements and compared it to PeCO2 measurements from the Respironics NICO2 Monitor during simulated ventilation.

Methods:
A Drager XL Ventilator set to Vt = 500mL, RR = 10, Insp. Time = .75 sec., Insp. Flow = 50 L/min, and FiO2 = .50 was attached to a single chamber of a Michigan Instruments Test Lung. The ventilator flow sensor and mainstream CO2 sensor, and the NICO2 monitor combined CO2 / flow sensor were calibrated and attached between the ventilator circuit and test lung chamber. 100% CO2 bleed-in to the test lung chamber was adjusted until an end tidal CO2 (ETCO2) of 36 - 37 mm Hg was displayed on the NICO2 monitor. PeCO2 calculated by NICO2 monitor and PeCO2 calculated from the ventilator measurements were recorded. Ventilation of the test lung was then changed in increments of 1 L/min by increasing the RR by 2 breaths per minute increments up to a RR of 30. PeCO2 calculated by the NICO2 monitor and PeCO2 calculated from the ventilator measurements were recorded at each step. The proximal and distal position of the ventilator CO2 sensor, and the NICO2 combined CO2 / flow sensor were then reversed and the measurements were repeated starting from an ETCO2 of 46 - 47 mm Hg and again from an ETCO2 of 63 - 65 mm Hg. A total of 33 measurements were recorded and compared.

Results:
There was a strong correlation between Drager XL Ventilator and NICO2 monitor measurements of PeCO2 (r = 0.9874, r squared = 0.9750, p < 0.0001).

Conclusion:
PeCO2 calculated directly from the Drager XL Ventilator measurements are in strong agreement with PeCO2 measurements from the NICO2 monitor during simulated ventilation. In-vivo comparison of the two PeCO2 measurements needs to be validated.