The Science Journal of the American Association for Respiratory Care

2007 OPEN FORUM Abstracts

FUNCTIONAL RESIDUAL CAPACITY MEASUREMENT USING A NEW, ON-AIRWAY OXYGEN SENSOR

L. Brewer1, J. Orr1


Background:
Functional Residual Capacity (FRC) is the volume of gas left in the lungs at the end of each expiration; having optimal FRC is critical for gas exchange. FRC may be a valuable parameter for use during PEEP titration in accordance with the goals of an open-lung ventilation strategy. The nitrogen washout method of FRC measurement evaluates the volume of excreted N2 and the corresponding change in N2 concentration to calculate FRC during mechanical ventilation. The aim of this study was to use a new, integrated O2, CO2 and flow sensor to measure FRC in a bench setup.

Methods:
The new, fast on-airway oxygen sensor (Respironics, Inc, Wallingford, CT) employs the photo-luminescence quenching principle. This new O2 sensor, along with the integrated CO2 and flow sensors, was used to calculate nitrogen level and volume in order to measure the FRC of a simulated lung (TTL, Michigan Instruments, Grand Rapids, MI). We calculated N2 concentration as the balance gas, assuming all gas that is neither O2 nor CO2 is N2. The test lung was modified to simulate gas exchange by adding a propane burner which consumes O2 while producing CO2 at precise rates and proportion.

The lung simulator was ventilated with the Esprit ventilator (Respironics, Inc, Carlsbad, CA) with two different settings: 1) VT of 520 mL; RR of 13 and 2) VT of 750 mL; RR of 9. The inspired O2 was decreased from 70% to 30% and the subsequent N2 wash-in signal was analyzed to calculate the FRC at three different FRC volume settings. FRC was calculated as the ratio of the volume of N2 eliminated to the corresponding change in N2 concentration. Regression analysis was used to compare the measured and actual FRC volumes.

Results:
Regression analysis yielded an r2 of 0.989 and a slope of 0.914 (Figure 1). Compared to the actual values, the average error was -6 mL. No significant differences were observed between the bias, regression coefficient, or slope for either of the ventilation settings.

Conclusions:
The on-airway O2 sensor provided reliable signals that were useful for accurate FRC measurements. The integrated flow and CO2 sensors, which if used in combination with the O2 signal, could make possible several other measures related to cardiopulmonary health. Advantages of the on-airway sensors include lack of possible sample tube obstruction and no need for sidestream sampling signal delay correction. Future work with this sensor will involve FRC measurement in human subjects.


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