The Science Journal of the American Association for Respiratory Care

2008 OPEN FORUM Abstracts

REPEATABILITY OF BEDSIDE FUNCTIONAL RESIDUAL CAPACITY MEASUREMENTS

Lara Brewer1, Joseph Orr1



Background: Functional Residual Capacity (FRC) is the volume of gas which remains in the lungs between breaths. The ability of the heart and lungs to work together to achieve adequate O2 uptake and CO2 removal is reduced if the FRC is smaller than normal. FRC volume can be reduced in cases such as with acute lung injury or obesity. When adjusting ventilation for patients such as these, it may be valuable to make use of the FRC measurement.
The nitrogen washout FRC measurement method evaluates the volume of excreted N2 and the corresponding change in N2 concentration to calculate FRC during mechanical ventilation. The N2 wash-in signal may be similarly evaluated. We used mainstream sensors to measure O2, CO2 and flow and assumed all remaining gas was N2 for the N2 washout/wash-in calculations. We previously reported the accuracy of this method for bench and animal studies1.

Methods:
We used the fast, on-airway oxygen sensor (Philips Medical, Wallingford, CT) which is based on photo-luminescence quenching. This new O2 sensor, along with the integrated on-airway CO2 and flow sensors, was used to calculate N2 levels and volumes in order to measure the FRC of 20 mechanically ventilated ICU patients according to an institution-approved protocol. For each measurement, the inspired O2 was increased from the baseline level to 100% for up to five minutes and was subsequently returned to the baseline level. FRC was calculated based on the concentration change of N2 relative to the washout volume. We analyzed both the N2 washout and the subsequent wash-in signals. The repeatability of the measurements was evaluated by comparing each measurement to the subsequent measurement taken in the same patient.

Results:
Regression analysis between the first and second measurements yielded an r2 of 0.96 and a slope of 0.98. The bias between repeated measurements was 40.3 mL (1.3 %) ± 209.8 mL (7.1 %). The limits of agreement were between -370.9 and 451.6 mL (-12.7 to 15.2%).

Discussion:
This method demonstrated satisfactory accuracy in previous bench and volunteer testing. The acceptable repeatability observed in this study of mechanically ventilated patients indicates the method may be reliable for monitoring FRC and changes in FRC volume with time at the bedside.


Reference:
1) Brewer LM, Haryadi DG, Orr JA. Measurement of functional residual capacity of the lung by partial CO2 rebreathing method during acute lung injury in animals. Respir Care. 2007 Nov;52(11):1480-9.