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A1680
October 22, 2008 9:00 AM - 11:00 AM Room Hall E2-Area G, |
| Functional Residual Capacity Monitored at the Bedside Using Mainstream Sensors |
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Lara Brewer, M.S., Earl Fulcher, R.R.T., Boaz Markewitz, M.D., Joe Orr, Ph.D. Anesthesiology, University of Utah Health Sciences Center, Salt Lake City, Utah |
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Background: Functional Residual Capacity (FRC) is the volume of gas left in the lungs at the end of a breath. If the FRC is reduced in size, such as in the presence of acute lung injury or obesity, it is difficult to maintain sufficient gas exchange. The FRC measurement may be a valuable parameter when adjusting ventilation for patients such as these. The nitrogen washout FRC measurement method evaluates the volume of excreted nitrogen and the corresponding change in nitrogen concentration to calculate FRC during mechanical ventilation. The nitrogen wash-in signal may likewise be used for an FRC measurement. We used mainstream sensors to measure O2 and CO2 and assumed all remaining gas was N2 for the N2 washout/wash-in calculations. We previously tested this method in bench and animal studies1. Methods: We used the fast, on-airway oxygen sensor (Philips Medical, Wallingford, CT) which is based on the photo-luminescence quenching principle. This new O2 sensor, along with the integrated on-airway CO2 and flow sensors, was used to calculate nitrogen 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 oxygen 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 nitrogen relative to the washout volume. We analyzed both the nitrogen washout and the subsequent wash-in for this study. 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.[figure1]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 411.3 mL (-12.7 to 15.2%).[figure2]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 useful for guidance in adjusting ventilation or for monitoring the change in FRC volume with time. 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. Anesthesiology 2008; 109 A1680 |
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