The Science Journal of the American Association for Respiratory Care

2011 OPEN FORUM Abstracts


Lara Brewer, Joseph Orr; Anesthesiology, University of Utah Health Sciences, Salt Lake City, UT

Background: There is a need for an automated bedside functional residual capacity (FRC) monitoring system that can continually track the size of a patient's end-expiratory lung volume during mechanical ventilation without necessitating a step change in inspired oxygen (FIO2). Such a system would be useful for measuring FRC in patients who cannot tolerate a change in FIO2 because they require high levels of oxygen to maintain arterial oxygen saturation. We have developed a method for FRC monitoring that does not require step increases or decreases in FIO2. The FRC measurement signals are provided by a volumetric capnometer (partial pressure of end-tidal carbon dioxide (PetCO2) and volume of CO2 eliminated (VCO2) (NICO2, Philips-Respironics, Wallingford, CT). The CO2 washout observed during the single-breath transition from steady-state partial rebreathing to non-rebreathing is automatically actuated by the rebreathing loop of the NICO2 monitor once every three minutes. This small observational study was designed to assess the accuracy, precision and repeatability of the proposed FRC measurement system. Methods: Accuracy and precision of FRC measurements were assessed by comparing the CO2 rebreathing FRC values to the reference method, multiple breath nitrogen washout, in nine intensive care patients whose lungs were under mechanical ventilation. Repeatability was assessed by comparing subsequent individual measurements. Results: Compared to the reference method, the accuracy (bias) was -0.05 L and precision (1 SD of the differences) was 0.34 L (-2.6% +/- 17.5%) (Figure 1). The difference between repeated measurements was 0.020 +/- 0.42 L (mean +/- SD) (1.1 +/- 23.4 %), n=58. Conclusions: The CO2 rebreathing method for FRC measurement provides acceptable accuracy and repeatability compared to the reference method during mechanical ventilation. These results indicate it should be possible to detect clinically important changes in FRC and automatically track the changes over a time course of hours or days. The precision of the measurement could likely be improved by averaging several measurements. Further study of the CO2 rebreathing FRC method is needed to learn how accurate the method is for extremely injured lungs. Since both FRC and cardiac output can be monitored noninvasively by the NICO2 monitor during mechanical ventilation, the measurements may be useful for guiding ventilator setting changes such as adjustments to positive end-expiratory pressure (PEEP).
Sponsored Research - Philips-Respironics