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: Functional residual capacity (FRC) volume is critical for both the delivery of oxygen to the body and the removal of carbon dioxide from the tissues since it provides the mechanism for gas exchange between the lungs and the blood. During mechanical ventilation, anesthesia, and lung pathophysiology, the volume of the FRC is compromised, which leads to reduced gas exchange. Knowledge of FRC size is useful during adjustment of positive end-expiratory pressure and other ventilator settings in which FRC volume is affected. Several systems have been proposed for FRC measurement based on step changes in inspired oxygen. However, there remains a need for an automated bedside FRC measurement which can be used for patients who cannot tolerate a change in inspired oxygen fraction to complete a measurement. We evaluated accuracy of a novel FRC measurement technique based on partial CO2 rebreathing which can be used at any fixed level of inspired oxygen. Methods: For eight healthy volunteers, accuracy and precision of CO2 rebreathing FRC measurements were assessed by comparing the CO2 rebreathing FRC values to the reference method, body plethysmography. Results: Compared to body plethysmography, the accuracy (mean error) was 0.01 L and precision (1 SD of the differences) was 0.26 L (0.4% +/- 7.0%) The limits of agreement were between -0.49 and 0.52 L (-13.4 to 14.2%) (Figure 1). Linear regression analysis showed an r2 of 0.92 and a slope of 0.99. Conclusions: The novel CO2 rebreathing FRC measurement showed acceptable accuracy and precision compared to the clinical gold standard, body plethysmography, for spontaneously breathing volunteers in this small study. Based on these results, it appears the automated bedside method may provide accurate FRC measurements during stable ventilation. The CO2 rebreathing FRC measurement does not require a step change in inspired oxygen and is therefore useful for patients who cannot tolerate an increase or a decrease in the prescribed inspired oxygen fraction. There is potential for the novel CO2 rebreathing method to also be used with circle breathing systems which are common in the operating room. Circle breathing systems cannot initiate a step change in oxygen and are therefore not compatible with nitrogen washout FRC measurement systems. Further testing is warranted to evaluate how accurate and repeatable the CO2 rebreathing method is during mechanical ventilation and for patients with significant lung injury.
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