Measurement of Functional Residual Capacity of the Lung by Nitrogen Washout, Carbon Dioxide Rebreathing and Body Plethysmography in Healthy Volunteers

Background: We measured Functional Residual Capacity (FRC) of the lungs with three methods in healthy volunteers. The three techniques included a CO2 partial rebreathing technique, nitrogen washout technique, and the reference technique for ambulatory patients, body plethysmography. Materials and Methods: After granting consent to an IRB-approved protocol, each of the 20 healthy volunteers participated in FRC measurement by three methods, including body plethysmography, carbon dioxide (CO2) rebreathing, and nitrogen washout. Gas concentration and volume data were collected from the distal side of a mouthpiece during spontaneous ventilation for the washout and rebreathing measurements. The FRC was measured twice with a nitrogen washout measurement technique and then signals from five partial CO2 rebreathing measurement cycles were collected. Finally, the nitrogen washout FRC measurements were repeated twice. We compared the average CO2 rebreathing FRC measurements and the average nitrogen washout FRC measurements to the body plethysmography FRC measurements for each subject through statistical methods of linear regression analysis and BlandAltman Analysis. Results: The squared correlation coefficient for the linear regression between nitrogen washout and body plethysmography measurements was r = 0.91 (n = 35). The bias ± Standard Deviation was 0.054 ± 0.373 L. Conclusion: These results indicate FRC measurement by nitrogen washout correlate well with the body plethysmography reference standard in ambulatory, spontaneously breathing subjects. This method could possibly be used in space to monitor lung function. Introduction Functional Residual Capacity (FRC) measurements may be useful as a guide for ventilation management or as a way to follow disease progression for patients with acute lung injury and ARDS. While traditional methods of FRC measurement are valuable for researching disease progression and for monitoring ambulatory patients, they have been of limited utility at the bedside due to difficulty of use, high expense, and impracticality during mechanical ventilation. FRC measurement at the bedside for mechanically and spontaneously ventilated patients has recently been reported by several researchers, who evaluated systems based on nitrogen washout and Electrical Impedance Tomography. The nitrogen washout and CO2 rebreathing methods have previously been validated by our research group with the use of a bench lung model. We also previously evaluated both methods in an animal study. The aim of this study was to evaluate the accuracy of the two FRC measurement systems compared to the clinical gold standard for ambulatory patients, body plethysmography, in healthy, spontaneously breathing volunteers.