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 Bland-Altman Analysis. Results: The squared correlation coefficient for the linear regression between nitrogen washout and body plethysmography measurements was r2 = 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.