OBJECTIVE: To validate a new system for functional residual capacity (FRC) measurements using oxygen washin/washout in spontaneously breathing humans. The system (LUFU, Drägerwerk AG, Lübeck, Germany) consists of an unmodified EVITA 4 ventilator, a side-stream paramagnetic oxygen sensor and a dedicated software. DESIGN: Laboratory study and measurements in spontaneously breathing volunteers. SETTING: Pulmonary function laboratory of a university hospital. PARTICIPANTS: 20 healthy and 15 lung diseased volunteers. INTERVENTIONS: FRC was measured by LUFU (LUFU-FRC) and by helium dilution (He-FRC); intra-thoracic gas volume (ITGV) was determined by body plethysmography. Each measurement cycle consisted of four independent LUFU-FRC determinations (step change of FiO(2) from 0.21 to 0.5 and back and from 0.21 to 1.0 and back), two helium-dilution runs and two body box measurements. Repeatability and agreement between methods were determined by comparing different measurements of one technique and by comparing different techniques among each other. MEASUREMENTS AND RESULTS: Repeatability of LUFU-FRC was estimated by comparing washin to washout and the different FiO(2)steps. The difference of the means was 3.7% at the most. Agreement between methods resulted in the following differences (mean+/-standard deviation of differences) for healthy and lung-diseased volunteers, respectively: LUFU-FRC vs. He-FRC -0.40+/-0.50 L (0.02+/-0.95 L), LUFU-FRC vs. ITGV -0.43+/-0.54 L (-0.18+/-0.61 L) and He-FRC vs. ITGV -0.03+/-0.43 L (-0.20+/-0.98 L). CONCLUSIONS: LUFU is a non-invasive method for the determination of FRC that requires only minor additional equipment and no modification to the ventilator. It can be used in difficult conditions such as breathing patterns with variations from breath to breath. The results of this study show that LUFU is sufficiently reliable and repeatable to warrant its clinical application.
