The Accuracy of the Oxygen Washout Technique for Functional Residual Capacity Assessment During Spontaneous Breathing

Abstract

In Brief BACKGROUND: Measurement of functional residual capacity (FRC) is of considerable interest for monitoring patients with lung injury. The lack of instruments has impeded routine bedside FRC measurement. Recently, a simple automated method for FRC assessment by O2 washout has been introduced. We designed this study to evaluate the accuracy of FRC measurement using the O2 washout technique. METHODS: The LUFU system (Draeger, Luebeck, Germany) estimates FRC by O2 washout, a variant of multiple breath nitrogen washout. This technique uses a sidestream O2-analyzer to calculate FRC from end-inspired and end-expired O2 concentrations during fast changes of Fio2. We measured FRC in 23 healthy, spontaneously breathing volunteers in the sitting position using three techniques: 1) helium dilution (FRC-He), 2) body plethysmography (FRC-bp), 3) oxygen washout (FRC-O2). RESULTS: FRC-O2 (mean 4.1 ± 1.1 L, range 2.4–6.9 L) corresponds with FRC-He (mean 4.0 ± 1.0 L, range 2.4–6.2 L; bias of FRC-O2: −0.2 ± 0.4 L) and FRC-bp (mean 4.2 ± 1.0 L, range 2.8–6.1 L; bias of FRC-O2: 0.1 ± 0.6 L). CONCLUSIONS: The bias and precision of the O2 washout technique using the LUFU system were clinically acceptable when compared with FRC-He and FRC-bp for FRC assessment in spontaneously breathing volunteers. IMPLICATIONS: Functional residual capacity (FRC) measurement by the oxygen washout method in healthy spontaneously breathing volunteers has an acceptable bias when compared with FRC measurements using helium dilution and body plethysmography, although limits of agreement could be improved.