The Impact of Different Step Changes of Inspiratory Fraction of Oxygen on Functional Residual Capacity Measurements Using the Oxygen Washout Technique in Ventilated Patients

In Brief BACKGROUND: Functional residual capacity (FRC) measurements may help to guide respiratory therapy. Using the oxygen washout technique, FRC can be assessed at bedside during spontaneous breathing. High repeatability, crucial for monitoring, has not been shown in ventilated patients. A large step change of inspiratory fraction of oxygen (Fio2) (ΔFio2) may impede the clinical use in patients ventilated with high Fio2. We investigated the repeatability of FRC measurements and the impact of different ΔFio2 on this repeatability. METHODS: The LUFU system (Draeger Medical, Luebeck, Germany) estimates FRC by oxygen washout, a variant of multiple-breath-nitrogen-washout during a fast ΔFio2. In 20 postoperative cardiac surgery patients, FRC was measured in duplicate using ΔFio2 of 0.1, 0.2, and 0.6. RESULTS: There were no differences between repeated measurements of FRC, neither using a ΔFio2 of 0.1, 0.2 nor 0.6(Δ0.1: 2.62 L ± 0.58, 2.62 L ± 0.59, P = 0.995; Δ0.2: 2.70 L ± 0.59, 2.66 L ± 0.56, P = 0.258; Δ0.6: 2.61 L ± 0.58, 2.59 L ± 0.58, P = 0,639). Coefficients of variation were 6.6%, 5.6%, and 6.6%, respectively. CONCLUSIONS: FRC can be measured in ventilated patients using the oxygen washout technique with a clinically acceptable repeatability. Repeatability is not significantly influenced whether using a ΔFio2 of 0.1, 0.2, or 0.6. IMPLICATIONS: Clinically acceptable repeatability of functional residual capacity (FRC) measurements using a small step change in inspiratory fraction of oxygen (Fio2) of 0.1 has been demonstrated in 20 postoperative cardiac patients with relatively normal pulmonary function. Acceptable measurements with a small step change in Fio2 make it feasible to perform FRC measurements at the bedside for patients with acute lung injury, although more study is needed to verify the technique in these patients.