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 (FiO(2)) (DeltaFiO(2)) may impede the clinical 
use in patients ventilated with high FiO(2). We investigated the repeatability 
of FRC measurements and the impact of different DeltaFiO(2) 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 
DeltaFiO(2). In 20 postoperative cardiac surgery patients, FRC was measured in 
duplicate using DeltaFiO(2) of 0.1, 0.2, and 0.6.
RESULTS: There were no differences between repeated measurements of FRC, neither 
using a DeltaFiO(2) of 0.1, 0.2 nor 0.6(Delta0.1: 2.62 L +/- 0.58, 2.62 L +/- 
0.59, P = 0.995; Delta0.2: 2.70 L +/- 0.59, 2.66 L +/- 0.56, P = 0.258; 
Delta0.6: 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 DeltaFiO(2) of 0.1, 0.2, or 0.6.