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TECHNOLOGY, COMPUTING, AND SIMULATION

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

From the Department of Anaesthesiology, University of Luebeck, Luebeck, Germany.

Address correspondence and reprint requests to Dr. Hermann Heinze, Department of Anaesthesiology, University of Luebeck, Ratzeburger Allee 160, 23538 Luebeck, Germany. Address e-mail to Hermannheinze{at}ngi.de.

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) (Fio₂) may impede the clinical use in patients ventilated with high Fio₂. We investigated the repeatability of FRC measurements and the impact of different Fio₂ 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 Fio₂. In 20 postoperative cardiac surgery patients, FRC was measured in duplicate using Fio₂ of 0.1, 0.2, and 0.6.

RESULTS: There were no differences between repeated measurements of FRC, neither using a Fio₂ of 0.1, 0.2 nor 0.6(0.1: 2.62 L ± 0.58, 2.62 L ± 0.59, P = 0.995; 0.2: 2.70 L ± 0.59, 2.66 L ± 0.56, P = 0.258; 0.6: 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 Fio₂ of 0.1, 0.2, or 0.6.

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