Modelling mixing within the dead space of the lung improves predictions of functional residual capacity

Harrison, Chris D. and Phan, Phi Anh and Zhang, Cathy and Geer, Daniel and Farmery, Andrew D. (2017) Modelling mixing within the dead space of the lung improves predictions of functional residual capacity. Respiratory Physiology and Neurobiology. ISSN 1569-9048 (https://doi.org/10.1016/j.resp.2017.03.006)

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Abstract

Routine estimation of functional residual capacity (FRC) in ventilated patients has been a long held goal, with many methods previously proposed, but none have been used in routine clinical practice. This paper proposes three models for determining FRC using the nitrous oxide concentration from the entire expired breath in order to improve the precision of the estimate. Of the three models proposed, a dead space with two mixing compartments provided the best results, reducing the mean limits of agreement with the FRC measured by whole body plethysmography by up to 41%. This moves away from traditional lung models, which do not account for mixing within the dead space. Compared to literature values for FRC, the results are similar to those obtained using helium dilution and better than the LUFU device (Dräger Medical, Lubeck, Germany), with significantly better limits of agreement compared to plethysmography.

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