Skip to main content

Does inspiration of exhaled CO2 explain improved oxygenation with a face mask plus high-flow nasal cannula oxygen in severe COVID-19 infection?

Dear Sir,

I read with interest the research letter by Dogani et al. [1] on the improvement in arterial oxygenation in spontaneously breathing patients with severe COVID-19 infection requiring high-flow nasal cannula oxygen (80% O2 at 40 l/min) with the application of a simple oxygen mask but with no further O2 added. The resultant striking rise in SpO2 from 90 to 95% in eighteen patients was associated with a non-significant 0.15 kPa (1.1 mmHg) rise in PaCO2 and a non-significant fall in respiratory rate of 2 breath/min. The authors ascribe these changes to some degree of elimination of ambient air entrainment by the mask such that the effective FIO2 was higher. While no argument can be made against this interpretation as a contributor, it is unlikely that with an already very high inspiratory flow rate of 40 l/min that minimization of ambient air entrainment with respiratory rates in the 20s fully explains the improvement. Another possibility is re-inspiration of exhaled CO2. This would be consistent with the slight rise in PaCO2. Addition of inspired CO2 improves regional ventilation-perfusion matching and oxygenation with unchanged minute ventilation and tidal volume [2] by its several actions on pulmonary vascular resistance, airways resistance and parenchymal compliance [3]. Additionally, even the slight rise in PaCO2 stimulates ventilation by increasing tidal volume which will reduce VD/VT and dead space ventilation by decreasing the fraction of the inspired volume needed to clear the anatomic (conducting airways) dead space. Whatever the explanation(s) the finding, if verified in other studies, is clinically important. Adding a mask is easy to administer and in situations of oxygen scarcity, a simple means to reduce use of a limited resource.

Availability of data and materials

Not applicable to a letter to the editor in response to a published paper.

References

  1. 1.

    Dogani B, Mansson F, Resman F, Hartman H, Tham J, Torisson G. The application of an oxygen mask without supplemental oxygen, improved oxygenation in patients with COVID-19 already treated with high-flow nasal cannula. Crit Care. 2021;25:319.

    Article  Google Scholar 

  2. 2.

    Swenson ER, Robertson HT, Hlastala MP. Effects of inspired CO2 on V/Q-matching in normoxia, hypoxia and hyperoxia. Am J Respir Crit Care Med. 1994;149:1563–9.

    CAS  Article  Google Scholar 

  3. 3.

    Swenson ER. Unappreciated role of CO2 in ventilation-perfusion matching. Anesthesiol. 2019;131:226–8.

    Article  Google Scholar 

Download references

Acknowledgements

None.

Funding

Not applicable to a letter to the editor in response to a published paper.

Author information

Affiliations

Authors

Contributions

As sole author I wrote the entirety of this letter. The author read and approved the final manuscript.

Corresponding author

Correspondence to Erik R. Swenson.

Ethics declarations

Ethics approval and consent to participate

Not applicable to a letter to the editor in response to a published paper.

Consent for publication

As sole author I give my consent to publish this letter.

Competing interests

The author declares no competing interests in the subject matter of this letter.

Additional information

Publisher's Note

Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

Rights and permissions

Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article's Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article's Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://creativecommons.org/licenses/by/4.0/. The Creative Commons Public Domain Dedication waiver (http://creativecommons.org/publicdomain/zero/1.0/) applies to the data made available in this article, unless otherwise stated in a credit line to the data.

Reprints and Permissions

About this article

Verify currency and authenticity via CrossMark

Cite this article

Swenson, E.R. Does inspiration of exhaled CO2 explain improved oxygenation with a face mask plus high-flow nasal cannula oxygen in severe COVID-19 infection?. Crit Care 25, 343 (2021). https://doi.org/10.1186/s13054-021-03771-7

Download citation