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. 2021 May 25;16(1):759.
doi: 10.4081/mrm.2021.759. eCollection 2021 Jan 15.

Worsening of gas exchange parameters at high FiO2 in COVID-19: misleading or informative?

Federico Raimondi  1   2 , Luca Novelli  1 , Gianmariano Marchesi  3 , Fabrizio Fabretti  3 , Lorenzo Grazioli  4 , Ivano Riva  3 , Chiara Allegri  1   2 , Roberta Biza  1   2 , Chiara Galimberti  2 , Ferdinando Luca Lorini  4 , Fabiano Di Marco  1   2
Affiliations

Worsening of gas exchange parameters at high FiO2 in COVID-19: misleading or informative?

Federico Raimondi et al. Multidiscip Respir Med. .

Abstract

Background: In COVID-19, higher than expected level of intrapulmonary shunt has been described, in association with a discrepancy between the initial relatively preserved lung mechanics and the hypoxia severity. This study aim was to measure the shunt fraction and variations of PaO2/FiO2 ratio and oxygen alveolar-arterial gradient (A-a O2) at different FiO2.

Methods: Shunt was measured by a non-invasive system during spontaneous breathing in 12 patients hospitalized at COVID-19 Semi-Intensive Care Unit of Papa Giovanni XXIII Hospital, Bergamo, Italy, between October 22 and November 23, 2020.

Results: Nine patients were men, mean age (±SD) 62±15 years, mean BMI 27.5±4.8 Kg/m2. Systemic hypertension, diabetes type 2 and previous myocardial infarction were referred in 33%, 17%, and 7%, respectively. Mean PaO2/FiO2 ratio was 234±66 and 11 patients presented a bilateral chest X-ray involvement. Mean shunt was 21±6%. Mainly in patients with a more severe respiratory failure, we found a progressive decrease of PaO2/FiO2 ratio with higher FiO2. Considering (A-a O2), we found a uniform tendency to increase with FiO2 increasing. Even in this case, the more severe were the patients, the higher was the slope, suggesting FiO2 insensitiveness due to a shunt effect, as strengthened by our measurements.

Conclusion: Relying on a single evaluation of PaO2/FiO2 ratio, especially at high FiO2, could be misleading in COVID-19. We propose a two steps evaluation, the first at low SpO2 value (e.g., 92-94%) and the second one at high FiO2 (i.e., >0.7), allowing to characterize both the amendable (ventilation/perfusion mismatch), and the fixed (shunt) contribution quote of respiratory impairment, respectively.

Keywords: ARDS; COVID-19; SARS-CoV-2; pneumonia; shunt.

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Figures

Figure 1.
Figure 1.
Relationship between (A-a) O2 gradient, PaO2/FiO2 ratio and FiO2. Black lines, oxygen alveolar-arterial gradient, (A-a) O2 gradient, at different FiO2; red lines, PaO2/FiO2 ratio at different FiO2.
Figure 2.
Figure 2.
Relationship between SatO2, and FiO2. The vertical axis represents SatO2, as measured by an arterial blood gas analysis (ABG) when expired gases stability was reached. Red line corresponds to SatO2 of 94%.
See this image and copyright information in PMC

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