Limitations of the ARDS criteria during high-flow oxygen or non-invasive ventilation: evidence from critically ill COVID-19 patients.

Background: The ratio of partial pressure of arterial oxygen to inspired oxygen fraction (PaO ₂ /F _I O ₂ ) during invasive mechanical ventilation (MV) is used as criteria to grade the severity of respiratory failure in acute respiratory distress syndrome (ARDS). During the SARS-CoV2 pandemic, the use of PaO ₂ /F _I O ₂ ratio has been increasingly used in non-invasive respiratory support such as high-flow nasal cannula (HFNC) and non-invasive ventilation (NIV). The grading of hypoxemia in non-invasively ventilated patients is uncertain. The main hypothesis, investigated in this study, was that the PaO ₂ /F _I O ₂ ratio does not change when switching between MV, NIV and HFNC.
Methods: We investigated respiratory function in critically ill patients with COVID-19 included in a single-center prospective observational study of patients admitted to the intensive care unit (ICU) at Uppsala University Hospital in Sweden. In a steady state condition, the PaO ₂ /F _I O ₂ ratio was recorded before and after any change between two of the studied respiratory support techniques (i.e., HFNC, NIV and MV).
Results: A total of 148 patients were included in the present analysis. We find that any change in respiratory support from or to HFNC caused a significant change in PaO ₂ /F _I O ₂ ratio. Changes in respiratory support between NIV and MV did not show consistent change in PaO ₂ /F _I O ₂ ratio. In patients classified as mild to moderate ARDS during MV, the change from HFNC to MV showed a variable increase in PaO ₂ /F _I O ₂ ratio ranging between 52 and 140 mmHg (median of 127 mmHg). This made prediction of ARDS severity during MV from the apparent ARDS grade during HFNC impossible.
Conclusions: HFNC is associated with lower PaO ₂ /F _I O ₂ ratio than either NIV or MV in the same patient, while NIV and MV provided similar PaO ₂ /F _I O ₂ and thus ARDS grade by Berlin definition. The large variation of PaO ₂ /F _I O ₂ ratio indicates that great caution should be used when estimating ARDS grade as a measure of pulmonary damage during HFNC.
(© 2022. The Author(s).)