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.