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

Abstract

Introduction The ratio of partial pressure of arterial oxygen to inspired oxygen fraction (PaO2/FIO2) 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 PaO2/FIO2 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 PaO2/FIO2 ratio does not change when switching between MV, NIV and HFNC. Methods This was a sub-study of a single-center prospective observational study of patients admitted to the intensive care unit (ICU) at Uppsala University Hospital in Sweden for critical COVID-19. In a steady state condition, the PaO2/FIO2 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 PaO2/FIO2 ratio (up to 48 mmHg, from HFNC to MV). Changes in respiratory support between NIV and MV did not show consistent change in PaO2/FIO2 ratio. In patients classified as mild to moderate ARDS during MV, the change from HFNC to MV showed a variable increase in PaO2/FIO2 ratio ranging between 52 and 140 mmHg (median of 127 mm Hg). This made prediction of ARDS severity during MV from the apparent ARDS grade during HFNC impossible. Conclusion HFNC is associated with lower PaO2/FIO2 ratio than either NIV or MV in the same patient, while NIV and MV provided similar PaO2/FIO2 and thus ARDS grade by Berlin definition. The large variation of PaO2/FIO2 ratio precludes using ARDS grade as a measure of pulmonary damage during HFNC.

Acute respiratory distress syndrome in adults: diagnosis, outcomes, long-term sequelae, and management

“…For instance, in a single centre prospective study of 148 patients, PaO 2 /FiO 2 was found to vary substantially after a change in respiratory support from high-flow nasal oxygen to invasive ventilation. 36 Furthermore, Ranieri and colleagues 37 showed that patients fulfilling ARDS criteria on high-flow nasal oxygen who transition to invasive mechanical ventilation might have lower mortality rates than patients on non-invasive ventilation who transition to invasive mechanical ventilation.…”