Validation of PaO₂:FiO₂ for predicting hospital mortality in critically ill patients with acute hypoxaemic respiratory failure: a retrospective binational registry-based study

Objective  To determine the optimal PaO₂:FiO₂ threshold in the first 24 hours of intensive care unit admission, and its associated discriminatory capacity, for prognostication of mortality among critically ill patients.

Methods  This bi-national registry included adult patients admitted to intensive care units in Australia and New Zealand from January-2018 to December-2022. The primary outcome was hospital mortality. Acute hypoxic respiratory failure was defined as PaO₂:FiO₂ of < 300 using the worst PaO₂:FiO₂ within the first 24 hours of intensive care unit admission. The unadjusted association between PaO₂:FiO₂ and hospital mortality was evaluated using restricted cubic splines with four knots to allow for continuous, non-linear associations. To determine the optimal threshold of the PaO₂:FiO₂ for predicting hospital mortality, Youden’s method was used to identify the maximum sum of sensitivity and specificity. The area under the receiver operating characteristic curve and Youden’s J-index were calculated to compare pre-specified subgroups.

Results  Among the 662,612 included patients, acute hypoxic respiratory failure was not present in 324,761 (49%) patients, mild in 181,499 (27%) patients, moderate in 128,277 (19%) patients, and severe in 28,125 (4%) patients. The hospital mortality rates, respectively, were 4.9% (15,797/324,761), 7.9% (14,291/181,499), 14% (18,247/128,277), and 31% (8,717/28,125). The association between PaO₂:FiO₂ and hospital mortality was non-linear with an inflection point at PaO₂:FiO₂ = 200. The area under the ROC curve was 0_.677 (95%CI 0.675 - 0.679) with an optimum PaO₂:FiO₂ threshold of 230. (Youden’s J-index of 0.267, sensitivity 56.1% and specificity 70.6%). The area under the ROC curve was 0.627 for patients who required invasive ventilation during their intensive care unit stay, compared with 0.698 for those who did not.

Conclusion  The optimal PaO₂:FiO₂ threshold for predicting hospital mortality was 230. PaO₂:FiO₂ has low discriminatory capacity in predicting hospital mortality among intensive care unit patients.

Keywords
Critical illness; Hospital mortality; Hypoxia; Hypoxaemia; Noninvasive ventilation; Oxygen; Respiratory insufficiency; Registries