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

Abstract

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. 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. 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. 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.