This was a secondary analysis of a multicenter prospective observational study performed to update HACOR scoring [17]. It was conducted in 18 hospitals in China and Turkey from September 2017 to September 2021. The relevant ethics committees approved the study and informed consent was obtained from the patients or their family members. We enrolled patients with de novo acute respiratory failure. However, hypercapnic patients were excluded. De novo acute respiratory failure was defined as occurrence of respiratory failure without chronic respiratory disease, chronic heart disease, asthma, cardiogenic pulmonary edema, cardiac problems other than cardiogenic pulmonary edema, or postoperative hypoxemia [2,3,4, 17, 18].
Patients who were admitted to the participating centers were managed by the attending physicians, respiratory therapists, and nurses in charge. NIV was used to avert respiratory failure if the respiratory rate (RR) was > 25 breaths/min, if a clinical presentation of breathlessness at rest emerged (such as active contraction of the accessory inspiratory muscles or paradoxical abdominal motion), or PaO2 fell to < 60 mmHg at room air pressure or PaO2/FiO2 fell to < 300 mmHg with supplemental oxygen [17]. The formula of 21 + 4 × flow (L/min) was used to estimate the FiO2 if supplemental oxygen was used [19, 20]. A face mask or nasal mask was used to connect the patient to the ventilator. If NIV intolerance occurred, HFNC was used as an alternative strategy to prevent intubation. NIV intolerance was defined as termination of NIV due to discomfort, even in case of intermittent use [21].
We collected diagnoses and underlying diseases at admission. Pneumonia was assessed as new or increasing pulmonary infiltrate in chest radiographs coupled with clinical findings suggesting infection, such as new onset of fever, purulent sputum, cough, chest pain, leukocytosis, decline in oxygenation, and so on [22]. Acute respiratory distress syndrome (ARDS) was diagnosed as follows: (1) presence of acute hypoxemic respiratory failure with PaO2/FiO2 less than 300 mmHg; (2) within 1 week of a clinical insult or the presence of new (within 7 days) or worsening respiratory symptoms; (3) bilateral opacities in computed tomographic (CT) scans or chest X-rays not fully explained by effusions, lobar or lung collapse, or nodules; and (4) respiratory failure not fully explained by cardiogenic pulmonary edema or fluid overload [23, 24].
Consciousness was assessed using the Glasgow Coma Scale (GCS). GCS, heart rate, RR, blood pressure, pH, PaCO2, PaO2/FiO2, and SpO2 were collected before treatment and after 1–2, 12, and 24 h NIV. Disease severity was assessed with the sequential organ failure assessment (SOFA) score. The primary outcome was NIV failure, which was defined as the requirement of intubation [17]. The secondary outcomes were duration of ICU stay and duration of hospital stay.
We used MedCalc (MedCalc Software Ltd, Ostend, Belgium) and SPSS (version 25.0; IBM Corp., Armonk, NY) to analyze the data. Normally distributed continuous variables were analyzed using Student’s t test, and abnormally distributed continuous variables were analyzed using the Mann–Whitney U test when appropriate. Categorical variables were analyzed using the chi-squared test or Fisher’s exact test, where appropriate. The ability to predict NIV failure was tested with the AUC. The Hanley and McNeil method was used to test the difference in AUC between the ROX index and PaO2/FiO2 or between the ROX index and PaO2/FiO2/RR [25]. Three cutoff values were selected for clinical reference at probabilities of NIV failure equal to 25%, 50%, and 75% [26]. Patients with probabilities of NIV failure less than 25%, 25–50%, 50–75%, and more than 75% after 1–2 h NIV were termed the low, moderate, high, and very high risk for NIV failure groups, respectively. A p value less than 0.05 was considered to indicate statistical significance.