A Novel Index to Predict the Failure of High-Flow Nasal Cannula in Patients with Acute Hypoxemic Respiratory Failure

Study design This is a multi-center prospective observational study performed over a 12-month period (from August 2022 to August 2023). The research premise was granted approval by the local Ethics Committee (2020ZDSYLL303-P02).

VOX index The VOX index was defined as the ratio of SpO2/FiO2 over VT. Investigators briefly interrupted HFNC (3 minutes) to measure VT using a mechanical ventilator (SV800, Mindray) in noninvasive ventilation (NIV) mode, as an "NIV test". Inspiratory support was set at 5 cmH2O and 5 cmH2O positive end-expiratory pressure level for all patients, and initial oxygen concentration was set as in HFNC. NIV was delivered through a face mask (ZS-MZ-A, Zhongshan Medical) and a double-pipe system, while minimizing leaks. In consideration of variations in VT, we recorded mean VT and respiratory rate for 1 minute under stable conditions.

Study protocol HFNC therapy was started within 15 minutes after recruitment and initiated with a 30-40 LPM minimum flow. Investigators adjusted FiO2 as suitable, targeting SpO2 of 92% or more, and the rate of flow set on the basis of the physician's decision. HFNC discontinuation and invasive mechanical ventilation (IMV) initiation were based on the intubation criteria defined in investigators clinical protocol, finial decisions were made by the physicians in charge, who were blinded to the VT during NIV test. HFNC failure was defined as a need for IMV, on account of NIV is not employed as the second line of ventila,ory support in the event of HFNC failure, in the participating units. Patient demographics, comorbidities, and chest radiographs prior to HFNC initiation were documented upon inclusion into the research analysis. The acute physiologic assessment and chronic health evaluation II (APACHE II) score along with the sequential organ failure assessment score (SOFA) were documented based on the highest scores in the 24h previous HFNC initiation. The time of HFNC onset was defined as 0h. Vital signs; HFNC settings including FiO2, flow rate, and temperature; clinical respiratory variables including RR, VT, and SpO2; diaphragmatic ultrasonography including diaphragmatic displacement, diaphragm thickness (tdi), and Δtdi% (was calculated as [tdi end-inspiration-tdi end-expiration/tdi end-expiration] ×100) were recorded at 0, 2, 6, 12, 18, and each 24h pre day following initiation of HFNC treatment until day 7. Variables of pulmonary gas exchange variables with the arterial line were documented at 0, 6, 12 and each 24h pre day following initiation of HFNC treatment until day 7 after initiation of HFNC treatment.