This is a single-center, ambispective observational cohort study of adults with acute hypoxemic respiratory failure (AHRF) receiving high-flow nasal oxygen (HFNO) as initial respiratory support. The study includes three stages. Stage 1 (retrospective derivation): the first 57 consecutive patients (May-September 2025) were retrospectively included to develop and lock a dual-domain logistic model using baseline EFI, baseline PaCO2, and 0-30 minute changes in respiratory rate (ΔRR) and SpO2 (ΔSpO2). Stage 2 (retrospective temporal validation): the later 58 patients (October 2025-March 2026) were retrospectively included, and the frozen model was directly applied without refitting; results of Stages 1 and 2 have been reported. Stage 3 (prospective validation): an independent prospective validation cohort is currently being enrolled, and the frozen model will be applied without refitting. The primary outcome is endotracheal intubation within 72 hours after HFNO initiation. This registration covers the complete study; the retrospective stages have been reported, and the prospective stage is ongoing.
Age ≥18 years
Acute hypoxemic respiratory failure (AHRF) requiring HFNO initiation
At least one of the following high-risk criteria: a) PaO₂/FiO₂ ≤200 mmHg or FiO₂ ≥0.40 to maintain SpO₂ ≥92%; b) Respiratory rate ≥25/min; c) APACHE II score ≥12; d) Bilateral infiltrates on chest imaging
Neuromuscular disease affecting spontaneous breathing
Pregnancy
Contraindication to EIT (e.g., chest wall wounds, pacemaker)
Do-not-intubate order
Adult patients with acute respiratory failure receiving HFNO
Cohort
Other
High-Flow Nasal Oxygen Therapy
Other interventionPatients received HFNO as part of standard clinical care for acute hypoxemic respiratory failure. The treatment was titrated by the clinical team based on physiological parameters (SpO2, RR, ABG), aiming to maintain SpO2 ≥ 92% and reduce signs of respiratory distress.
Electrical Impedance Tomography Monitoring
DevicePulmoVista 500 EIT device (Dräger Medical, Lübeck, Germany) was used for continuous real-time monitoring of regional lung ventilation and to derive the EIT-derived flow index (EFI) at baseline and 30 minutes.
HFNO Failure (Endotracheal Intubation Within 72 Hours)
HFNO failure was defined as endotracheal intubation within 72 hours after HFNO initiation due to refractory hypoxemia, progressive respiratory acidosis, severe respiratory distress, or hemodynamic instability. Escalation to noninvasive ventilation (NIV) without endotracheal intubation and within-tier adjustments (increasing flow or FiO2 without changing support modality) were NOT considered the primary failure event. The model is intended for reassessment at 30 minutes, but the outcome is assessed within 72 hours.Unit of Measure: Proportion of participants.
Model Discrimination in Retrospective Temporal Validation
Area under the receiver operating characteristic curve (AUC) of the frozen dual-domain model for predicting endotracheal intubation within 72 hours in the retrospective temporal validation cohort.Unit of Measure: AUC (dimensionless, 0-1).
Model Calibration in Retrospective Temporal Validation
Calibration intercept and slope of the frozen dual-domain model in the retrospective temporal validation cohort. Unit of Measure: Dimensionless.
Model Discrimination in Prospective Validation
AUC of the frozen dual-domain model for predicting endotracheal intubation within 72 hours in the ongoing prospective validation cohort.Unit of Measure: AUC (dimensionless, 0-1).
Model Calibration in Prospective Validation
Calibration intercept and slope of the frozen dual-domain model in the ongoing prospective validation cohort. Unit of Measure: Dimensionless.
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