Predicting High-Flow Nasal Oxygen Failure Within 72 Hours Using a Physiology-Informed Dual-Domain Model at 30 Minutes

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.

Inclusion Criteria (must meet all): * 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 Exclusion Criteria: * Neuromuscular disease affecting spontaneous breathing * Pregnancy * Contraindication to EIT (e.g., chest wall wounds, pacemaker) * Do-not-intubate order * Hypercapnic respiratory failure as primary diagnosis (PaCO₂ \>50 mmHg with pH \