Participants undergo an assessment-only diagnostic intervention: diaphragmatic speckle-tracking ultrasound performed at predefined time points during routine care to evaluate feasibility and measurement properties (e.g., foundational speckle-tracking metrics and inter-/intra-operator reproducibility). Images of the bilateral zone of apposition are acquired and analyzed offline for the contraction synchrony index, end-diaphragmatic residual contraction (EDRC), displacement velocity, and maximal displacement. Ultrasound findings are not used for clinical decision-making; respiratory support (e.g., HFNC/ventilator settings) is determined independently by the treating clinicians.

Healthy volunteers undergo protocolized, assessment-only diaphragmatic speckle-tracking ultrasound to characterize normal diaphragmatic physiology and establish reference ranges; no feasibility or reproducibility endpoints are collected. Bilateral zone-of-apposition images are acquired and analyzed offline for DPSI, contraction synchrony index, end-diaphragmatic residual contraction (EDRC), displacement velocity, and maximal displacement. No therapeutic interventions are delivered, and participation does not alter clinical management.

Participants receive HFNC flows 20→30→40→60 L/min across four periods. Each period maintains the assigned flow for a predefined steady window; FiO₂ is adjusted per routine to meet target SpO₂. At the end of each period, protocolized diaphragmatic speckle-tracking ultrasound (bilateral zone of apposition) is performed with offline analysis of DPSI, contraction synchrony index, EDRC, displacement velocity, and maximal displacement; vital signs, respiratory rate, comfort/tolerance, and oxygenation are recorded. Safety overrides (e.g., hypoxemia, distress, intolerance) permit clinicians to modify or terminate the period.

Delivers heated, humidified blended oxygen via HFNC with real-time titration based on diaphragmatic speckle-tracking metrics (e.g., DPSI, contraction synchrony). Flow is adjusted in predefined increments to reach target diaphragmatic physiology while FiO₂ is titrated to maintain target SpO₂. Ultrasound feedback is used for bedside decisions; safety triggers allow clinical override.

The objective of this prospective observational study is to explore the Diaphragmatic Physiology Similarity Index (DPSI), derived from speckle tracking ultrasound, in guiding the adjustment of high-flow nasal cannula (HFNC) flow settings, and to assess its application in patients with acute respiratory failure.