High-flow nasal cannula (HFNC) delivers heated, humidified oxygen at flow rates of 20-60 L/min through large-bore nasal prongs, providing several physiological benefits beyond conventional oxygen therapy. HFNC delivers a more precise FiO₂ (21-100%) by meeting or exceeding the patient's peak inspiratory flow rate (~30-40 L/min at rest, up to 60-120 L/min in respiratory distress), preventing room air entrainment that dilutes the delivered FiO₂ with standard nasal cannula. It generates a low level of continuous positive airway pressure (CPAP-like effect of 2-5 cm H₂O, flow-dependent) that provides mild alveolar recruitment, increases functional residual capacity, and reduces work of breathing. The heated humidification (34-37°C, 100% relative humidity) preserves mucociliary function, prevents airway desiccation, enhances mucociliary clearance of secretions, and reduces the metabolic cost of gas conditioning. HFNC creates a nasopharyngeal dead space washout effect — the high flow washes out CO₂-rich gas from the anatomical dead space, improving alveolar ventilation efficiency without increasing minute ventilation. Clinical indications include acute hypoxemic respiratory failure (type 1), post-extubation respiratory support, pre-oxygenation for intubation, and bridge therapy for patients who decline intubation. The ROX index [(SpO₂/FiO₂)/respiratory...