High-Flow Nasal Cannula in Adult Acute Care: Physiological Mechanisms, Clinical Evidence, and Persistent Misconceptions

High-flow nasal cannula (HFNC) oxygen therapy is widely used in emergency, intensive care, perioperative, and procedural settings, but it is often conceptually assimilated to non-invasive positive-pressure ventilation (NIPPV). This interpretation is physiologically misleading. HFNC is an open high-flow oxygen system whose main effects derive from dead-space washout, improved stability of inspired oxygen fraction (FiO2), heated humidification, patient comfort, and only modest flow-dependent positive airway pressure. Its open interface also facilitates eating, drinking, speaking, and secretion clearance. Clinical evidence supports HFNC mainly in selected patients with acute hypoxemic respiratory failure, where it may reduce intubation compared with conventional oxygen therapy. However, evidence is more heterogeneous in hypercapnic respiratory failure, cardiogenic pulmonary edema, immunocompromised patients, obesity, and settings requiring substantial pressure support or ventilatory unloading. NIPPV remains preferred in acidotic hypercapnic COPD exacerbations, while CPAP or NIPPV is indicated in selected pressure-responsive conditions such as cardiogenic pulmonary edema. Persistent excessive effort or clinical deterioration requires timely assessment for invasive ventilation. HFNC should therefore be regarded as optimized high-flow oxygen therapy rather than as “non-invasive ventilation through nasal prongs.” Recognizing this distinction may improve patient selection, guide escalation, and avoid overinterpretation of its clinical effects.