Physical and Physiological Mechanisms of Emergent Hydrodynamic Pressure in High-Flow Nasal Cannula Therapy

High-flow nasal cannula (HFNC) therapy is frequently described as a positive pressure modality, yet this classification lacks mechanistic support. This critical narrative review integrates experimental, computational, and clinical evidence to examine the established physiological mechanisms underlying HFNC, with emphasis on precise terminology. The study clarifies that labeling HFNC as "positive pressure" is conceptually inaccurate, as the system delivers transient, flow-dependent pressures characteristic of open-circuit administration. Evidence is synthesized to quantify the relative contributions of nasopharyngeal dead-space clearance versus emergent pressure generation. Unlike CPAP, HFNC produces pressures ranging from 0.2 to 13.5 cmH₂O, determined by airway geometry, leak magnitude, and mouth position. Fluid dynamic modeling using Bernoulli and Darcy-Weisbach equations demonstrates oscillatory rather than sustained pressures, with magnitudes linked to nasopharyngeal Reynolds numbers (2400-6000) and turbulent energy dissipation (30-60%). Clinical efficacy persists despite variable pressures, reflecting synergistic mechanisms: inspiratory flow matching (40-50% reduction in work of breathing), dead-space clearance (CO₂ reduction, r = -0.77, p < 0.05), emergent pressure effects (10-20%), and thermal humidification (10-20%). Electrical impedance tomography reveals heterogeneous alveolar recruitment, with high-potential (54%) and low-potential (46%) phenotypes. Based on these mechanistic insights, this review proposes the term "emergent hydrodynamic pressure" to accurately describe HFNC's transient, flow-dependent pressures. This terminology differentiates HFNC from conventional positive pressure systems and aligns language with the principles of fluid dynamics and respiratory physiology.