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

## TL;DR

High-flow nasal cannula therapy creates temporary pressures due to fluid dynamics, not sustained positive pressure, and its main benefits come from clearing dead space and matching breathing flow.

## Contribution

Introduces 'emergent hydrodynamic pressure' as a precise term to describe HFNC's transient pressure effects, clarifying its mechanism and terminology.

## Key findings

- HFNC generates transient pressures (0.2–13.5 cmH2O) influenced by airway geometry and flow dynamics, not sustained positive pressure.
- Therapeutic benefits include dead-space washout (CO2 reduction, r = −0.77) and flow matching, reducing work of breathing by 40–50%.
- HFNC's pressure effects are secondary, with clinical efficacy driven by multiple synergistic mechanisms including humidification and alveolar recruitment.

## Abstract

What are the main findings?
HFNC generates transient, flow-dependent pressures, not sustained positive pressure, due to its open-system design and hydrodynamic principles.The primary therapeutic benefit of HFNC arises from dead-space washout and flow matching, not from pressure generation.

HFNC generates transient, flow-dependent pressures, not sustained positive pressure, due to its open-system design and hydrodynamic principles.

The primary therapeutic benefit of HFNC arises from dead-space washout and flow matching, not from pressure generation.

What are the implications of the main findings?
Accurate terminology distinguishes HFNC as a flow-based system with secondary pressure effects, not a pressure modality; terms such as “emergent hydrodynamic pressure,” “flow-dependent transient pressure,” or “dynamic airway pressure” clarify this distinction.Individualized HFNC optimization should prioritize flow and patient anatomy over assumptions of pressure-driven effects.

Accurate terminology distinguishes HFNC as a flow-based system with secondary pressure effects, not a pressure modality; terms such as “emergent hydrodynamic pressure,” “flow-dependent transient pressure,” or “dynamic airway pressure” clarify this distinction.

Individualized HFNC optimization should prioritize flow and patient anatomy over assumptions of pressure-driven effects.

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.