4.5 HFNC Physiology, Sizing & Flow

4.5 HFNC Physiology, Sizing & Flow

High-Flow Nasal Cannula (HFNC) therapy delivers heated, fully humidified medical gas mixtures at flow rates that equal or exceed the patient's peak inspiratory flow demand. Over the last two decades, HFNC has emerged as a cornerstone of noninvasive respiratory support across neonatal and pediatric intensive care units. To safely deploy HFNC, the respiratory care practitioner must understand its underlying mechanisms of action, precise cannula sizing safety boundaries, weight-based titration protocols, and objective criteria for failure.

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The Four Physiological Mechanisms of HFNC

HFNC provides respiratory support through four interrelated physiological mechanisms:

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|                                    THE FOUR PILLARS OF HFNC PHYSIOLOGY                                      |
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| 1. Anatomical Deadspace Washout  --> Flushes CO2 from nasopharynx; creates high-O2, low-CO2 upper reservoir |
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| 2. Optimal Gas Conditioning      --> Heated (37°C), 100% RH, 44 mg/L H2O preserves mucociliary function     |
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| 3. Decreased Airway Resistance   --> Flow matches or exceeds peak inspiratory demand; blunts suction collapse |
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| 4. Dynamic End-Expiratory PEEP   --> Expiratory backpressure recruits collapsed alveoli and maintains FRC  |
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1. Anatomical Nasopharyngeal Deadspace Washout

The extrathoracic upper airway is a clinically important portion of anatomical dead space in infants and young children. At end-exhalation it contains expired gas.

• Continuous flow can wash some expired gas from the nasopharynx and reduce rebreathing on the next inspiration.
• The magnitude of washout varies with flow relative to patient demand, prong fit, leak, mouth position, and airway anatomy.
• Improved ventilatory efficiency may reduce work of breathing in a responder, but HFNC does not guarantee CO2 clearance and must not delay escalation when ventilation worsens.

2. Optimal Gas Conditioning (37°C, 44 mg H2O/L, 100% Relative Humidity)

Conventional high-flow gas without humidification would cause rapid mucosal drying, epithelial desiccation, and hypothermia. HFNC relies on active heated passover humidifiers with heated-wire breathing circuits to condition gas to Body Temperature and Pressure Saturated (BTPS: $37^\circ\text{C}$, $44\text{ mg }\text{H}_2\text{O/L}$, $100%$ relative humidity).

• Preserves the mucociliary escalator, enhances clearance of purulent secretions, and prevents thick mucus encrustation.
• Reduces the heat and moisture burden of conditioning dry medical gas and may improve comfort and secretion handling.

3. Reduction of Inspiratory Upper Airway Resistance

In spontaneous breathing, inspiratory effort creates negative intraluminal pressure within the pharynx, which can cause compliant infant pharyngeal tissues to collapse inward. When delivered flow meets a meaningful portion of inspiratory demand, HFNC can reduce room-air entrainment and part of the resistive work of drawing gas through the upper airway. Benefit varies with the patient's demand and does not eliminate inspiratory effort.

4. Dynamic Positive End-Expiratory Pressure (PEEP)

As the patient exhales against the incoming high-velocity gas stream, backpressure is created across the upper airway, generating dynamic positive end-expiratory pressure (PEEP).

• Distending pressure is variable and rises with flow, larger prongs, a tighter leak, smaller patient size, and mouth closure; it cannot be predicted from one flow-to-pressure conversion.
• In a responder, this pressure and flow support may improve FRC and ventilation-perfusion matching. Because pressure is not set or continuously measured, reassess work of breathing, oxygen need, air leak, and comfort.
• Crucial Clinical Limitation: HFNC is an open, unsealed system. Unlike continuous positive airway pressure (CPAP), delivered airway pressure is unregulated, unmonitored, and highly variable. Opening the mouth increases leak and can reduce delivered pharyngeal pressure substantially and unpredictably.

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Cannula Sizing & Preserving an Expiratory Leak

Correct cannula sizing is an important HFNC safety check because excessive occlusion can raise pressure unpredictably and impair leak.

       [ INCORRECT: OCCLUSIVE FIT ]                     [ CORRECT: OPEN, DEVICE-SIZED FIT ]
   ┌──────────────────────────────────┐             ┌──────────────────────────────────┐
   │  ██████████████████████████████  │             │  ░░░░░░  ██████████████  ░░░░░░  │
   │  █████████[PRONG]██████████████  │             │  ░LEAK░  █   [PRONG]  █  ░LEAK░  │
   │  ██████████████████████████████  │             │  ░░░░░░  ██████████████  ░░░░░░  │
   └──────────────────────────────────┘             └──────────────────────────────────┘
    Total Nares Occlusion (> 50-100%)                Prong Fit Per Device Sizing Gauge
   • Expiratory leak pathway blocked                • Continuous expiratory leak pathway
   • Uncontrolled pressure spikes                   • Safe dynamic PEEP generation
   • TENSION PNEUMOTHORAX RISK                      • Pressure risk reduced and monitored

Applying the Sizing Rule

• Use the HFNC manufacturer's gauge or fit instructions. Many pediatric pathways select a prong external diameter no greater than roughly half the naris, while other systems specify a different allowable occlusion.
• HFNC is intended to remain an open system with visible leak. CPAP interfaces are also selected according to their own manufacturer and circuit—not by assuming every CPAP prong creates a 100% seal.
• An oversized or sealed HFNC interface can create unintended pressure and skin injury and has been associated with air leak. Inspect the nares, preserve the specified leak, monitor response, and change size or support mode when a controlled pressure is required.

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Dosing & Flow Titration Guidelines

Neonatal Titration Parameters

• Flow Range: $2\text{ to }8\text{ L/min}$ (rarely exceeding $8\text{ L/min}$ in neonates).
• Starting Flow: Typically initiated at $4\text{ to }6\text{ L/min}$.
• $\text{FiO}_2$ is titrated to maintain target $\text{SpO}_2$ between $90%\text{ and }95%$ in preterm infants, or $\ge 94%$ in term infants.

Pediatric Weight-Based Flow Dosing

Many pediatric pathways use a weight-based starting range, but formulas differ and flows are capped by device, cannula, age, and tolerance. One commonly used pathway is:

$\mathbf{\text{First } 10\text{ kg:}} \quad 1.5\text{ to }2.0\text{ L/kg/min}$ $\mathbf{\text{Each kg above } 10\text{ kg:}} \quad +0.5\text{ L/kg/min}$

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