Dead-space washout and CO2 reduction with HFNC

Frank explains how HFNC flushes upper-airway dead space, improving efficiency and modestly lowering CO2 levels.

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🧭 REBEL Rundown

🗝️ Key Points

💨 NIV = Support without a tube: CPAP, BiPAP, and HFNC improve oxygenation and reduce the work of breathing.
🫁 CPAP = Continuous pressure: Best for hypoxemic patients (e.g., pulmonary edema, OSA).
️ BiPAP = Two pressures (IPAP/EPAP): Great for hypercapnic failure (e.g., COPD, obesity hypoventilation).
🌬️ HFNC = Heated, humidified high flow: Reduces effort, improves comfort, and enhances oxygen delivery.
🩺 Supportive, not definitive: NIV stabilizes patients while the underlying cause is treated.

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📝 Introduction

Non-invasive ventilation (NIV) refers to respiratory support provided without endotracheal intubation. The most common modalities include continuous positive airway pressure (CPAP), bilevel positive airway pressure (BiPAP), and high-flow nasal cannula (HFNC). These therapies aim to improve oxygenation, reduce the work of breathing, and potentially prevent invasive mechanical ventilation.

💨 CPAP and BiPAP

CPAP delivers a single, continuous pressure during inspiration and expiration. This pressure (commonly 5–10 cm H₂O) helps recruit atelectatic alveoli, reduce shunt, and improve oxygenation. It is commonly used for conditions like pulmonary edema, obstructive sleep apnea, or mild hypoxemia without significant ventilatory failure.
BiPAP alternates between two pressures:
- Inspiratory positive airway pressure (IPAP), augments tidal volume and unloads inspiratory muscles.
- Expiratory positive airway pressure (EPAP), maintains alveolar recruitment and improves oxygenation.
  The differential between IPAP and EPAP is critical for reducing hypercapnia in patients with COPD exacerbations or acute hypercapnic respiratory failure.
Indications
- CPAP: hypoxemia without major ventilatory failure (e.g., cardiogenic pulmonary edema, atelectasis, OSA).
- BiPAP: hypercapnia with increased work of breathing (e.g., COPD exacerbation, neuromuscular weakness, obesity hypoventilation).
A helpful way to conceptualize CPAP and BiPAP is through the hairdryer analogy. Imagine placing a hairdryer in your mouth:

🩺 Clinical Considerations

Masks can be uncomfortable, impair secretion clearance, and limit oral intake.
Some patients require sedation to tolerate NIV, but this carries risks in patients with unprotected airways.
NIV is thus a high-stakes intervention requiring close monitoring.
Common starting dose to understand titration, but start at the level appropriate for your patient: IPAP 10 cm H₂O / EPAP 5 cm H₂O (“10/5”) and are titrated:
- Increase IPAP to improve tidal volume and CO₂ clearance.
- Increase EPAP to recruit alveoli and improve oxygenation.
- Both may be raised simultaneously if the patient is both hypoxemic and hypercapnic.

🚀 High-Flow Nasal Cannula (HFNC)

H: Heated & humidified – improves mucociliary clearance, prevents airway drying, and enhances tolerance. I: Inspiratory flow – high flow meets or exceeds patient demand, reducing respiratory rate and effort.
F: Functional residual capacity – modest generation of positive end-expiratory pressure (PEEP), promoting alveolar recruitment.
L: Lighter – generally more comfortable and less restrictive than mask-based NIV.
O: Oxygen dilution – minimizes entrainment of room air, delivering higher and more predictable FiO₂.
W: Washout – flushes anatomical dead space, reducing CO₂ rebreathing.
HFNC delivers heated, humidified oxygen at high flow rates (30–60 L/min) through wide-bore nasal prongs. A mnemonic, H-I-F-L-O-W, helps summarize its mechanisms:
Indications: Traditionally used for acute hypoxemic respiratory failure (e.g., pneumonia), HFNC is increasingly studied for hypercapnic failure as well, with trials suggesting non-inferiority to BiPAP in select populations.