Oxygen Therapy - Methods and Devices

Understanding Low-Flow vs. High-Flow

• Roberts and Hedges' Clinical Procedures in Emergency Medicine, p. 89

1. Nasal Cannula (NC)

• How it works: Two small prongs sit in the nostrils and deliver a continuous flow of O₂ that accumulates in the nasopharynx, creating a small O₂ reservoir for each inspiration.
• Flow rate: Typically 1-6 L/min
• FiO₂ delivered: ~24-44% (roughly, each 1 L/min increase raises FiO₂ by ~4%)
• Common use: Mild hypoxemia, long-term O₂ therapy, patients who need to eat/speak
• Key point: At standard 2-4 L/min, delivers approximately 30-35% FiO₂. Patients must not smoke while using it - serious burns can occur.

2. Simple Face Mask

• How it works: The mask itself (approx. 200 mL volume) acts as the O₂ reservoir. O₂ flows in continuously; during expiration, some exhaled gas fills the mask but is partially washed out before the next breath. Room air is also entrained through side vent holes.
• Flow rate: 5-10 L/min (minimum 5 L/min to prevent CO₂ rebreathing)
• FiO₂ delivered: ~35-55% at 5-10 L/min; can reach 80-90% at 30 L/min
• Common use: Moderate hypoxemia, short-term use
• Key point: FiO₂ is variable and decreases significantly in patients in respiratory distress (higher minute ventilation = more air entrainment).

3. Partial Non-Rebreather Mask (PNRB)

• How it works: A bag reservoir (~1 L) is attached to the mask to provide additional O₂ during inspiration. Unlike the true non-rebreather, the valve between the bag and mask is absent or non-functional, allowing some exhaled gas to re-enter the bag.
• Flow rate: 10-15 L/min (keep bag inflated)
• FiO₂ delivered: ~50-70%

4. Non-Rebreather Mask (NRB)

• How it works: Similar to PNRB but has one-way valves: one between the reservoir bag and mask (prevents exhaled gas entering the bag) and two on the side ports (allow exhalation but prevent outside air entry). In practice, one side valve is often removed as a safety measure.
• Flow rate: 10-15 L/min; can push to "flush" (~40+ L/min on most wall meters)
• FiO₂ delivered: ~70% at 15 L/min; rises to ~90% at 45 L/min
• Common use: CO poisoning (100% O₂ target), severe hypoxemia, significant trauma
• Important misconception: Many clinicians think 15 L/min gives ~100% FiO₂ - in practice it gives ~70%. True near-100% FiO₂ requires flow rates of 45+ L/min.

For CO poisoning: 100% O₂ via NRB reduces carboxyhemoglobin half-life from 4-5 hours (room air) to ~1 hour.

5. Venturi Mask (Air-Entrainment Mask)

• How it works: Uses the Venturi (Bernoulli) principle - a jet of O₂ at high velocity entrains room air through side ports in a fixed, precise ratio. The total gas flow (O₂ + entrained air) exceeds the patient's inspiratory flow, so no extra air is drawn in around the mask edges - making FiO₂ highly consistent.
• Flow rate: Device-dependent (set per color-coded jet insert)
• FiO₂ delivered: Precisely 24%, 28%, 31%, 35%, 40%, 60% (color-coded jets)
• Common use: COPD with chronic hypercapnia, any situation requiring precise FiO₂ control
• Key points:
  • Cannot deliver FiO₂ above 60% (not above 35% in many descriptions)
  • FiO₂ stays constant regardless of the patient's breathing pattern - its main advantage
  • Color-coded jets (e.g., blue = 24% at 2 L/min, white = 28% at 4 L/min, green = 60% at 15 L/min)
• Roberts and Hedges', p. 91

6. High-Flow Nasal Cannula (HFNC)

• How it works: Delivers heated, humidified O₂ through wide-bore nasal prongs at very high flow rates (up to 60 L/min). The nasopharynx acts as a natural reservoir that refills with O₂ after each breath. Three required components: wide-bore prongs, a humidifier, and a gas delivery system.
• Flow rate: 5-60 L/min
• FiO₂ delivered: ~70-80% at 15 L/min; ~90%+ at 45 L/min; near 100% achievable
• Additional benefits:
  • Low-level positive airway pressure (1-3 cm H₂O, increases with flow rate)
  • Increases tidal volume and end-expiratory lung volumes
  • Better tolerated than face masks or NPPV
  • FiO₂ stays relatively constant even with mouth open
• Common use: Acute hypoxemic respiratory failure, post-extubation support, neonatal/pediatric ICU
• Must be humidified - dry high-flow O₂ is not tolerated
• Commercial systems: Vapotherm, Fisher & Paykel Nasal High Flow, AquinOx
• Roberts and Hedges', p. 91

7. Non-Invasive Positive Pressure Ventilation (NPPV) - CPAP / BiPAP

• CPAP (Continuous Positive Airway Pressure): Delivers a constant positive pressure throughout the breathing cycle via a tight-fitting face or nasal mask. Keeps alveoli open (prevents collapse), improves V/Q matching.
• BiPAP (Bilevel Positive Airway Pressure): Delivers higher pressure during inspiration (IPAP) and lower during expiration (EPAP), supporting ventilation in addition to oxygenation.
• FiO₂: Titrable, can deliver up to 100% FiO₂
• Common use: Acute pulmonary edema, COPD exacerbation, obstructive sleep apnea, hypoxemic respiratory failure when intubation not yet needed
• Requires: Tight mask seal, patient cooperation

8. Bag-Valve-Mask (BVM / Bag-Mask Ventilation)

• How it works: A self-inflating bag connected to a face mask and O₂ source, with a one-way valve. The provider manually squeezes the bag to deliver positive pressure breaths. With a reservoir bag and O₂ at 15 L/min, delivers near 100% FiO₂.
• Use: Apneic patients, cardiac/respiratory arrest, preoxygenation before intubation, rescue ventilation after failed intubation
• Critical skill: Proper mask seal + airway positioning is required. Good BVM technique is prerequisite to advanced airway management.
• Roberts and Hedges', p. 92

9. Endotracheal Tube (ETT) / Mechanical Ventilation

• How it works: A cuffed tube inserted into the trachea bypasses the upper airway entirely, allowing precise control of FiO₂ (21-100%), tidal volume, PEEP, and respiratory rate.
• FiO₂: 21-100%, precisely set on the ventilator
• Use: Respiratory failure requiring ventilatory support, apnea, inability to protect airway, severe hypoxemia unresponsive to other methods
• Key settings: FiO₂, tidal volume (6 mL/kg IBW), PEEP, rate

10. Hyperbaric Oxygen Therapy (HBO)

• The patient breathes 100% O₂ inside a pressurized chamber at pressures above 1 atmosphere (typically 2-3 ATA).
• Dramatically increases dissolved O₂ in plasma.
• Indications: CO poisoning (severe), gas gangrene, necrotizing fasciitis, decompression sickness, refractory wound healing, radiation necrosis
• Not available at bedside; requires specialized facility

Quick Reference Summary Table

Device Selection Guide

• COPD with hypercapnia - Venturi mask (precise low FiO₂: 24-28%)
• CO poisoning - NRB at high flow (100% O₂ target)
• Mild hypoxemia - Nasal cannula
• Severe hypoxemia, respiratory distress - HFNC or NRB at high flow
• Acute pulmonary edema - CPAP/BiPAP
• Apnea/cardiac arrest - BVM + 100% O₂, then ETT
• Respiratory failure needing full support - ETT + mechanical ventilation