Types of Oxygen Therapy

Figure: Noninvasive oxygenation interfaces - Murray & Nadel's Textbook of Respiratory Medicine

1. Low-Flow (Variable-Performance) Devices

A. Nasal Cannula (Standard)

• Flow: 1-6 L/min
• FiO2: ~24-44% (each L/min adds approximately 4% FiO2 above room air)
• The most commonly used and best-tolerated device
• Above 4-5 L/min, patients often find the nasal jet flow uncomfortable; humidification is advised to prevent mucosal drying
• Requires nasal patency; FiO2 is unpredictable with mouth breathing

• Moustache-style reservoir cannula - a small (~20 mL) reservoir fills with oxygen during exhalation; delivers an oxygen bolus early in inspiration, reducing waste; patients can be maintained at a slightly lower flow rate
• Pendant reservoir cannula - a chest-worn reservoir (~same function) attached to standard nasal cannula tubing

B. Enhanced (Hudson) Nasal Cannula

• Flow: up to 15 L/min
• FiO2: 50-70%
• A three-channel design that allows higher flows than standard nasal prongs; not the same as heated high-flow nasal cannula (HFNC)

C. Simple Face Mask (Hudson Mask)

• Flow: 5-10 L/min (minimum 5 L/min to flush expired CO2)
• FiO2: ~35-60%
• Provides a larger oxygen reservoir than nasal cannulae (100-300 mL dead space of the mask cavity)
• FiO2 is variable, dependent on patient's minute ventilation
• Limitations: interferes with eating/drinking, risk of aspiration if vomiting, drying with prolonged use

2. Reservoir Mask Systems

D. Partial Rebreathing Mask

• Flow: 10-15 L/min
• FiO2: ~50-75%
• The patient exhales the first third of tidal volume (mostly dead-space gas, high O2/low CO2) back into the reservoir bag, which then mixes with fresh oxygen for the next breath
• No one-way valves
• FiO2 still variable; largely replaced by the non-rebreather mask in clinical practice

E. Non-Rebreather Mask (NRB)

• Flow: ≥15 L/min (enough to keep reservoir bag at least half-full)
• FiO2: up to 80-95% (up to 60-80% with very high minute ventilation)
• A one-way valve between the reservoir bag and the mask prevents exhaled air from re-entering the reservoir; side-port one-way valves minimize ambient air inhalation
• First-line device for patients presenting with severe hypoxemia (SpO2 < 85%)
• Risk: prolonged high FiO2 can cause oxygen toxicity (pulmonary inflammation and reactive oxygen species injury)

3. High-Flow (Fixed-Performance) Devices

F. Venturi Mask (Air-Entrainment Mask)

• Flow: variable (set by adapter/nozzle)
• FiO2: precisely 24%, 28%, 31%, 35%, 40%, or 50% (depending on the entrainment port/adapter used)
• Developed by E.J. Campbell (1967) based on the Bernoulli/Venturi principle: oxygen passes through a narrow jet nozzle at high velocity, creating a low-pressure zone that entrains a fixed, predictable ratio of ambient air
• Ideal for COPD patients requiring controlled, titrated oxygen (to avoid suppressing hypoxic drive) and for any condition where precise FiO2 is essential
• Humidification can slightly alter FiO2 (increases density of entrained air, resulting in a slightly higher FiO2)

G. High-Flow Nasal Cannula (HFNC)

• Flow: 40-60 L/min
• FiO2: 30-100% (fully adjustable and precise)
• Delivers heated (37°C), humidified (99.9% relative humidity) oxygen at very high flow rates through wide-bore nasal prongs
• Key physiological effects:
  • FiO2 delivered equals that set on the device (flow exceeds patient's inspiratory demand)
  • Produces a small CPAP effect (~1-3 cmH2O) from the high-velocity nasopharyngeal gas flow
  • Flushes nasopharyngeal dead space with fresh oxygen-rich gas (CO2 washout)
  • Reduces work of breathing
• Indications: hypoxemic respiratory failure (especially pneumonia, acute respiratory distress), post-extubation support, bridge before intubation
• FLORALI trial (2015): HFNC reduced intubation rate in patients with P/F ratio ≤200 compared with NIV or NRB mask
• Note: used cautiously in infectious conditions (e.g. COVID-19) due to aerosol generation concerns

4. Noninvasive Positive Pressure Devices

H. CPAP (Continuous Positive Airway Pressure)

• Delivers a fixed positive pressure throughout the respiratory cycle
• Recruits atelectatic alveoli, improves functional residual capacity (FRC), reduces work of breathing
• Used in OSA, cardiogenic pulmonary edema, postoperative hypoxemia

I. BiPAP / NIPPV (Non-Invasive Positive Pressure Ventilation)

• Delivers different pressures during inspiration (IPAP) and expiration (EPAP)
• Provides ventilatory support in addition to oxygenation
• Indications: COPD exacerbation with hypercapnia, acute cardiogenic pulmonary edema, immunocompromised patients with respiratory failure
• Delivered via full face mask, nasal mask, or nasal pillows

5. Invasive Mechanical Ventilation

• Endotracheal tube (ETT) or tracheostomy connected to a ventilator
• FiO2: 21-100% (precisely controlled)
• Reserved for patients failing all noninvasive methods, with severe respiratory failure, altered consciousness, or inability to protect airway

6. Hyperbaric Oxygen Therapy (HBO)

• Patient breathes 100% oxygen at pressures greater than 1 atmosphere (typically 2-3 atm) inside a sealed chamber
• At 2 atm: plasma oxygen content increases by ~125%; tissue oxygen tension increases 10-fold vs. room air
• Also produces vasoconstriction, reducing edema while maintaining enhanced oxygen delivery
• Indications: carbon monoxide poisoning, decompression sickness, necrotizing fasciitis, osteomyelitis, crush injuries, compromised wound healing, gas gangrene, central retinal artery occlusion, sudden sensorineural hearing loss

Quick Reference: FiO2 by Device

• Fishman's Pulmonary Diseases and Disorders, Chapter 143
• Murray & Nadel's Textbook of Respiratory Medicine, Chapter 137
• Comprehensive Clinical Nephrology, 7th Edition, Table 15.3
• Miller's Anesthesia, 10th Edition
• Tintinalli's Emergency Medicine