Oxygen Therapy Devices

Classification Overview

1. Low-Flow (Variable-Performance) Devices

Nasal Cannula

• The most widely used outpatient oxygen delivery device; comfortable and allows eating, drinking, and speech.
• Available in standard (1-6 L/min) and enhanced (up to 15 L/min) configurations.
• FiO2 rises approximately 2-4% per L/min above room air: at 1 L/min FiO2 ~24%, at 6 L/min FiO2 ~44%.
• Flows >5-6 L/min are poorly tolerated - they jet gas uncomfortably into the nasal cavity and dry the mucosa; humidification is advised for long-term use.
• ~80% of delivered oxygen is wasted during expiration.
• Limitation: FiO2 is unpredictable in tachypneic or mouth-breathing patients.

Simple Face Mask (Hudson Mask)

• Flow rates of 5-10 L/min, FiO2 approximately 35-60%.
• Has a 100-300 mL internal reservoir (the mask cavity itself).
• Minimum flow of 5 L/min is required to flush exhaled CO2 from the mask; flows below this cause CO2 rebreathing and increased inspiratory resistance.
• Suitable for short-term use (transport, PACU, ED) but not for profoundly hypoxemic or tachypneic patients.

2. Reservoir Systems

Partial Rebreathing Mask

• A 600-1000 mL reservoir bag attached to a simple face mask; source oxygen at 10-15 L/min.
• During exhalation, initial expired gas (largely dead-space air, low in CO2) fills the bag; on the next breath, this is rebreathed mixed with fresh O2.
• FiO2 approximately 50-75%, depending on minute ventilation.
• Largely replaced by the non-rebreathing mask in practice.

Non-rebreathing Mask (NRB)

• Same as above but with a one-way valve between the reservoir bag and mask, preventing exhaled gas from entering the bag.
• Source flow: 15 L/min; FiO2 can reach approximately 60-95% (up to ~1.0 under ideal conditions).
• First-line oxygen delivery for severe hypoxemia (SpO2 <85%) in the acute setting.
• Risk of hyperoxia-related pulmonary and systemic toxicity with prolonged use.

3. High-Flow (Fixed-Performance) Devices

Venturi Mask

• Developed by E.J. Campbell in 1967; operates on Bernoulli's principle (Venturi effect).
• High-velocity 100% O2 passes through a narrow jet, creating negative pressure that entrains a precise ratio of ambient air through calibrated apertures.
• Color-coded adapters deliver fixed FiO2 of 24%, 28%, 31%, 35%, 40%, or 50%.
• Total gas flow is 15-50 L/min, which flushes the mask and prevents CO2 accumulation even in tachypnea.
• Key advantage: Precise, reliable FiO2 delivery - essential in hypercapnic patients (e.g., COPD exacerbation) where uncontrolled oxygen can blunt hypoxic drive.
• Currently first-line for patients at risk of hypercarbia.

Bag-Valve-Mask (BVM) / Anesthesia Bag Systems

• Self-inflating bags (~1.5 L) with oxygen inlet reservoir, capable of FiO2 approaching 1.0 when correctly assembled and sealed.
• Used for manual ventilation in resuscitation or pre-intubation; requires a tight mask seal.
• A tailpiece inlet valve in self-inflating bags requires negative pressure recoil to open - a spontaneously breathing patient cannot reliably trigger it, so care is needed to avoid misleading the clinician.

4. High-Flow Nasal Cannula (HFNC)

• Delivers heated, humidified oxygen at 40-60 L/min through wide-bore nasal prongs.
• An air-oxygen blender allows FiO2 titration from 0.21 to 1.00.
• Humidification to near 100% relative humidity at 37°C is mandatory for tolerance at these flow rates.
• Prongs should not fully occlude the nares - a 50% gap is recommended for adequate exhalation and to prevent gastric distention.

• Each 10 L/min increase in flow generates approximately 0.3-1.0 cmH2O of CPAP effect.
• Reduces respiratory rate, minute ventilation, and work of breathing.
• Washes out upper airway dead space, providing a small CO2 clearance benefit.
• Increases end-expiratory lung volume and improves ventilation distribution.

5. CPAP and Non-invasive Ventilation (NIV)

• CPAP delivers continuous positive pressure throughout the respiratory cycle via a full face mask, total face mask, or nasal mask.
• It supports oxygenation by increasing mean airway pressure and alveolar recruitment, but does not augment ventilation per se.
• Indications: COPD exacerbations, cardiogenic pulmonary edema, obstructive sleep apnea, and post-extubation support.
• BiPAP and other NIV modes add inspiratory pressure support on top of PEEP, assisting ventilation as well as oxygenation.

6. Special Considerations

Hypercapnic Patients (e.g., COPD)

Long-Term Oxygen Therapy (LTOT)

• Indicated when resting PaO2 ≤55 mmHg or SpO2 ≤88%, or PaO2 56-59 mmHg with cor pulmonale or polycythemia.
• Delivered via nasal cannula for ≥15 hours/day - based on the MRC and NOTT trials, this threshold improves survival in COPD.
• The 2016 LOTT trial found no benefit in patients with moderate resting or exercise-induced desaturation (SpO2 89-93%).

Air Travel

• Murray & Nadel's Textbook of Respiratory Medicine (Chapter 137)
• Morgan & Mikhail's Clinical Anesthesiology, 7e (Chapter 58)
• Fishman's Pulmonary Diseases and Disorders (Chapter on LTOT)