What is the fraction of inspired oxygen (FIO2) provided by nasal prong oxygen?

FiO2 Delivered by Nasal Prong Oxygen

Nasal cannula oxygen at flow rates of 1-6 L/min delivers approximately 24-50% FiO2, with each liter per minute increasing FiO2 by roughly 3-4%, though the actual delivered concentration varies significantly based on the patient's breathing pattern, respiratory rate, tidal volume, and whether the mouth is open or closed. 1, 2

Flow Rate to FiO2 Conversion

The British Thoracic Society guidelines provide the following framework for standard nasal cannula oxygen delivery 1, 2:

• 1-2 L/min: Approximately 24-28% FiO2 (equivalent to low-concentration Venturi masks) 1, 2
• 2-4 L/min: Approximately 28-40% FiO2 2, 3
• 4 L/min: Approximately 36-40% FiO2 1
• 5 L/min: Approximately 40% FiO2 2
• 6 L/min: Up to 44-50% FiO2 (maximum for standard nasal cannula) 1, 2

Research data confirms these ranges, with measured pharyngeal FiO2 ranging from 0.26-0.54 at rest during 1-6 L/min flow rates 4. However, the actual delivered FiO2 increases by only 2.5-3.8% per liter of oxygen flow, which is lower than traditionally taught 5, 6.

Critical Factors Affecting FiO2 Delivery

Respiratory Pattern Effects

The delivered FiO2 is not fixed and varies substantially based on 4, 6:

• Respiratory rate: Higher rates (>20 breaths/min) decrease FiO2 by approximately 0.012 per 5-breath increase 6
• Tidal volume: Larger tidal volumes dilute the oxygen concentration with more room air 4, 7
• Minute ventilation: Increased ventilation decreases delivered FiO2 6
• Inspiratory flow rate: Higher inspiratory flows entrain more room air, reducing FiO2 4, 7

Mouth Position

Mouth-open breathing significantly increases delivered FiO2 compared to mouth-closed breathing (p < 0.05), though paradoxically decreases mean FiO2 by approximately 0.024 in some measurements due to increased air entrainment 4, 6. This creates substantial inter-patient and intra-patient variability.

Clinical Implications and Limitations

When Nasal Cannula is Appropriate

The British Thoracic Society recommends nasal cannula as the preferred device for medium-concentration oxygen therapy (targeting SpO2 94-98%) because 1, 3:

• Superior patient comfort and tolerance 1, 2
• Can be worn during meals and speech 2, 3
• Less likely to be removed by patients 2, 3
• Lower inspiratory resistance than face masks 2, 3
• No risk of CO2 rebreathing 2, 3
• More cost-effective 1, 2

When Nasal Cannula is NOT Appropriate

Do not use nasal cannula when precise FiO2 control is required 1, 2:

• Patients at risk of hypercapnic respiratory failure (use Venturi masks at 24-28% instead) 1, 3
• When calculating alveolar-arterial oxygen gradients (requires known FiO2) 2
• Patients with respiratory rate >30 breaths/min who need reliable oxygen delivery 1
• When FiO2 >50% is needed (use reservoir mask at 15 L/min) 3

Common Pitfalls

The "4% rule" (each L/min adds 4% FiO2) is inaccurate - actual increases are only 2.5-3.8% per liter and vary widely between patients 5, 6. Standard deviation ranges from ±0.04 to ±0.15, making precise FiO2 prediction impossible 4.

Flow rates above 4-6 L/min may cause nasal discomfort and dryness, though many patients tolerate this well 2. Never assume a specific FiO2 value for clinical calculations without direct measurement 2, 4.

High-Flow Nasal Cannula Comparison

High-flow nasal cannula (HFNC) systems deliver 30-70 L/min and provide more predictable FiO2 (up to 100%) because the high flow rate exceeds patient inspiratory demands 1, 2. At 60 L/min HFNC flow, measured FiO2 closely matches set FiO2 at concentrations of 30-50%, though some deviation occurs at 70% with larger tidal volumes 7. HFNC should be considered for patients requiring medium-to-high concentration oxygen who are not at risk of hypercapnia 1.

Practical Algorithm for Oxygen Titration

For patients without risk of hypercapnia 1, 3:

1. Start nasal cannula at 2-4 L/min
2. Titrate by 1-2 L/min increments to achieve SpO2 94-98%
3. Maximum 6 L/min for standard nasal cannula
4. If inadequate at 6 L/min, escalate to reservoir mask at 15 L/min or consider HFNC

For patients at risk of hypercapnia (COPD, obesity hypoventilation) 1, 3:

1. Use Venturi mask at 24% or 28% (preferred for precision)
2. Alternative: nasal cannula at 1-2 L/min
3. Target SpO2 88-92%
4. Monitor with arterial blood gas within 30-60 minutes