FiO2 in Oxygen Delivery Devices

Core Concept: Low-Flow vs. High-Flow Systems

• Morgan & Mikhail's Clinical Anesthesiology, 7e
• Roberts and Hedges' Clinical Procedures in Emergency Medicine

1. Nasal Cannula (Variable-Performance / Low-Flow)

• Small prongs sit in the nares; oxygen accumulates in the nasopharynx as a reservoir
• Rule of thumb: FiO2 increases by ~4% (some sources say ~2-4%) per 1 L/min increase in flow
• Comfortable, allows eating and speaking; ~80% of gas is wasted during expiration
• Flows >5 L/min poorly tolerated (nasal dryness, discomfort)

2. Simple (Hudson) Face Mask (Variable-Performance / Low-Flow)

• Covers nose and mouth; the mask body acts as an oxygen reservoir (~200 mL)
• Has side vent holes allowing room air entrainment during inspiration
• Minimum flow of 5 L/min required to flush CO2 from the mask and avoid rebreathing
• FiO2 is unpredictable due to variable mask seal and breathing pattern

3. Partial Rebreathing Mask

• Simple mask + reservoir bag (~600 mL); bag fills with the first portion of exhaled gas (mainly dead space, so little CO2 rebreathing)
• No valves between bag and mask
• Higher FiO2 than simple mask due to larger reservoir

4. Non-Rebreathing Mask (NRM)

• Reservoir bag + one-way valves: a valve between bag and mask prevents exhaled gas from entering bag; side valves prevent room air inhalation
• In practice, one side valve is often removed as a safety measure (allows inhalation if O2 supply fails)
• A critical clinical point: at 15 L/min, actual FiO2 is ~70%, not 100% as commonly assumed, due to imperfect facial seal
• Higher FiO2 (>90%) is achievable by increasing flow to 45 L/min using a flush-capable flowmeter

5. Venturi Mask (Air-Entrainment Mask) - Fixed-Performance / High-Flow

• Uses the Bernoulli/Venturi principle: a jet of O2 through a narrow orifice entrains room air through side ports in a fixed ratio, providing a precise FiO2
• The total gas flow (O2 + entrained air) meets or exceeds the patient's inspiratory flow, so FiO2 is largely independent of breathing pattern
• Color-coded interchangeable jets/diluters; each jet specifies the required wall O2 flow
• Best device for COPD patients needing controlled, precise low-concentration oxygen

6. High-Flow Nasal Cannula (HFNC)

• Delivers heated, humidified O2 at high flow rates (up to 60 L/min) via wide-bore nasal prongs
• Requires 3 components: wide-bore prongs, humidifier, and a high-flow delivery device (e.g., Optiflow/Fisher & Paykel)
• FiO2 remains relatively stable even with the mouth open (nasopharynx acts as an anatomical reservoir)
• Better tolerated than face masks or NPPV; requires brief setup

7. Bag-Valve-Mask (BVM) / Anesthesia Bag

• Self-inflating BVM: ~1.5 L bag; delivers FiO2 up to 0.9+ when oxygen reservoir is used
• Anesthesia bag (non-self-inflating): requires continuous flow; FiO2 approaches 1.0 with proper technique and a tight mask seal
• During cardiac arrest: 100% FiO2 should be administered (AHA guidelines)
• After ROSC: titrate FiO2 to target SpO2 ≥94% to avoid reperfusion-related hyperoxia injury

Summary Table

Clinical Pearls

1. Low-flow vs. high-flow is about the system, not the flow number. A nasal cannula at 6 L/min is still "low-flow" because patient inspiratory flow exceeds supply flow; a Venturi mask at 4 L/min may be "high-flow" because total entrainment flow meets inspiratory demand.
2. Tachypnea reduces FiO2 in all variable-performance devices. The faster a patient breathes, the more room air is entrained, diluting the delivered O2.
3. NRM does not deliver 100% FiO2 at 15 L/min - real-world FiO2 is ~70% due to facial seal leaks. Increasing flow to the flush setting (~45 L/min) can achieve >90%.
4. Venturi mask is preferred in COPD when precise, controlled low-concentration oxygen is needed to avoid hypercapnic respiratory failure.
5. HFNC is better tolerated than face masks and NPPV, and delivers reliably high FiO2 at high flow rates, making it increasingly preferred for moderate-to-severe hypoxemia in spontaneously breathing patients.