A. Low-Flow Systems
- Low-flow systems provide less oxygen flow than the patient’s inspiratory flow (normal 20-30 L/min).
- For every 1 L/min of flow, FiO2 increases by ~4% over room air (21%).
Nasal Cannula
Flow rate: 1-6 L/min (FiO2 24-44%)
Advantages:
- Convenient (patients can talk, eat, drink)
- Easy to use, inexpensive, disposable
Limitations:
- Less effective in mouth breathers, blocked nostrils, deviated septum, or nasal polyps
- Can cause dry nasal passages if flow exceeds 4 L/min
Simple Face Mask
Flow rate: 5-10 L/min (FiO2 40-60%)
Advantages:
- Provides a higher oxygen concentration than a nasal cannula.
Limitations:
- Less comfortable, can't exceed 10 L/min
- Risk of rebreathing CO2 at low flow rates
Clinical Tip:
Ensure a minimum of 5 L/min to prevent CO2 rebreathing.
Non-Rebreather Mask (NRB)
Flow rate: 10-15 L/min (FiO2 80-95%)
Advantages:
- Delivers high concentrations of oxygen; a one-way valve prevents rebreathing expired air.
Limitations:
- Risk of suffocation if oxygen flow is interrupted
- Only for short-term use (e.g., emergency pre-oxygenation before intubation)
- Requires a tightly sealed mask for full effectiveness
Clinical Tip:
Ensure the reservoir bag is fully inflated before placing the mask on the patient.
B. High-Flow Systems
High-flow systems exceed the patient's inspiratory demand, delivering precise FiO2. They are beneficial in situations requiring accurate oxygen delivery.
Venturi Mask
Colors and FiO2 settings:
- Blue: 24% (2 L/min)
- Yellow: 28% (4 L/min)
- Green: 35% (8 L/min)
- Pink: 40% (10 L/min)
- Red: 60% (15 L/min)
Mechanism:
Uses the Venturi effect to entrain air, ensuring a precise FiO2 regardless of the patient's breathing pattern.
Advantages:
- Accurate and adjustable FiO2
- Useful in COPD patients where hyperoxia should be avoided
Limitations:
° Limited to FiO2 of 60%
° FiO2 can be diluted in patients with high inspiratory flow (e.g., severe respiratory distress)
Clinical Tip:
Ideal for COPD patients requiring precise, controlled oxygen delivery.
High-Flow Nasal Cannula (HFNC)
Flow rate:
Can deliver up to 40-60 L/min
FiO2:
Adjustable (up to 100%)
Mechanism:
Provides heated, humidified oxygen, reduces work of breathing, and can wash out dead space.
Advantages:
- Comfortable compared to NIV mask
- Can match high inspiratory demands
- Provides a small degree of positive end-expiratory pressure (PEEP)
Limitations:
° Does not provide a constant PEEP like CPAP
° Requires special equipment and expertise
Clinical Tip:
Often used as a bridge before NIV or intubation in acute respiratory failure.