Nasal Prongs (Nasal Cannula)
1. Definition
Nasal prongs (nasal cannula) are a low-flow oxygen delivery device Key characteristics:
- Low-flow oxygen device
- Delivers oxygen through nasal route
- Allows patient to talk, eat, and drink
- Provides variable FiO₂ depending on patient ventilation
2. Principle of Oxygen Delivery
Nasal prongs deliver oxygen by entraining room air during inspiration.
The actual FiO₂ depends on:
- Oxygen flow rate
- Patient tidal volume
- Respiratory rate
- Inspiratory flow
- Mouth breathing
Because room air mixes with oxygen, FiO₂ is variable and unpredictable.
3. Oxygen Flow Rate and FiO₂
Classical ICU Rule (4% Rule)
Each 1 L/min increases FiO₂ by ~4% above room air.
|
Oxygen Flow |
Approximate FiO₂ |
|
Room air |
21% |
|
1 L/min |
24% |
|
2 L/min |
28% |
|
3 L/min |
32% |
|
4 L/min |
36% |
|
5 L/min |
40% |
|
6 L/min |
44% |
4. Maximum Recommended Flow
Standard nasal cannula:
|
Parameter |
Value |
|
Minimum flow |
1 L/min |
|
Typical range |
1–4 L/min |
|
Maximum flow |
6 L/min |
Higher flows cause:
- Nasal dryness
- Epistaxis
- Mucosal irritation
5. Anatomical Oxygen Reservoir Effect
The nasopharynx acts as a small oxygen reservoir.
During expiration:
- Oxygen accumulates in nasopharynx
During next inspiration:
- Stored oxygen is inhaled first
This increases effective oxygen delivery efficiency.
6. Contraindications
Absolute contraindications are rare but include:
|
Contraindication |
Reason |
|
Severe respiratory distress |
Insufficient FiO₂ |
|
Nasal obstruction |
Oxygen cannot enter |
|
Facial trauma |
Cannula cannot be placed |
|
Severe hypoxemia |
Requires higher FiO₂ device |
|
Apnea |
No spontaneous breathing |
7. Advantages
|
Advantage |
Explanation |
|
Comfortable |
Least intrusive oxygen device |
|
Allows speech |
Patient can talk normally |
|
Allows eating |
No mask obstruction |
|
Easy to apply |
Minimal training required |
|
Cheap |
Low cost device |
|
Mobility |
Patient can ambulate |
8. Disadvantages
|
Disadvantage |
Explanation |
|
Variable FiO₂ |
Depends on breathing pattern |
|
Limited FiO₂ |
Maximum ~44% |
|
Nasal dryness |
Especially without humidification |
|
Epistaxis |
With high flows |
|
Mouth breathing reduces efficacy |
Oxygen bypassed |
9. Humidification
Humidification is usually not required at low flow.
Recommended when:
|
Flow rate |
Recommendation |
|
≤4 L/min |
Usually not needed |
|
>4 L/min |
Humidification preferred |
|
Long-term oxygen therapy |
Recommended |
Humidification prevents:
- Mucosal drying
- Thick secretions
- Epistaxis
10. Comparison with Other Oxygen Devices
|
Device |
FiO₂ |
Flow |
Type |
|
Nasal cannula |
24–44% |
1–6 L/min |
Low flow |
|
Simple face mask |
35–60% |
5–10 L/min |
Low flow |
|
Venturi mask |
24–60% |
Variable |
High precision |
|
Non-rebreather mask |
60–95% |
10–15 L/min |
High FiO₂ |
|
HFNC |
Up to 100% |
30–70 L/min |
High flow |
|
Feature |
Standard Cannula |
HFNC |
|
Flow |
≤6 L/min |
30–70 L/min |
|
FiO₂ |
≤44% |
Up to 100% |
|
Humidification |
Optional |
Heated humidified |
|
Positive pressure |
No |
Mild PEEP effect |
|
Dead space washout |
Minimal |
Significant |
2. Target saturation (Guidelines)
|
Patient group |
Target SpO₂ |
|
Most ICU patients |
92–96% |
|
COPD/CO₂ retainers |
88–92% |
|
Post cardiac arrest |
94–98% |
(WHO / BTS / ERS guidelines)