The nasal cannula is a widely used, low-flow oxygen delivery device employed across various healthcare settings. It consists of a lightweight tube with two prongs that rest just inside the nostrils, providing supplemental oxygen to patients with mild to moderate respiratory needs. Proper placement is essential for patient safety and therapeutic effectiveness.
Understanding How Nasal Cannulas Work
The nasal cannula operates on a low-flow principle, typically delivering oxygen at rates between one and six liters per minute (\(\text{L}/\text{min}\)). This flow rate is significantly lower than the patient’s maximum inspiratory flow rate, meaning the oxygen is always diluted with ambient room air, which contains 21% oxygen.
The device’s effectiveness relies on the anatomical dead space of the nose and pharynx, which acts as a small oxygen reservoir. During the pause between breaths, the low-flow oxygen accumulates in the nasal cavity. When the patient inhales, this concentrated volume is drawn into the lungs before room air is entrained.
This reservoir effect allows the fraction of inspired oxygen (\(\text{FiO}_2\)) to increase above 21%. For every \(\text{L}/\text{min}\) increase in flow, the \(\text{FiO}_2\) increases by approximately 4%, reaching a maximum of about 44% at 6 \(\text{L}/\text{min}\). The patient’s respiratory rate and the volume of each breath significantly influence the actual concentration of oxygen that reaches the lungs.
The Risks and Ineffectiveness of Mouth Placement
Placing the prongs inside the mouth is ineffective and introduces risks. This failure occurs because the oral cavity disrupts the oxygen reservoir mechanism the device relies upon. The mouth is a large, open space that cannot trap and concentrate oxygen before inhalation.
When oxygen is delivered orally, it is immediately diluted by the vast volume of room air inhaled simultaneously. The low-flow stream becomes a negligible fraction of the total air volume, resulting in an insignificant increase in the \(\text{FiO}_2\).
Furthermore, administering dry oxygen directly into the mouth poses a physical risk to the delicate mucous membranes. The high-velocity stream can rapidly dry out the oral mucosa, leading to discomfort, irritation, and inflammation. Using the mouth as a delivery site negates the physiological benefit and does not improve oxygen saturation.
Correct Usage and Troubleshooting Common Issues
Standard nasal cannulas are designed for use only in the nostrils, with prongs curved downward to follow the nasal passages. Securing the tubing behind the ears and under the chin ensures the prongs remain correctly seated. Routinely check that the tubing is free of kinks or obstructions.
A frequent complaint is dryness and irritation of the nasal passages because the administered oxygen is typically dry and strips natural moisture. To combat this, a humidification bottle is recommended, particularly when flow rates exceed 4 \(\text{L}/\text{min}\). Humidification adds water vapor to the oxygen stream, maintaining the health of the nasal lining.
Do not attempt to increase the oxygen flow rate for minor congestion, as this rarely improves saturation and increases the risk of mucosal damage. If symptoms or oxygen saturation levels do not improve with correct usage, a different delivery method is required.
Medical Alternatives to Standard Nasal Delivery
When a patient cannot effectively use a standard nasal cannula, such as chronic mouth breathers or those requiring higher oxygen concentrations, a medical alternative is necessary. Switching devices must be decided by a healthcare provider.
Simple Face Mask
The simple face mask is a low-flow device that covers both the nose and mouth. It delivers a moderate \(\text{FiO}_2\), typically 35% to 55%, at a flow rate of 5 to 12 \(\text{L}/\text{min}\). The flow rate must be set at a minimum of 5 \(\text{L}/\text{min}\) to ensure exhaled carbon dioxide is flushed from the mask, preventing rebreathing. This is suitable for patients who breathe primarily through their mouth.
Non-Rebreather and Venturi Masks
For patients requiring a higher concentration of oxygen, a non-rebreather mask (NRB) may be used. This mask features a reservoir bag and one-way valves, allowing delivery of a much higher \(\text{FiO}_2\), often 60% to 90%, at flow rates of 10 to 15 \(\text{L}/\text{min}\).
If a precise and stable oxygen concentration is needed, the Venturi mask is an option. It uses color-coded adaptors to blend oxygen with room air and deliver a fixed, consistent \(\text{FiO}_2}\), regardless of the patient’s breathing pattern.