How does high-flow nasal cannula (HFNC) work?
High-flow nasal cannula (HFNC) is a heated and humidified system that allows prescribed fraction of inspired oxygen (FIO2) levels to be delivered at very high flow rates. Let’s now consider how HFNC can help our patients improve their respiratory disease.
There are three main proposed benefits of HFNC:
- Precise oxygen delivery
- Functional residual capacity enhancement
- Dead space washout
Precise oxygen delivery
Traditional nasal cannula delivers flow rates of 2–6 L / min. But patients with respiratory distress can have much higher peak inspiratory flow rates. If the inspiratory flow rate of the patient is greater than what is being provided by the cannula, the patient will entrain room air into the lungs. This results in oxygen dilution, and the patient will not be receiving the precise amount of oxygen that is desired.
When high-flow nasal cannula, or HFNC, is used to deliver oxygen, the flow rates are much higher than can be achieved with traditional nasal cannula. This results in a greater delivery of prescribed oxygen into the lungs, and less entrainment of room air. The oxygen you want to deliver to your patients is not prone to the same effect of dilution!
Figure 1. Differences in oxygen delivery between traditional nasal cannula and high-flow nasal cannula (HFNC), a) when using the traditional nasal cannula, if the inspiratory flow rate of the patient is greater than what is being provided by the cannula, the patient will entrain room air into the lungs, resulting in oxygen dilution, b) when using HFNC, inspiratory flow rates of oxygen are higher, resulting in greater delivery of prescribed oxygen into the lungs, and less entrainment of room air.
Functional residual capacity enhancement
HFNC also exerts its effect by providing some variable positive pressure.
When measured with manometers in the posterior oropharynx with closed mouth breathing, the pressures are modest, but the effect nearly goes away when patients open their mouths.
Despite this discrepancy, studies show that the functional residual capacity (FRC) increases by about 25% when HFNC is applied.
Figure 2. By providing variable positive pressure, use of high-flow nasal cannula (HFNC) increases functional residual capacity (FRC) by about 25% compared to use of traditional nasal cannula.
Dead space washout
Lastly, when HFNC is applied, the constant high flow of oxygen provides a washout of the anatomical dead space of the oropharynx and proximal tracheobronchial tree, which results in more efficient breathing.
Figure 3. Compared to traditional nasal cannula, the use of high-flow nasal cannula (HFNC) results in more efficient breathing due to a washout of anatomical dead space in the oropharynx and proximal tracheobronchial tree.
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