How Does High Flow Nasal Cannula (HFNC) Work?

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Answer

It improves oxygenation in these three ways. First, it washes out the CO2 from the end expiration. Every time a person exhales, they have end-expiratory gas that remains in the upper airway. Whenever they exhale, there is a little bit of CO2 that is left in that end expiration. Whenever we use higher flows, we wash out CO2 to have a more oxygen-rich environment when the patient goes to inspire, and there are fewer traces of those end-expiratory gases. Ultimately, this increases the oxygen that is being delivered to the patient.

We can also reduce dead space. Dead space is anywhere that oxygen exchange cannot occur. Oxygen exchange cannot happen in any upper airways like the nose, sinuses, or trachea. Oxygen exchange cannot occur until deep in the lungs, where it crosses over in the alveoli and membranes there. If we can reduce dead space by generating flows that get the oxygen down to where it can be absorbed, not only are we going to improve oxygenation, but chances are we will also improve the work of breathing. The patient is not having to work as hard to overcome their normal anatomical dead space to get that oxygen down into their lungs where it needs to be.

Over the last few years, there have been varying studies about whether high flow therapy provides PEEP or positive end-expiratory pressure. Dres and Demoule (2017) reported that optimal PEEP levels were directly linked to a regular breathing pattern and a closed mouth. This means that if the patient has a normal rate and pattern, they can get up to approximately seven centimeters of pressure. However, we know that that is not always the case. Sometimes our patients are talking, eating, or taking medications, and their mouth is open. We cannot get them to keep their mouth closed at all times. So yes, PEEP is possible.

We know that PEEP increases the alveoli's surface area, creating lung expansion and improving the potential for oxygenation. However, it is a catch-22 when we look at it as a non-invasive support device. I will talk about that in a little bit more detail later. Another interesting aspect I found about this study is that they also reported not only positive end-expiratory pressure but also for every 10 liters of the flow used on a high flow therapy device, there was an increase in central airway pressures. This was a .69 increase if the patient's mouth was closed and a .35 increase for the same user if the patient's mouth was open. These are not huge increases, but if you have a patient with high main airway pressures, you need to be mindful of this.

This Ask the Expert is an edited excerpt from the course, High Flow Nasal Oxygen: What’s What!, presented by Jessica Fino, EdD, RRT.