Why Not All High Flow is the Same
In recent years, High Flow Nasal Cannula (HFNC) has gained popularity with clinicians treating patients in respiratory distress.
The general consensus about HFNC is that it is often viewed as an option that is more effective than standard oxygen therapy among hypoxemic patients and also more comfortable when compared to mask-based noninvasive ventilation (NIV). In the 2018 article A Systematic Review of the High-flow Nasal Cannula for Adult Patients, authors Yigal and Einav conclude that HFNC has been found to be “non-inferior to NIV in most studies”.
But, it’s critical to note that the studies examined in the literature review were focused on hypoxemia, not hypercapnia. This exclusion of hypercapnia seems to encompass the current understanding of HFNC as a tool that provides largely effective oxygenation support, but has limitations when it comes to helping hypercapnic patients as well as all-comers in undifferentiated respiratory distress.
As awareness of HFNC continues to spread, it can be easy to assume that the growing body of research on the topic is in essence describing a singular technology—with devices manufactured by different companies, but that are ultimately interchangeable. However, there are multiple design elements at play in these devices — and differences in each impact what is being delivered to the patient.
This is why it’s important for practitioners to know what to look out for when comparing HFNC devices.
What is HFNC?
It’s a little hard to pin down what we mean by HFNC because there is no standardized definition of a HFNC device. But these are the commonly accepted mechanisms of action (MOA) for HFNC therapy that provide a useful breakdown when we look at how devices achieve their efficacy:
- Washout of upper airway dead space
- Reduction of work of breathing (WOB) through provision of adequate flow
- Improvement of lung mechanics through adequately humidified and warmed gas
- Reduction of energy cost for conditioning (heating and humidifying) gas
- Provision of mild distending pressure (akin to CPAP)
To make difference-spotting easier, we’ll further break this down into:
- How does it deliver flow? (that is, MOA 1 & 2)
- How does it deliver humidity? (that is, MOA 3 & 4)
Although point 5, regarding the provision of distending pressure, is a secondary MOA, it’s difficult to measure and compare, as it also depends on whether the patient’s mouth is open or closed. This article will therefore focus on the first four MOA as broken down above.
How does the device deliver flow?
One key design element to look out for is whether your device is designed to work with a large-bore or small-bore cannula. This is because a small diameter increases velocity to more efficiently flush CO2 at lower and more comfortable flows. Miller and colleagues showed in a computational fluid dynamics model that small bore imparts higher velocities, and thus flushes the airway more completely in any given time. In that study, for example, upper airway dead space was purged in 1/3 less time via the small bore.
Figure 1 shows the difference between velocity achieved through a small- versus large-bore cannula.
How does the device deliver humidity?
As with the delivery of flow, the delivery of humidification also depends on design elements that differ across HFNC devices. One common method of heating and humidifying gas is through pass-over humidification, where breathing gas passes over highly heated water to add moisture and humidify the gas. Devices using this method use higher than set temperatures in both the creation and maintenance of humidification.
An alternative, used in Vapotherm® high velocity therapy is membrane humidification, wherein water molecules are delivered to the gas path across a membrane to produce energetically stable vapor without high heat.
In addition to these different ways of humidifying the gas, there are also key design elements in delivering the humidified gas all the way to the patient.
One common method is a delivery tube with heated wire circuits. The common downside of this design is that because the tube isn’t heated uniformly, rainout can happen when gas comes into contact with cooler surfaces.
An alternative, designed to minimize rainout, is the triple lumen delivery tube created by Vapotherm. It uses the safe, insulating heat of warm water to maintain gas temperature and humidity all the way to the patient. The tube feels warm — near body temperature — and patients will not get burned even if prolonged skin contact with it should occur.
Figure 2 shows these different designs via heat mapping.
What FDA product category is the device in?
Finally, you can also look to FDA product categories to see that not all HFNC devices are the same. Commodity high flow oxygen products are in a respiratory humidifier product category (BTT). However, the FDA recently created a new category which includes a product that uses high velocity nasal insufflation and has a label that states the product can be used to augment breathing in spontaneously breathing patients suffering from respiratory distress in a hospital setting. At the time of publication, Vapotherm’s Precision Flow Hi-VNI™ system is the only device placed in this FDA category.
The key to keep in mind when comparing all these differences is that they not only impact the look and feel of the device, but ultimately also the efficacy and clinical outcomes that you can deliver for your patient. You can learn more about those outcome differences in part two of our post: Why It Matters That Not All HFNC Are the Same.
Learn why it matters that not all HFNC are the same.
 Helviz,Yigal and Sharon Einav. A Systematic Review of the High-flow Nasal Cannula for Adult Patients. Crit Care. 2018; 22: 71. Published online 2018 Mar 20. doi: 10.1186/s13054-018-1990-4.
 Dysart K, Miller T, Wolfson M, Shaffer T. Research in high flow therapy: Mechanisms of action. Respiratory Medicine. 2009; 103: 1400-05.
 Miller TL, Saberi B, Saberi S (2016) Computational Fluid Dynamics Modeling of Extrathoracic Airway Flush: Evaluation of High Flow Nasal Cannula Design Elements. J Pulm Respir Med 6:376. doi: 10.4172/2161-105X.1000376. https://www.omicsonline.org/open-access/computational-fluid-dynamics-modeling-of-extrathoracic-airway-flush-evaluation-of-high-flow-nasal-cannula-design-elements-2161-105X-1000376.php?aid=81462