Why It Matters That Not All High Flow is the Same
In a previous piece we discussed the various design elements that create differences among types of High Flow Nasal Cannulas (HFNC). Given the increasing popularity of HFNC, it’s important to not just be aware of what makes one device different from another, but also to understand the difference in their clinical outcomes.
Not all HFNC achieve the same results.
The key categorical difference we’ll discuss in this piece concerns commodity high flow oxygen products versus high velocity therapy, which is also known as high velocity nasal insufflation (HVNI).
The Impact of Faster Flush Time
As we all know, the primary mechanism of action for HFNC devices is an effective upper airway purge. The device replaces the CO2 rich end-expiratory gas found in the anatomical dead space with fresh, oxygen-rich gas, helping patients breathe from their nasopharynx rather than through it.
High velocity therapy, which uses small-bore cannulas, flushes the upper airway dead space faster than commodity HFNC using large-bore cannulas.1 This speed of washout matters because the higher a patient’s respiratory rate, the less time there is between breaths to wash out the CO2 from the dead space, and thereby provide effective treatment.
A computational fluid dynamics model study by Miller and colleagues illustrates this impact well. The study was comparing 20 L/min of flow through a small-bore vs a large-bore cannula in the same model.1 Figure 1 depicts how much CO2 is left in the airway comparing the two cannula options at different respiratory rates.
It becomes clear that high velocity therapy has a better washout efficacy than commodity HFNC. As impressive as these visuals may be in highlighting the difference, they are done on a computational model, not a real human being. So, let’s look at studies conducted on real patients to further understand how high velocity therapy outcomes are different from HFNC ones.
High Velocity Therapy Outcomes Are Comparable to Bi-Level PAP
Randomized controlled trial evidence showed high velocity therapy has outcomes comparable to Non-Invasive Positive Pressure Ventilation (NiPPV)/Bi-Level PAP when treating adult emergency department patients in undifferentiated respiratory distress.2 You can read a summary of the study here, but the key take-away is that respiratory distress patients presenting in the ED and traditionally deemed in need of NiPPV, can be effectively treated using high velocity therapy without an increased risk in intubation. Additionally, a subgroup analysis of these patients with an ADHF/CHF diagnosis at discharge found that there was no difference in the efficacy of treatment or risk for intubation between NiPPV and high velocity therapy.3
It is currently not possible to say how commodity HFNC would fare in relation to NiPPV because studies comparing the two tend to focus only on patients with symptoms of hypoxemia and exclude hypercapnia.4,5 A systemic review of studies comparing commodity HFNC performance to noninvasive ventilation in adults found that most studies show HFNC to have comparable outcomes to NIV for hypoxemic patients.6 The review does not address hypercapnic patients or patients in undifferentiated respiratory distress.
Looking at the neonatal population, clinical studies conducted with high velocity therapy have shown noninferiority to CPAP as well as Bi-Level PAP for primary respiratory support.7,8 In other words, high velocity therapy is as safe and effective as nCPAP or Bi-Level PAP in treating premature neonates with RDS.
Similar studies conducted with commodity HFNC have not been able to show these results.9,10
Figure 2 shows at a glance a summary of recent randomized controlled trials in neonates.
The Impact of Optimal Humidification with Minimal Rainout
In addition to being non-inferior to NiPPV, high velocity therapy also shows better outcomes when it comes to rainout.11 Humidification is a key component to HFNC devices as optimal humidification is needed to preserve mucociliary health.12 In general, the less rainout there is, the more humidity is delivered to the patient.
In their study “Risks Associated With Conventional Humidifiers Adapted for High-Flow Nasal Cannula Therapy in Human Infants: Results of a Time and Motion Study”, Tero and colleagues looked at the work load associated with condensate management between a conventional HFNC and high velocity therapy.11 They concluded that commodity HFNC “was associated with greater staff workload and patient risk related to the management of the rainout compared with [Vapotherm®]” high velocity therapy.
In short, the differences between high velocity therapy and commodity HFNC add up and become apparent in patient outcomes and staff workload. High velocity therapy is an appealing mask-free alternative to NiPPV. It is a system that combines the comfort of high flow with the efficacy of NiPPV.
References