High Velocity Nasal Insufflation and Humidification: A Summary of Mechanisms of Action, Technology and Research
Vapotherm, Inc. is the inventor and provider of Hi-VNI® Technology – high-flow, thermally-controlled, humidification systems for High Flow Nasal Cannula therapy. Vapotherm devices are cleared for delivery of breathing gases by nasal cannula at flow rates of up to 8 L/min in infants and 40 L/min in adults, providing what is known as High Velocity Nasal Insufflation (HVNI).
What Is High Velocity Nasal Insufflation (HVNI)?
High Velocity Nasal Insufflation (HVNI) is a refined form of High Flow Nasal Cannula that flushes the anatomical dead space of expiratory gas between each breath, creating a fresh gas reservoir that facilitates oxygenation and alveolar ventilation.
HVNI differs from generic high flow nasal cannula by the higher velocity gas flows it delivers through a small-bore cannula system, which introduces a dynamic energy to support mixing of gas. This effect allows HVNI to more efficiently flush the upper airway of expiratory gas high in CO2 and replace it with optimally conditioned gas.This improved flush efficiency results in HVNI being able to be utilized as a tool for treating the signs and symptoms of respiratory distress, including hypoxia, hypercapnia, and dyspnea.
￼￼Respiratory Physiology and Alveolar Ventilation
In order to understand the mechanisms behind HVNI, it is helpful to review some fundamental respiratory physiology. Under normal breathing conditions, approximately 30% of an inspired tidal volume represents anatomical dead space in adults and up to 50% in neonates. At the start of an inspiration, this dead space is filled with expiratory gas remaining from the previous expiration. While this anatomical dead space volume is essential to warming and humidifying inspiratory gas and conducting this gas to the thorax and dispersing to lung regions, the contribution of dead- space (expiratory gas) to a new breath does impact breathing efficiency.
In a healthy person, alveolar oxygen concentrations are lower than ambient air and alveolar carbon dioxide concentrations are greater than ambient air. This difference between ambient and alveolar gas is a function of alveolar ventilation as well as blood gas content. Alveolar ventilation differs from the more familiar term minute ventilation as a function of dead space, where:
Minute Ventilation = Tidal Volume x Respiratory Rate
Alveolar Ventilation = (Tidal Volume – Dead Space) x Respiratory Rate
Based on the relationship between ventilation parameters, a reduction in dead space volume results in lower minute ventilation required to achieve adequate alveolar ventilation. Therefore, dead space volume directly impacts tidal volume and/or respiratory rate requirements, and thus breathing effort, even in healthy people. In this regard, HVNI via small-bore cannula can enhance respiratory efficiency by maximizing the flush of extra- thoracic dead space and supporting respiratory work efforts. But first, ideal gas conditioning must be achieved.
￼￼Importance of Gas Warming and Humidification
The mucosal tissue of the nasopharyngeal space is designed to warm and humidify breathing gas prior to entering the lower respiratory tract1. This is accomplished anatomically by achieving a large surface area to interact with inspiratory gas, coupled with cyclic exhalation where warmth and humidity can be recycled back to these tissues. Exposing the nasopharyngeal tissues to a continuous flow of gas that is below body temperature and the water vapor saturation point (i.e., below 100% relative humidity at 37°C) can overload these tissues. Such an overload of the nasopharyngeal tissues result in significant dysfunction, drying and damaging the nasal mucosa2-5, which likely contributes to staphylococcal sepsis6. Even at low flows, conventional nasal cannula therapy is uncomfortable and raises numerous patient complaints, particularly related to dry nose and mouth7.
Ideally, inspiratory gas should be warmed to approximately body temperature (37oC) and humidified to 100% relative humidity8,9. Furthermore, humidification with vapor versus aerosolized water is the least likely to cause airway and lung injury by latent ￼￼￼￼￼￼￼￼￼￼￼￼￼￼￼￼￼￼￼￼￼￼￼￼￼￼￼￼￼￼￼￼￼￼￼￼￼￼￼￼￼￼￼￼￼￼￼￼￼￼￼￼￼￼￼￼￼￼￼￼￼￼￼￼￼￼￼￼￼￼￼￼￼heat loss and deposition of water droplets12. Vapotherm membrane technology facilitates the passage of water into the breathing gas in a vapor phase, and as demonstrated in a bench test by Waugh and Granger, provides respiratory gases at body temperature and 99.9% relative humidity throughout the designated flow range up to 40 L/min.
Vapotherm devices incorporate a patented vapor transfer cartridge system that allows water vapor to diffuse into the respiratory gas stream while heating the gases to the prescribed temperature (typically 35°C to 37°C, per patient comfort). This system is fundamentally different from the conventional heated plate humidifier systems. The Vapotherm devices also employ a triple lumen ‘jacketed’ delivery tube and proprietary nasal cannulae designed to maintain temperature and to minimize condensation and to eliminate rainout. These later two features protect the state of respiratory gases so that the gas reaches the patient.
Circuit Design and the Impact on Flow and Velocity
Volumetric flow is the volume of gas that passes over a unit of time. The speed at which that volume travels is the velocity. Velocity, at a constant volumetric flow, varies inversely with the cross sectional area of a tube. The smaller the cross section the higher the velocity.
Applying this concept to cannula design explains the higher velocity flows Vapotherm’s small-bore cannula system delivers compared to larger bore cannula systems. The graph below shows the calculated velocity difference between a Vapotherm adult cannula and a commercially available large bore cannula. At a volumetric flow of 40 L/min, Vapotherm delivers a velocity almost six times that of the large bore cannula system.
Physiologic Impact of Velocity
In a patient in respiratory failure, time is critical. HVNI has its greatest impact during the exhalation phase of breathing, when it resets a significant portion of the anatomical dead space as a reservoir of fresh, CO2 depleted, gas for inhalation. As the patient’s respiratory rate increases, the time to flush the expiratory gas from the upper airway decreases. For example, an adult patient with a respiratory rate of 20 breaths per minute or above has 1.5 seconds or less of time between inspired breaths, depicted in the graph below.
As respiratory rate increases, the patient becomes less efficient at eliminating CO2 from the anatomical dead space. The importance of velocity is that velocity introduces dynamic energy that facilitates the mixing of gas in the upper airway to expedite purge of expiratory gas.
Therefore, the advantage of a system that delivers high velocity flowsis the ability to treat the signs and symptoms of respiratory distress in patients who are experiencing CO2 retention, which may reduce the need for more invasive therapies (i.e. NIPPV and Mechanical Ventilation).