High Flow Nasal Cannula Oxygen Therapy - Part 1

This is an automatically translated article.

The article was written by Dr., Doctor Truong Ngoc Hai, Department of Emergency Resuscitation, Vinmec Central Park International General Hospital
Nasal high-flow oxygen therapy helps to provide positive pressure support to the oropharynx, creating positive end-expiratory pressure for the lower airways. This effect works similarly to continuous positive airway pressure ventilation, which exerts force to keep the alveolar airways from collapsing due to increased surface tension during expiration.
Oxygen therapy is one of the commonly prescribed interventions in the care of patients with acute tissue hypoxia. Low-flow nasal cannula (LFNC) oxygen can only provide a maximum efficiency of 4 to 6 liters/min, equivalent to a FiO2 concentration of about 37-45%. Long-term use of more than 4-6 liters of oxygen per minute will irritate the nasal mucosa, dry the airways and increase the risk of nosebleeds. In this therapy, FiO2 levels are directly related to oxygen flow. To increase FiO2, oxygen flow must be increased. Low-flow oxygen delivery through the nasal cannula, is an open system, so the amount of oxygen leakage is high, the therapeutic effect is limited.
High-flow nasal cannula (HFNC) oxygen therapy is an oxygen delivery system capable of delivering up to 100% warmed and humidified oxygen, with flow rates up to 60 liters/ minute. In HFNC, all settings are independently controlled, allowing for greater control over the FiO2 delivery and several benefits of use [1][2].
Basic components include a flow generator providing up to 60 l/min air flow, an oxygen-air mixer to achieve a 21% to 100% increase in FIO2 regardless of air flow. and the saturating humidifier humidifies the gas mixture at a temperature of 31 to 37oC. To minimize condensation, warm, humidified air is delivered to the patient via heat pipes to the wide-bore nasal prong.
Currently, there are 5 physiological mechanisms that are thought to be responsible for the effectiveness of Nasal High Flow Oxygen Therapy. Includes:
Elimination of emissions at physiological dead space, including CO2; Decreased breathing rate; Positive end-expiratory pressure; Increased tidal volume; Increase end-expiratory volume. Physiological dead space accounts for about one-third of the tidal volume. Inefficient ventilation leads to accumulation of CO2 and reduced availability of oxygen (O2) in inhaled air. The high airflow rates in Nasal Catheter High Flow Oxygen Therapy can deliver a volume that exceeds the patient's physiological breathing rate, increasing ventilation and allowing O2 to replace stagnant CO2. Increases PAO2 and improves patient oxygenation.

Nasopharyngeal high-flow oxygen therapy can reduce nasopharyngeal airway resistance, leading to improved ventilation and blood oxygenation through positive airway pressure. The resistance of the airways obeys Hagen-Poiseuille's law and is calculated as follows:
R = 8nl / 3.14 r4
where l is the length of the airway, n is the dynamic viscosity of the air and r is the radius of the airway. Physiologically, the pharynx is a dynamic environment that allows for the expansion and contraction of the airway radius. By creating positive airway pressure, Nasal Catheter High Flow Oxygen Therapy pressurizes the oropharynx from the inside out. This dilates the radius of the oropharyngeal airway and significantly reduces airway resistance to airflow, increasing ventilation and oxygenation of the blood.
There have been a number of physiological studies showing improvement in respiratory mechanics by decreasing respiratory rate and increasing tidal volume. [3]
In addition to providing positive pressure support to the oropharyngeal region, nasogastric high-flow oxygen therapy also produces positive end-expiratory pressure to the lower airways. This effect works similarly to continuous positive airway pressure, which exerts force to keep the alveolar airways from collapsing due to increased surface tension. during exhalation. In addition, this therapy allows improved recruitment of alveoli, increasing the effective surface area in the lungs for gas diffusion in the alveolar-capillary membrane. However, it is important to note that patients must keep their mouths closed in order to obtain the maximum benefit of PEEP from nasogastric high-flow oxygen therapy. An approximate estimate of PEEP produced with the mouth closed is about 1 cm H2O for a gas flow rate of 10 liters. An increase in PEEP results in a corresponding increase in end-expiratory volume [4] [5]. One of the challenges is that it is difficult for patients to close their mouths while in respiratory failure.
Humidifying and warming the inhaled air is extremely important in creating an efficient oxygenation system. Mainly creating comfort in ventilation. Classically, nasogastric low-flow oxygen therapy blows cool, dry air directly into the nasal passages. Leads to mucosal dryness, irritation, nosebleeds and cracking of tissue barriers, causing discomfort and poor adherence. Many nasal cannula high-flow oxygen delivery systems are designed with inline warming and humidifying systems to provide a well-moisturized and temperature-controlled airflow without irritating the mucosa, increasing the comfortable for the patient (31oC to 37oC). Increased comfort leads to improved compliance and therefore better treatment outcomes. [sixty seven]

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References
[1] Segovia B,Velasco D,Jaureguizar Oriol A,Díaz Lobato S, Combination Therapy in Patients with Acute Respiratory Failure: High-Flow Nasal Cannula and Non-Invasive Mechanical Ventilation. Archivos de bronconeumologia. 2018 Jul 12 [PubMed PMID: 30017253]
[2] de Jong A,Calvet L,Lemiale V,Demoule A,Mokart D,Darmon M,Jaber S,Azoulay E, The challenge of avoiding intubation in immunocompromised patients with acute respiratory failure . Expert review of respiratory medicine. 2018 Aug 12 [PubMed PMID: 30101630]
[3] Mündel T,Feng S,Tatkov S,Schneider H, Mechanisms of nasal high flow on ventilation during wakefulness and sleep. Journal of applied physiology (Bethesda, Md. : 1985). 2013 Apr; [PubMed PMID: 23412897]
[4] Parke RL,McGuinness SP, Pressures delivered by nasal high flow oxygen during all phases of the respiratory cycle. Respiratory care. 2013 Oct; [PubMed PMID: 23513246]
[5] Parke RL,Bloch A,McGuinness SP, Effect of Very-High-Flow Nasal Therapy on Airway Pressure and End-Expiratory Lung Impedance in Healthy Volunteers. Respiratory care. 2015 Oct; [PubMed PMID: 26329355]
[6] Esquinas AM,Karim HMR,Soo Hoo GW, Insight to the growing utilizations of high flow nasal oxygen therapy over non-invasive ventilation in community teaching hospital: alternative or complementary? Hospital practice (1995). 2018 Aug 9 [PubMed PMID: 30092679]
[7] Di Mussi R,Spadaro S,Stripoli T,Volta CA,Trerotoli P,Pierucci P,Staffieri F,Bruno F,Camporota L,Grasso S, High-flow nasal cannula oxygen therapy talking postextubation neuroventilatory drive and work of breathing in patients with chronic obstructive pulmonary disease. Critical care (London, England). 2018 Aug 2 [PubMed PMID: 30071876]
Synopsis from Sharma S, Danckers M, Sanghavi D, et al. High Flow Nasal Cannula. [Updated 2020 Jul 2]. Print: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2020 Jan-.