High-Flow Nasal Oxygen
The use of heated and humidified high-flow nasal oxygen (HFNO) has become increasingly popular in both the treatment of patients with airway obstruction, and to facilitate a variety of procedures in adults and in children. In this article, Dr Anil Patel Consultant Anaesthetist at University College London Hospital, Royal National Throat, Nose and Ear Hospital and Dr Reza Nouraei, Consultant laryngologist & tracheal surgeon at Poole Hospital NHS Trust, teach us about HFNO.
The uses of High-Flow Nasal Oxygen
HFNO isn’t just a standard nasal cannula turned up to very high flow rates. It takes gas, is able to heat it to 37oC with a 100% relative humidity, and can deliver 21-100% fraction of inspired oxygen (FiO2) at flow rates of up to 70 L/min. The flow rate and in some machines the FiO2 can be independently titrated based on a patient’s flow and FiO2 requirements.
This table summarises the physiological benefits of HFNO using warmed humidified oxygen:
|Feature of HFNO||Physiological Effect|
|Warmed humidified gas||Reduces airway surface dehydration, improves secretion clearance, decreases atelectasis|
|Gas flow of up to 70 L/min||CO2 washout, reduces anatomical dead space, provides an oxygen reservoir, enables delivery of FiO2 close to 100%|
|Continuous Positive Pressure Ventilation (CPAP)||Increases end-expiratory lung volume, allows alveolar recruitment|
HFNO is a useful tool in patients with airway obstruction, during pre-oxygenation for rapid sequence induction particularly in the critically ill, for awake tracheal intubation or as part of a safe extubation in difficult airways, as well as to provide apnoeic oxygenation and ventilation during procedures in the form of in the form of Transnasal Humidified Rapid Insufflation Ventilatory Exchange (THRIVE).
What is THRIVE?
THRIVE is a physiological mechanism utilising humidified high-flow nasal oxygen at flow rates of 30-70 L/min for oxygenating and ventilating adult patients who are under general anaesthesia and who have diminished or absent respiratory effort (apnoea).
The high flow oxygen results in three main features: continuous positive airway pressure (CPAP), apnoeic oxygenation, apneoic ventilation:
The Physiology of THRIVE
A patient who had been breathing air will desaturate to dangerous levels within minutes of being rendered apnoeic. Filling the pharynx with 100% oxygen, without any respiratory movements, will prolong apnoea time up to a point, but it does not clear CO2. This is known as ‘apnoeic oxygenation’.
In 2012 Weingart and Levitan described using a nasal cannula and 15 L/min of oxygen during Rapid Sequence Induction to delay desaturation. This is an application of apnoeic oxygenation, called nasal oxygenation during efforts securing a tube (NODESAT) . However, cold and dry oxygen can cause mucosal trauma and is painful. Conversely, warmed and humidified oxygen can be delivered at much higher rates to awake patients without causing significant discomfort or trauma .
At flow rates of 60 L/min, CO2 is entirely flushed out of the airway and 100% oxygen is present at the supraglottic area. In THRIVE, high-flow nasal oxygen loops around the soft palate, and exits through the mouth, creating a highly turbulent ‘primary supraglottic vortex‘ which constantly replenishes the pharynx with oxygen and prevents entrainment of room air. This vortex effectively bypasses the upper airways, hence reducing resistance and work of breathing by approximately 50%. These high flows also generate a positive airway pressure even if the mouth is open, which in turn reduces upper airway collapse and distal airway atelectasis.
The primary vortex does not, however, extend deep into the trachea and cannot by itself account for the observed level of gas exchange.
‘Apnoeic ventilation’ is the result of an interaction between the primary supraglottic vortex from above and cardiogenic oscillations from below.
Cardiogenic oscillations result from compression and expansion of the small airways, brought about by blood leaving and entering the thoracic cavity with each heartbeat. Ordinarily, cardiogenic oscillations result in small-volume (10-40ml per heartbeat) mass movement of gases within the trachea. If we assume the volume of a ‘cardiogenic breath’ to be 20ml per heartbeat, and a heart rate of 70 per minute, 1400ml of gas which contains CO2 is removed, and is replaced with 100% oxygen. This is not enough to achieve full CO2 clearance which is why it still accumulates, albeit at a slower rate.
Using THRIVE to assist intubation
The principal role of THRIVE lies in changing the nature of securing a definitive airway in emergency and difficult airways, from a pressured stop-start process to a smooth and unhurried undertaking. Through pre-oxygenation (optimising the oxygen reservoir prior to apnoea) and per-oxygenation (continued oxygen delivery during laryngoscopy/intubation attempts via the mechanisms of apnoeic oxygenation and apnoeic ventilation), THRIVE maintains arterial oxygenation and prolongs the safe apnoea period, thereby improving the margin of safety for securing the airway, including during awake tracheal intubation  and rapid sequence induction .
Using THRIVE in shared-airway surgery
The benefit for ENT surgery is that a volume of gas can pass through a critical stricture with minimal respiratory effort and without the need for the patient to actually be taking breaths through a compromised airway. This, combined with the bypassing of the nose-to-glottis resistor, upper airway splinting through positive intrapharyngeal pressure, and reduced lower airway atelectasis, translates to a reduction in the work of breathing, increasing patient comfort and diminishing stridor in patients with acute airway compromise.
THRIVE: How to do it
The patient is placed in head-up position for optimum preoxygenation. HFNO starts as soon as the patient is positioned and is continued whilst monitoring is attached and intravenous access is established. The patient is advised to keep their mouth closed, breathe through the nose and periodically take deep breaths. Once consciousness is lost a full jaw-thrust must be maintained, and the surgeon undertakes surgical laryngoscopy. If a difficult view is encountered, the laryngoscope can be placed in suspension within the pharynx, making enough space to create an outflow tract. In this situation, over several minutes the suspension can be gradually increased to help drain any tissue fluid from the base of tongue, which can improve the view of the glottis. At the end of the procedure a supraglottic airway device (SAD) is placed and attached to a breathing circuit which is connected to 6-10 L/min of oxygen. The anaesthetist must remain with the patient until spontaneous ventilation is re-established.
It is important to note that THRIVE may fail to prolong the apnoea time as anticipated in patients with morbid obesity, leading to desaturation. Also, the delivery high concentration of oxygen in the presence of an ignition source, represents a fire hazard, as we will learn later this week in the steps on LASER.
Have you used High Flow Nasal Oxygen and or THRIVE? What are your experiences? What went well? What problems did you encounter?
Please remember when discussing patients in the forums to keep all identifiable information confidential.
In the “Downloads” section you can find an infographic summary on HFNO.
Patel A, Nouraei SAR. Transnasal Humidified Rapid-Insufflation Ventilatory Exchange (THRIVE): a physiological method of increasing apnoea time in patients with difficult airways. Anaesthesia 2015;70:323- 9.
Hermez LA, Spence CJ, Payton MJ, Nouraei SAR, Patel A, Barnes TH. A physiological study to determine the mechanism of carbon dioxide clearance during apnoea when using transnasal humidified rapid insufflation ventilatory exchange (THRIVE). Anaesthesia. 2019 Apr;74(4):441-449.
Badiger S, John M, Fearnley RA, Ahmad I. Optimizing oxygenation and intubation conditions during awake fibre-optic intubation using a high-flow nasal oxygen delivery system. Br J Anaesth 2015;115:629-32.
Mir F, Patel A, Iqbal R, Cecconi M, Nouraei SAR, A randomised controlled trial comparing transnasal humidified rapid insufflation ventilatory exchange (THRIVE) pre‐oxygenation with facemask pre‐oxygenation in patients undergoing rapid sequence induction of anaesthesia. Anaesthesia. 2017 Apr;72(4):439-443.