Airway management in the adult patient with COVID-19: High flow nasal oxygen or not? A summary of evidence and local expert opinion

Abstract

The use of high flow nasal oxygen in the care of COVID-19-positive adult patients remains an area of contention. Early guidelines have discouraged the use of high flow nasal oxygen therapy in this setting due to the risk of viral spread to healthcare workers. However, there is the need to balance the relative risks of increased aerosol generation and virus transmission to healthcare workers against the role high flow nasal oxygen has in reducing hypoxaemia when managing the airway in high-risk patients during intubation or sedation procedures. The authors of this article undertook a narrative review to present results from several recent papers. Surrogate outcome studies suggest that the risk of high flow nasal oxygen in dispersing aerosol-sized particles is probably not as great as first perceived. Smoke laser-visualisation experiments and particle counter studies suggest that the generation and dispersion of bio-aerosols via high flow nasal oxygen with flow rates up to 60 l/min is similar to standard oxygen therapies. The risk appears to be similar to oxygen supplementation via a Hudson mask at 15 l/min and significantly less than low flow nasal prong oxygen 1–5 l/min, nasal continuous positive airway pressure with ill-fitting masks, bilevel positive airway pressure, or from a coughing patient. However, given the limited safety data, we recommend a cautious approach. For intubation in the COVID-positive or suspected COVID-positive patient we support the use of high flow nasal oxygen to extend time to desaturation in the at-risk groups, which include the morbidly obese, those with predicted difficult airways and patients with significant hypoxaemia, ensuring well-fitted high flow nasal oxygen prongs with staff wearing full personal protective equipment. For sedation cases, we support the use of high flow nasal oxygen when there is an elevated risk of hypoxaemia (e.g. bariatric endoscopy or prone-positioned procedures), but recommend securing the airway with a cuffed endotracheal tube for the longer duration procedures when theatre staff remain in close proximity to the upper airway, or considering the use of a surgical mask to reduce the risk of exhaled particle dispersion.

Keywords

Aerosols anaesthesia healthcare workers high flow nasal oxygen hypoxia non-invasive ventilation oxygen SARS-CoV-2

Introduction

The novel coronavirus disease named COVID-19 by the World Health Organization (WHO) and identified in early December 2019 in Wuhan City, Hubei Province, China, remains a global pandemic. The responsible pathogen designated severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), results in a viral pneumonitis which can progress to an acute respiratory distress syndrome (ARDS) in critically ill patients which is largely responsible for patients’ morbidity and mortality. The aim of this article was to review and present the current literature around the risks posed to healthcare workers (HCWs) associated with the use of high flow nasal oxygen (HFNO) when anaesthetising and managing the airway in COVID suspected or confirmed patients.

Gattinoni et al. suggest that HFNO can reduce the need for intubation and mechanical ventilation and therefore reduce mortality in the severe COVID-19 patient group.¹ HFNO has also shown benefits of reduced rates of hypoxaemia at induction of anaesthesia and improved patient safety during intubation, especially in potential difficult airway scenarios.^2–4 Similarly, HFNO may be useful for sedation procedures with an elevated risk of hypoxaemia (such as bariatric endoscopy, or prone-positioned endoscopic retrograde cholangiopancreatography procedures),⁵ as well as for respiratory support on extubation. However, where COVID-19-positive and suspected COVID-19-positive patients are unwell enough to require intubation, the use of HFNO remains an area of contention due to presumed risks of increased aerosol generation and virus transmission to HCWs.^6–8

Profound hypoxaemic respiratory failure secondary to ARDS is the dominant clinical feature among critically ill patients with COVID-19, accounting for mechanical ventilation in 30% to 100%^9–12 of these patients. For those who are able to avoid mechanical ventilation, a conservative oxygenation strategy targeting oxygen saturation (SpO₂) of 90% or greater is recommended by the WHO.¹³ However, even this may require higher flows of oxygen via various devices such as a non-rebreather or Venturi facemask, non-invasive ventilation (NIV) systems such as continuous positive airway pressure (CPAP) or bilevel positive airway pressure (BiPAP), and high flow nasal cannulae (HFNC), all of which may promote microorganism aerosolisation and thereby increase infectious risk to HCWs and other patients.⁸

HFNO delivers high flows of heated humidified gases up to 60 l/min via a specially designed nasal cannula. The device reduces dead space and provides an element of positive end-expiratory pressure. Its usage was controversial during the early stages of the pandemic due to the unquantified risk of aerosol dispersion, cross infection and the perceived high likelihood of patients ultimately deteriorating and requiring mechanical ventilation. Although an association between HFNO and a reduction in the need for invasive ventilation in COVID-19 patients with acute hypoxaemic respiratory failure has not been clearly demonstrated,¹⁴ one trial suggests that HFNO potentially decreases the need for mechanical ventilation in these patients.¹⁵

In the case of HFNO during intubation, there is a paucity of high level evidence in COVID-19 patients as a therapy aimed at reducing apnoea-induced hypoxaemia. On the other hand, there have not been any published case reports of SARS-CoV-2 transmission with HFNO to date. Early guidelines in both Australia and the UK were cautious in their approach to HFNO during intubation.⁶,¹⁶ Some experts, however, now advise to consider HFNO after carefully weighing the risks and benefits to the patient, resource availability, and exposure risks to HCWs.¹⁷

Currently, HFNO is still considered to be an aerosol-generating procedure and personal protective equipment (PPE) recommendations are therefore for airborne precaution requirements to reduce the risk of transmission to HCWs.⁶,⁷ At present, there is in fact very little clinical data supporting the assertion that the use of HFNO therapy during airway instrumentation increases the risk of aerosol generation or infection transmission when compared with conventional oxygen supplementation strategies during airway management of COVID-19 patients.⁸ Studies that might directly answer the questions above are either technically difficult (in the case of demonstrating whether HFNO creates more aerosolisation of SARS-CoV-2 virus in COVID-positive patients than other oxygenation devices), or ethically problematic (comparing risks posed to HCWs by HFNO and other oxygen delivery methods).

This paper aims to identify the key studies on whether HFNO increases both aerosolisation of the SARS-CoV-2 virus and transmission of COVID-19. It also aims to make recommendations for the use of HFNO during airway management based on the current evidence and the opinions of an expert group of airway clinicians.

Identification of key studies

Key studies were identified by a literature search by two authors. References of identified papers were then screened by the same authors to identify further relevant studies.

The key papers were presented to the Australian and New Zealand College of Anaesthetists (ANZCA) Victorian and Tasmania Airway Leads Committee for discussion and formation of an expert recommendation for clinical practice.

Summary of key studies

Recent studies have reported methods that include smoke laser-visualisation of particles, direct particle counter studies, or demonstrations of levels of bacterial growth or yeast dispersion in proximity to HFNO. A number of smoke laser-visualisation experiments on manikins suggest that generation and dispersion of bio-aerosols via HFNO is similar to standard oxygen therapies. One study by Hui et al.¹⁸ showed that air dispersion in the sagittal plane for both HFNO and nasal CPAP was limited (maximum 17 and 33 cm, respectively), and negligible for oronasal CPAP, provided there was good mask interface fitting. Higher flow rates of HFNO slightly increased dispersion of exhaled air from 6.5 cm (at 10 l/min) to 17.2 cm (at 60 l/min) (although this was similarly demonstrated when nasal CPAP was increased from 5 to 20 cm water (H₂O)) and may lead to moderate increases in distance travelled by cough-expelled respiratory secretions.¹⁹ Of note, air leak increased when connections on any device were loose, with a 62 cm lateral leak from the HFNO. Interestingly, a previous study conducted by the same author, under similar conditions²⁰ revealed a greater exhalation spread with conventional nasal cannulae (from 66 to 100 cm sagittal with oxygen flows increasing from 1 to 5 l/min, respectively).

Other smoke laser-visualisation studies on commonly used simple oxygenation devices have demonstrated either similar or greater dispersion than HFNO in the study by Hui et al. above.¹⁸ Hui et al. also showed maximum exhaled air dispersion of 40 cm for a simple oxygen mask at 10 l/min, and 42 cm for oxygen via a nasal cannula and greater than 95 cm for BiPAP in another study.²¹ Ip et al.²² have demonstrated maximum exhaled dispersion of 12.5 cm for a simple oxygen mask at 10 l/min and 20.7 cm at 15 l/min.

A number of human volunteer particle counter studies have been conducted. McGain et al.²³ showed that both HFNO therapy (60 l/min) and simple facemask oxygen (15 l/min) only marginally increased particle counts at 65 cm from the patient (and therefore were not considered high aerosol-generating procedures). This increased particle count was significantly less than with BiPAP and nebulised oxygen (O₂) at 10 l/min. Jermy et al.²⁴ studied healthy participants during ‘quiet breathing’ and ‘vigorous breathing’ (i.e. coughing, snorting or sneezing) either unsupported or using HFNO. They showed that while HFNO caused a small increase in particle counts during quiet breathing, vigorous breathing itself caused a marked increase in particle counts, and no significant difference (with a tendency to less) with HFNO compared with unsupported vigorous breathing. Another human study conducted by Iwashyna et al.²⁵ showed that similar aerosol production levels and particle number concentrations were found with HFNO at 30 l/min and 60 l/min compared with spontaneous breathing, 6 l/min nasal cannula humidified, and non-rebreather mask at 15 l/min. Another recent study by Gaeckle et al.²⁶ measuring particles and droplet generation from the respiratory tracts of ten healthy volunteers receiving oxygen with various modes of delivery also demonstrated no difference with HFNO at 50 l/min compared with breathing room air, simple facemask at 15 l/min oxygen or BiPAP. The authors concluded that different respiratory patterns, individual characteristics and cough had more impact on aerosol-based respiratory infection transmission rather than the specific mode of oxygen therapies applied. Feng et al. also highlighted a fundamental concept that distances travelled by respiratory droplets and particles were greatly influenced by relative humidity and local ventilation in the room.²⁷

Based on these smoke laser-visualisation and particle counter studies, HFNO is similar to a Hudson mask up to 15 l/min, with smoke plume detected at 17 cm¹⁸ and only a small increase in particles measured at 65 cm.²³ The dispersion distance associated with HFNO use during such studies was significantly less than with nasal prongs at 5 l/min, nasal CPAP with an ill-fitting mask, BiPAP, or a coughing patient.

During coughing, a surgical mask placed on the patient has been shown to reduce dispersion distance significantly from 68 to 30 cm,²⁸ and reduce influenza virus–infected bio-aerosol 20 cm away from patients.²⁹ During normal breathing, a computational fluid dynamic simulation study showed that a surgical mask over a properly fitted HFNO device may be an effective option to reduce droplet deposition from exhaled gas flow.³⁰,³¹ However, another study of healthy volunteers comparing HFNO, oronasal CPAP and 6 l/min low flow nasal oxygen, although showing no difference in aerosol production between the three modes, also showed that the use of a surgical mask over the HFNO device did not change aerosolised particle spread during normal breathing or when coughing.³¹,³²

The bacterial growth study by Leung et al.³³ in patients with Gram-negative bacterial (GNB) pneumonia showed no difference between HFNO and oxygen mask for bacterial airborne or contact surface contamination with viable GNB on Petri dishes 0.4 and 1.5 m from the patients. The authors suggested that additional infection control measures may not be required given these results. They did highlight the inability to draw conclusions about viruses from their study findings. Similarly, another study by Kotoda et al. with HFNO in a manikin showed no significant dispersion of water or yeast droplets more than 60 cm away from the face, concluding that HFNO may not increase the risk of droplet infection.³⁴

There have been some theories put forward that may explain why HFNO does not significantly increase aerosol generation. One of these is that aerosols are formed from shear stress along the airway wall during turbulent flow and vibration of the vocal cords.³⁵,³⁶ While the exact flow rate to create a shear force over the mucus-lined respiratory tract sufficient enough to promote aerosolisation is unknown, it is possible that HFNO at flows of 60 l/min may not be enough compared with coughing which produces flows as high as 400 l/min.³⁷,³⁸ The second concept is the bronchiole fluid film burst model which describes how aerosols are generated from the opening of closed bronchioles in the lungs. In fact, according to this theory and highlighted in the discussion of Gaeckle et al.,²⁶ HFNO may actually decrease aerosol production by providing positive end-expiratory pressure that prevents the closure of small bronchioles. In addition, compared with other devices such as non-humidified low flow nasal cannulae oxygen, the tighter fitting interface of HFNC and the utilisation of humidification may generate larger droplets with a shorter trajectory path on exhalation.¹⁸ Furthermore, the resistance to exhalation created by HFNO (as evidenced by the generation of CPAP) may reduce exhalation distance.

Assessment of quality of evidence

The body of evidence is composed of small case-control trials and experimental studies, considered Level 2 and 3 evidence.³⁹

Limitations of these studies

The aforementioned studies use HFNO up to 60 l/min whereas the main HFNO device in use today, Optiflow™ (Fisher & Paykel Healthcare, Auckland, New Zealand), is calibrated to deliver up to 70 l/min, and although not recommended, can actually deliver 80–100 l/min if the flow meter is fully open. Although we lack data for this additional 17% to 67% increase in maximum flow rate, it is unlikely to affect the clinical risk significantly given that increases in HFNO flow rate between 100% and 600% in other studies caused a minimal or negligible increase in aerosol dispersion.^18,25

The studies identified are largely based on healthy volunteers or manikin simulators. The studies based on smoke visualisation assume the visible extent of exhaled smoke plumes is equivalent to the physical extent of exhaled air plumes. This could be inaccurate given that smoke visibility may rapidly decrease when blended with clean air and does not equate to the disappearance of smoke particles themselves. Furthermore, the particle size of smoke is less than 1 μm diameter¹⁸ and may not necessarily represent the full range of mass of bio-aerosol generated by patients. So, while human respiration produces suspended liquid and solid particles in the air which are generally around 1 μm, they range from 0.5 to 20 μm in size.²³,⁴⁰,⁴¹ If inhaled, particles in this full range of less than 5–20 μm act as aerosols and have the ability to reach the respiratory portion of the airways.²³,^40–43 Finally, the generally accepted but somewhat arbitrary definition of an aerosol is a respiratory particle less than 5 μm.⁴³ Put together, smoke visualisation may underestimate the true extent of aerosol dispersion. Nevertheless, it is reassuring that there was no greater dispersion with HFNO compared with other modalities under the same or similar conditions.

The particle counter studies measure particles of differing sizes more similar to the full range of human respiratory aerosols, but again with different detection limits of 5 μm,²³ 10 μm²⁵ and even 33 μm.²⁴ The studies showing growth of bacteria, or yeast dispersion at distances from the HFNO³³,³⁴ may be indicative but cannot deliver direct conclusions about viral spread. The studies were performed in conditions with a range of air changes per hour (ACH) generally between six and 15 ACHs. Therefore, these results may not be applicable to HFNO in areas with a poor ventilation system or low number of ACHs, for example a hospital ward.

Conclusion

The question of whether HFNO increases aerosolisation of the SARS-CoV-2 virus as well as increasing the risk of transmission to HCWs will be challenging to answer definitively. However, surrogate outcome studies suggest that the risk of HFNO in dispersing aerosol-sized particles and spreading infection (at least for bacteria) is not as great as originally perceived. The risk appears to be similar to a high flow Hudson mask at 15 l/min, and significantly less than low flow nasal prong oxygen at 1–5 l/min, nasal CPAP with ill-fitting mask, BiPAP, or from any patient who is coughing.

In the current climate of uncertainty around the use of HFNO in COVID-19-positive patients, the evidence presented in this review may alleviate some of the anxiety surrounding its use in anaesthesia (laryngoscopy and intubation, sedation procedures).

Recommendations

Given the limitations previously outlined, we still recommend a cautious approach, wearing full PPE with N95 respirator, until better safety data emerge. With time, these recommendations may change.

HFNO during laryngoscopy and intubation in the COVID-19-positive or suspected COVID-19-positive adult patient:

We support HFNO for preoxygenation and apnoeic oxygenation in at-risk groups including the morbidly obese, those with predicted difficult airways and those who are already significantly hypoxaemic.

Ensure well-fitted HFNC with no loose connections (the majority of patients would fit the medium size prongs).

Where practical, the use of a surgical facemask may reduce exhaled dispersion of particles.

Where possible, intubate in a negative pressure room and/or rooms with high ACH rates.

HFNO for sedation cases in the COVID-19-positive or suspected COVID-19-positive adult patient.

In addition to the above recommendations, consideration should be given to securing the airway with a cuffed endotracheal tube especially for the longer duration procedures and when theatre staff are in close proximity to the upper airway for other than brief periods (e.g. plastic and reconstructive surgery). This may reduce staff exposure to any aerosols generated during the procedure.

Similar precautions should be used when the patient has an active cough or when other airway management techniques such as high flow oxygen via the Hudson mask, oxygen via nasal prongs, nasal CPAP or BiPAP are utilised.

Footnotes

Author Contribution(s)

Sarah Lee: Investigation; Resources; Writing-original draft; Writing-review & editing.

W Pierre L Bradley: Investigation; Writing-review & editing.

David J Brewster: Investigation; Writing-review & editing.

Rani Chahal: Investigation; Writing-review & editing.

Laurence Poon: Investigation; Writing-review & editing.

Reny Segal: Investigation; Writing-review & editing.

Savas Totonidis: Investigation; Writing-review & editing.

David Tsang: Investigation; Writing-review & editing.

Mark Ng: Conceptualization; Investigation; Resources; Writing-original draft; Writing-review & editing.

Acknowledgements

The authors wish to thank all the airway leads in Victoria and Tasmania (M Balkin, V Bertram, Y Chen, A Jones, M Gerstman, J Graham, A Hague, V Kartha, C Lang, N Le, C O’Loughlin, H Roberts, S Sabato, C Wan) for their help and input into the discussions.

Declaration of conflicting interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) received no financial support for the research, authorship, and/or publication of this article.

ORCID iDs

Sarah Lee

W Pierre L Bradley

Laurence Poon

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