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Are tracheal intubation and extubation aerosol‐generating procedures?

. Correspondence to:M.G. Irwin Email: mgirwin@hku.hk Accepted: 2November 2020


1711-1776)
The COVID-19 pandemic has significantly impacted peri-operative care due to staff redeployment and a reduction in elective surgery secondary to depletion of resources and a requirement for increased infection control procedures in order to protect healthcare staff. As well as patients with severe COVID-19 infection who require tracheal intubation, patients may be relatively asymptomatic and present for surgery. COVID-19 is highly infectious and may be transmitted through small droplets and/or aerosols [1]. There is evidence that airway management may be a source of transmission of infection as these are potential aerosol-generating procedures and, therefore, use of full airborne personal protective equipment (PPE) has become standard practice in all patients. This has important implications as PPE shortages in some countries has impacted on the provision of elective surgery. The latest experiments for respiratory emissions indicate that a high concentration of droplets/aerosols (virus hosts) appears in the breathing zone ( Fig. 1) [2].
Recommendations are revised periodically and, given the rapid escalation of the pandemic, often have little evidence base, thereby adding to the cognitive load and anxiety of frontline healthcare workers. Experience from severe acute respiratory syndrome (SARS) and other infectious disease epidemics has been the main reason for adoption of airborne protective measures for aerosol-generating procedures, but there is a paucity of clinical data supporting this approach. Current infection control guidelines are based more on a precautionary than evidential approach [3]. Even the use of facemasks by the general public has been an area of much controversy and confusion.
Aerosol generation has been a major focus of guidelines underpinning the required level of PPE. It is imperative to recognise that many daily activities generate aerosols, but these do not necessitate staff to don the same high level of PPE [4]. Whether or not these pose a clinically significant risk depends on the number, size and concentration of infectious aerosols [5]. In this issue of Anaesthesia, there are two papers using robust scientific approaches to evaluate aerosol generation during tracheal intubation and extubation to address this gap in evidence [6,7]. From these two studies, it can be confirmed that tracheal intubation and extubation does produce a certain number of aerosols which can remain suspended and spread in the operation room air; but does this research provide any reassurance as to whether what we are currently doing is safe or appropriate?
Dhillon et al. [6] report their findings in cm -3 and, for the purpose of this editorial, we have converted their data to l -1 to allow direct comparison with Brown et al. [7]. Both groups recorded baseline aerosol concentration, but in order to assess infection transmission risk, the absolute number of aerosols generated is more significant than an increase from baseline. Brown et al. which is in excess of the average amount of aerosol generated by airway management in either study. Violent expiratory events, such as a cough, produce a mixture of droplets and aerosols that range from 0.5 lm to 20 lm in size [8,9]. In both studies, spikes can be seen in aerosol generation during cough episodes in the tracheal extubation sequence. The substantial differences in the aerosol concentrations of these two studies at baseline and following tracheal intubation/extubation could be attributed to: 1 The difference in the background ventilation systems (26 vs. 500-650 volume air exchanges.h -1 ): the degree of ventilation that influences underlying airflow could alter droplet/aerosol pathways as well as particle concentrations at the sampling points.  There is some contention over how to define aerosolgenerating procedures, the classification of aerosols into droplets and what is airborne. Small particles with diameters <5 lm can be considered aerosols, which can remain suspended in air for many hours, while droplets with diameters >5 lm will undergo gravitational settling in a short time [8]. It is known that <5% of all human aerosols are >5 lm [9]. Brown et al. define aerosol-generating procedures as procedures with a greater likelihood of producing aerosols than coughing, and this aligns with UK NHS guidance [10]. This bar is significantly higher than other institutions that also include activities such as talking and breathing in their definition [11]. Aerosols can be produced during a procedure without classifying the intervention as high-risk. A volitional cough produced 732 particles.l -1 in the study by Brown et al. and, using this as a reference point, neither the sequence of tracheal intubation nor extubation reached this level in either study. Coughing and sneezing produce more aerosols [12] than those generated from airway manoeuvres, but infection control guidelines do not mandate healthcare workers to don airborne precaution PPE for asymptomatic untested patients. Furthermore, a study during the H1N1 pandemic in the UK found that aerosol-generating procedures did not significantly increase the probability of sampling an H1N1-positive aerosol in the vicinity of an H1N1-positive patient [13].

Aerosols
Recent stability tests confirm that aerosol and fomite transmission of COVID-19 is evident because the virus can remain viable and infectious in small droplets for several hours [14]. The overall risk of transmission (airborne, droplet, fomite) is distinct from the risk from aerosolgenerating procedures. A crucial confounding variable is the physiological state of the patient. During SARS, transmission rates increased with higher acute physiology and chronic health evaluation-2 (APACHE-2) scores [15].
Healthcare staff performing non-aerosol-generating procedures were also at risk of contracting the disease simply by spending extended periods of time in close proximity to patients who were critically ill [16] Singapore, which found that over 70% of infections were attributable to super-spreader events. The number of aerosols produced also varies between individuals, with some producing far in excess of others. In one study, 99% of the bioaerosols produced came from 6 out of 11 patients investigated [19]. Rhinorrhoea, an unusual symptom of SARS, was also linked to super-spreading events [20].
Whether increased secretions or co-infection correspond to increased bioaerosol production needs to be investigated further.

Airborne transmission
Airborne transmission has been a global concern during this pandemic as this was seen in SARS where, for example, airborne spread was responsible for a large community Quantitative, objective measurement of neuromuscular function is a more accurate endpoint [21] and this should be routine if trying to reduce aerosol-   (Table 1).
Although such patients may still be infectious, the viral load is likely to be low and aerosol generation negligible.