A quantitative evaluation of aerosol generation during tracheal intubation and extubation: a reply

Abstract

We thank all the authors [1-3] for their interest in our study [4]. We have debated some of these issues in a recent Anaesthesia podcast (www.podbean.com/ew/pb-sm9g3-f0d570) but address the others herein. Our ethical permission enabled us to conduct environmental monitoring during patient care but not to modify the conduct of anaesthesia nor document the characteristics of the patients beyond noting the urgency and type of surgery. In future interventional work, we aim to analyse some of the patient level factors mentioned by Drs Schumacher and Greig [1]. Use of a cough as a reference provided standardisation across monitoring sessions and enabled the aerosol production during intubation and extubation to be put into a meaningful context. We used the average aerosol produced by a cough from a single investigator (JB), measured in sets of three at 1-min intervals under identical conditions as for patient recordings (Fig. 1 and twitter.com/i/status/1318890563303804928). Aerosol generation did not depend on cough sequence in a set (Fig. 1a) and comparing JB’s cough to other healthy volunteers indicated he was not an outlier. We agree with Drs Wong and Abramowicz [2] that using the patients’ own cough as the reference may be beneficial and are doing that in our current studies. We also agree that coughs from patients with COVID-19 may produce more aerosol, but this remains to be quantitatively verified. We found no evidence of aerosol generation during facemask ventilation as part of the intubation sequence (twitter.com/i/status/1319171324401668099). Of note, facemask ventilation was undertaken by anaesthetists of varying degrees of experience and it is likely some leak occurred, yet no aerosol was detected during our measurement. The hypothesised cause of bio-aerosol during facemask ventilation is opening/closing of terminal bronchioles, turbulence at the glottic inlet or high velocity airflow across mucosal surfaces [5]: it is plausible the bronchiole opening-closing cycles may be lessened by positive pressure ventilation [6] and the airflow velocities with manual ventilation are notably lower than during a cough, or even normal breathing. All these factors would tend to reduce, rather than increase, aerosol generation. Dhillon et al. [7] documented aerosol generation with facemask ventilation using quite comparable methodology, a difference which we cannot yet explain. In their study, aerosol generation occurred throughout the period of facemask ventilation which is puzzling if the anaesthetists did maintain a patent airway with a good facemask seal. To resolve this uncertainty, we are undertaking further studies using a protocol agreed with the Melbourne group. We studied 10 tracheal extubations with the collecting funnel positioned perpendicular to the mouth and 0.5 m distant. In a further four tracheal extubations, the collecting funnel was moved approximately 45° to the perpendicular above and behind the patient’s head, to where the anaesthetist normally positions themselves. In this funnel position, the aerosol generated when the patient coughed during tracheal extubation was not detected, reflecting the directionality of the aerosol emission. While extubation coughs are typically weaker than a volitional cough, and this might be anticipated because of the residual effects of anaesthetic agents, the presence of opioids and other drugs that attenuate the cough reflex, we cannot conclude that this is universally true. Our purpose was not to study how extubation coughs may be suppressed; however, we agree with the suggestion by Dr Cribben et al. [3] that this may be appropriate (see [8]), provided it does not add risk to the patient, so again further research is mandated. The majority of our measurements were made in an operating theatre with an ultraclean laminar flow ventilation system. Nonetheless, similar measurements made with the laminar flow system on standby (which stepped the flow down to that seen in a typical operating theatre) did not change the measured magnitude or rate of clearance of aerosol generated by a cough (Fig. 1b and c) or by tracheal intubation. We did not switch the laminar flow system into standby during the extubation phase but have no reason to believe – based on the effect on cough or intubation – that this would have made a material difference to our findings. We found that switching the ultraclean ventilation system off altogether led to a rapid increase in background particle counts to a level where even a cough is not resolvable against spontaneous fluctuations (Fig. 1c) which is likely predominantly from non-respiratory sources. This point is worth emphasising, this HEPA-filtered and ultraclean environment has enabled respiratory generated aerosol to be unambiguously distinguished from background aerosol. Based on our study, we concluded there should be a re-evaluation of whether tracheal intubation, extubation and bag/valve/mask ventilation are aerosol-generating procedures, as all generate considerably less aerosol than a volitional cough from a healthy individual. This is reassuring for those involved in airway management as it suggests that these procedures pose no additional risk above the risk of close contact with patients. However, while the findings are supportive of not needing airborne precautions in low-risk settings, we emphasise the importance of cough in aerosol generation. During airway management of patients who are known or suspected to be COVID-19-positive, there are many risks for the practitioner, not least that they will be entering an environment where the patient may have been coughing regularly, often in a poorly ventilated and confined space. Even in this setting, however, our findings suggest that the risk of aerosol generation from the tracheal intubation sequence itself is likely considerably less than from the patients’ coughs. The potential for both direct contact and airborne transmission must be regarded as being high and airborne precautions would still be appropriate regardless of any airway procedure.

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