How should aerosol generating procedures be defined?
BMJ 2022; 378 doi: https://doi.org/10.1136/bmj-2021-065903 (Published 18 August 2022) Cite this as: BMJ 2022;378:e065903
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Dear Editor
We read the review by Chiu and colleagues with considerable interest [1], and note it reflects the knowledge of aerosol generation and AGPs 16 months ago – a consequence of ending their literature review in April 2021. Much has changed in the intervening period regarding our understanding of procedural aerosol generation and the associated SARS-CoV-2 transmission risks.
The most important advance comes from clinical aerosol studies (including AERATOR) that have assessed aerosol generation for many healthcare procedures [2-5] and compared them against natural respiratory activities [6]. The IPC-Cell rapid review of this evidence [7] led to a substantial revision of the UK AGP list in May 2022 [8] with removal of tracheal intubation and extubation, facemask ventilation, non-invasive ventilation, and high-flow nasal oxygenation. It is regrettable this was not captured in this educational summary of AGP knowledge.
The authors offer a non-standard AGP definition: “any medical practice or technique that enables aerosols to be transmitted from one person to another.” As patients and healthcare workers generate respiratory aerosol during breathing and speaking, it is hard to imagine any medical procedure that does not fit within this definition. This definition is therefore inferior to that created by the W.H.O. in 2014 stating AGPs are “Medical procedures that have been reported to be aerosol-generating AND consistently associated with an increased risk of pathogen transmission” [9]. Although the AGP construct promotes an overemphasis on the procedure [10] we believe the current W.H.O. definition should be retained. It is clear and applies to procedures where respiratory sources of aerosol generation from coughing and breathing have been documented [2,3,5,11].
The statement “The mechanisms and quantities of aerosols generated are unknown, but the amount of aerosolisation is likely related to flow rate and volume of air exerted on a patient’s mucus-air interface” reflects concerns present early in the pandemic but does not match current evidence. Respiratory aerosol is generated via physiological mechanisms in both the upper or lower airways [12], as highlighted in the infographic however, particles generated from alveoli open-close cycles are not governed by air flow rates or turbulence but due to fluid-bursts during opening of distal air passages and alveoli. This mechanism produces the majority of the fine aerosol (<2µm) which has been shown to contain much of the infectious virus [13]. The continued focus on high velocity, turbulent gas passing over mucous membranes in the upper airway could mislead practitioners in their risk assessment.
The text and infographic suggest knowledge of the distance travelled by soot particles, dispersed by differing gas velocities during simulation studies, may help mitigate aerosol risks within healthcare. Such information has many implicit assumptions that are not realistic within healthcare settings and contrasts with clinical aerosol studies that have shown non-invasive ventilation and high-flow nasal oxygenation decrease respiratory aerosol generation [14-16]. We would suggest they are removed from what is an otherwise helpful infographic. The authors also advocate the use of aerosol enclosures despite the FDA having revoked the emergency use authorisation for these devices due to the potential for adverse events [17-22]. This article has also broached the topic of risk assessment without addressing the patient’s likely infective state, the healthcare worker’s immune status, the viral strain, the duration of the interaction or other environmental factors.
The evolving topic of AGPs remains important as we recover from the impacts of the pandemic however, it is regrettable that this educational article continues to promulgate old ideas and fears around the topic of nosocomial airborne transmission risks. We hope the authors and editors will seek to redress some of these issues in a response to the readership.
References
1. Chui J, Hui DS, Chan MT. How should aerosol generating procedures be defined? BMJ 2022: e065903.
2. Shrimpton AJ, Gregson FKA, Brown JM, et al. A quantitative evaluation of aerosol generation during supraglottic airway insertion and removal. Anaesthesia 2021.
3. Gregson FKA, Shrimpton AJ, Hamilton F, et al. Identification of the source events for aerosol generation during oesophago-gastro-duodenoscopy. GUT 2022; 71: 871-8.
4. Shrimpton AJ, Brown JM, Gregson FKA, et al. Quantitative evaluation of aerosol generation during manual facemask ventilation. Anaesthesia 2022; 77: 22-7.
5. Shrimpton AJ, Brown JM, Cook TM, Penfold CM, Reid JP, Pickering AE. Quantitative evaluation of aerosol generation from upper airway suctioning assessed during tracheal intubation and extubation sequences in anaesthetized patients. J Hosp Infect 2022; 124: 13-21.
6. Gregson FKA, Watson NA, Orton CM, et al. Comparing aerosol concentrations and particle size distributions generated by singing, speaking and breathing. Aerosol Science and Technology 2021; 55: 681-91.
7. UK IPC Cell. A rapid review of aerosol generating procedures (AGPs), 2022. https://www.england.nhs.uk/wp-content/uploads/2022/04/C1632_rapid-review... (accessed 27/09/2022).
8. NHS England. National infection prevention and control manual for England, 2022. https://www.england.nhs.uk/wp-content/uploads/2022/04/C1636-national-ipc... (accessed 27/09/2022).
9. W.H.O. Infection prevention and control of epidemic- and pandemic-prone acute respiratory infections in health care, 2014. https://www.who.int/publications/i/item/infection-prevention-and-control... (accessed 07/09/2022).
10. Hamilton F, Arnold D, Bzdek BR, et al. Aerosol generating procedures: are they of relevance for transmission of SARS-CoV-2? Lancet Respiratory Medicine 2021.
11. Brown J, Gregson FKA, Shrimpton A, et al. A quantitative evaluation of aerosol generation during tracheal intubation and extubation. Anaesthesia 2021; 76: 174-81.
12. Johnson GR, Morawska L, Ristovski ZD, et al. Modality of human expired aerosol size distributions. Journal of Aerosol Science 2011; 42: 839-51.
13. Coleman KK, Tay DJW, Tan KS, et al. Viral Load of Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2) in Respiratory Aerosols Emitted by Patients With Coronavirus Disease 2019 (COVID-19) While Breathing, Talking, and Singing. Clinical Infectious Diseases 2022; 74: 1722-8.
14. Gaeckle NT, Lee J, Park Y, Kreykes G, Evans MD, Hogan CJ, Jr. Aerosol Generation from the Respiratory Tract with Various Modes of Oxygen Delivery. American Journal of Respiratory and Critical Care Medicine 2020; 202: 1115-24.
15. Hamilton FW, Gregson FKA, Arnold DT, et al. Aerosol emission from the respiratory tract: an analysis of aerosol generation from oxygen delivery systems. Thorax 2022; 77: 276-82.
16. Wilson NM, Marks GB, Eckhardt A, et al. The effect of respiratory activity, non-invasive respiratory support and facemasks on aerosol generation and its relevance to COVID-19. Anaesthesia 2021; 76: 1465-74.
17. Lim ZJ, Ponnapa Reddy M, Karalapillai D, Shekar K, Subramaniam A. Impact of an aerosol box on time to tracheal intubation: systematic review and meta-analysis. British Journal of Anaesthesia 2021; 126: e122-e5.
18. Sorbello M, Rosenblatt W, Hofmeyr R, Greif R, Urdaneta F. Aerosol boxes and barrier enclosures for airway management in COVID-19 patients: a scoping review and narrative synthesis. Br J Anaesth 2020; 125: 880-94.
19. Begley JL, Lavery KE, Nickson CP, Brewster DJ. The aerosol box for intubation in coronavirus disease 2019 patients: an in‐situ simulation crossover study. Anaesthesia 2020; 75: 1014-21.
20. Noor Azhar M, Bustam A, Poh K, et al. COVID-19 aerosol box as protection from droplet and aerosol contaminations in healthcare workers performing airway intubation: a randomised cross-over simulation study. Emergency Medicine Journal 2021; 38: 111-7.
21. FDA. FDA In Brief: FDA revokes emergency use authorization for protective barrier enclosures without negative pressure due to potential risks., 2021. https://www.fda.gov/news-events/fda-brief/fda-brief-fda-revokes-emergenc... (accessed 27/09/2022).
22. Simpson JP, Wong DN, Verco L, Carter R, Dzidowski M, Chan PY. Measurement of airborne particle exposure during simulated tracheal intubation using various proposed aerosol containment devices during the COVID‐19 pandemic. Anaesthesia 2020; 75: 1587-95.
Competing interests: Mark Hull has nothing to declare. Andy Shrimpton is an NIHR-funded Doctoral Research Fellow (NIHR301520 grant). Tony Pickering and Andy Shrimpton are members of the AERATOR study group. The AERATOR study is registered in the ISRCTN registry (ISRCT N21447815) and is funded by an NIHR-UKRI rapid rolling grant (Ref. COV0333 ). Tony Pickering declares advisory board work for Lateral Pharma, and consultancy for and research grants from Eli Lilly for projects unrelated to this study.
Re: How should aerosol generating procedures be defined?
Dear Editor,
We thank Drs Shrimpton, Pickering and Hull for their thoughtful comments on our recently published article [1]. They measured aerosol generation with environmental sampling in a number of clinical settings [2–5] and have derived a list of aerosol generating procedures [6]. These studies produced useful data but are limited by the temporal and spatial resolution of the sampling devices. The procedures were performed in controlled environment with fixed temperature and constant humidity and did not account for concomitant patient activities such as talking, sneezing, and coughing during the procedure.
Consequently, conflicting result has been reported. For instance, in contrast to the current UK manual for National Infection Prevention and Control, aerosol generation from upper gastro-intestinal endoscopy was also identified substantially during general anaesthesia in other study [7] These discrepancies highlight the uncertainties as how aerosol generating procedures should be defined. Nevertheless, coughing appears to be a common source of aerosols in many procedures. Any manoeuvre that provokes coughing, such as tracheal extubation, may be considered as aerosol generating. Nonetheless, no study has linked these procedures to actual risk of pathogen transmission. We therefore believe it is imprecise to stratify risks of nosocomial transmission based on procedure alone. In this respect, addition or removal from the list of aerosol generating procedures in the height of pandemic, will likely overestimate or underestimate the risk, respectively.
We acknowledged that aerosols are generated from opening of mucus film in the small airways.[8] However, the amount of aerosols that travel to the nose and mouth is dependent on airflow. In a study of 13 patients who had coronavirus disease 2019, singing and talking increased small (<5 µm) aerosols by 10-fold, indicating the important contribution of high airflow in the generation viral-laden small aerosols. [9]
Dr Shrimpton and co-workers are also critical to our discussion on aerosol dispersion. It should be clear that while non-invasive ventilation and high flow nasal oxygenation may not increase aerosol generation, they may carry infected aerosols from the mouth and nose to a large distance, and our simulation studies [10–19] highlighted the possible maximum distance that aerosols could be dispersed.
Taken together, we believe it is far important understand how aerosols are being generated and dispersed. Instead of a fixed list of aerosol generating procedures, we believe it is more helpful for readers to design on the appropriate protective strategy to avoid nosocomial infection in individual patient having specific procedure.
Dr. Jason Chui 1
Dr. David SC Hui 2
Dr. Matthew TW Chan 3
1 Department of Anesthesia and Perioperative Medicine, University of Western Ontario, Canada
2 Department of Medicine and Therapeutics, The Chinese University of Hong Kong, Hong Kong Special Administrative Region, China
3 Department of Anesthesia and Intensive Care, The Chinese University of Hong Kong, Hong Kong Special Administrative Region, China
References
1 Chui J, Hui DS, Chan MT. How should aerosol generating procedures be defined? BMJ 2022;378:e065903.
2 Shrimpton AJ, Gregson FKA, Brown JM, et al. A quantitative evaluation of aerosol generation during supraglottic airway insertion and removal. Anaesthesia 2021;76:1577–84.
3 Gregson FKA, Shrimpton AJ, Hamilton F, et al. Identification of the source events for aerosol generation during oesophago-gastro-duodenoscopy. Gut 2022;71:871–8.
4 Shrimpton AJ, Brown JM, Gregson FKA, et al. Quantitative evaluation of aerosol generation during manual facemask ventilation. Anaesthesia 2022;77:22–7.
5 Shrimpton AJ, Brown JM, Cook TM, et al. Quantitative evaluation of aerosol generation from upper airway suctioning assessed during tracheal intubation and extubation sequences in anaesthetized patients. J Hosp Infect 2022;124:13–21.
6 AERATOR team. A rapid review of aerosol generating procedures (AGPs). Available at https://www.england.nhs.uk. Last accessed at 10th Oct 2022.
7 Chan SM, Ma TW, Chong MK-C, et al. A Proof of Concept Study: Esophagogastroduodenoscopy Is an Aerosol-Generating Procedure and Continuous Oral Suction During the Procedure Reduces the Amount of Aerosol Generated. Gastroenterology 2020;159:1949-1951.e4.
8 Wei J, Li Y. Airborne spread of infectious agents in the indoor environment. Am J Infect Control 2016;44:S102-8.
9 Coleman KK, Tay DJW, Tan KS, et al. Viral Load of SARS-CoV-2 in Respiratory Aerosols Emitted by COVID-19 Patients while Breathing, Talking, and Singing. Clin Infect Dis 2021;74
10 Hui DSC, Chan MTV, Chow B. Aerosol dispersion during various respiratory therapies: a risk assessment model of nosocomial infection to health care workers. HKMJ 2014;20 Suppl 4:9–13
11 Hui DSC, Hall S, Chan MTC, et al. Risks posed by the use of oxygen therapy and non-invasive positive pressure ventilation: a pilot study. HKMJ 2009;15 Suppl 8:4–7.
12 Hui DS, Chow BK, Chu LCY, et al. Exhaled air and aerosolized droplet dispersion during application of a jet nebulizer. Chest 2009;135:648–54.
13 Hui DS, Chow BK, Chu L, et al. Exhaled air dispersion and removal is influenced by isolation room size and ventilation settings during oxygen delivery via nasal cannula. Respirology 2011;16:1005–13.
14 Hui DS, Hall SD, Chan MTV, et al. Noninvasive positive-pressure ventilation: An experimental model to assess air and particle dispersion. Chest 2006;130:730–40.
15 Hui DS, Chow BK, Ng SS, et al. Exhaled air dispersion distances during noninvasive ventilation via different Respironics face masks. Chest 2009;136:998–1005.
16 Hui DS, Hall SD, Chan MTV, et al. Exhaled air dispersion during oxygen delivery via a simple oxygen mask. Chest 2007;132:540–6.
17 Hui DS, Chow BK, Lo T, et al. Exhaled air dispersion during noninvasive ventilation via helmets and a total facemask. Chest 2015;147:1336–43.
18 Chan MTV, Chow BK, Lo T, et al. Exhaled air dispersion during bag-mask ventilation and sputum suctioning - Implications for infection control. Sci Rep 2018;8:198–8.
19 Chan MTV, Chow BKM, Chu L, et al. Mask ventilation and dispersion of exhaled air. Am J Respir Crit Care Med 2013;187:e12-4.
Competing interests: No competing interests