Aerosols: are anaesthetists at risk of COVID‐19 or not? (2020)

In his excellent narrative review of the health risks from COVID-19 for anaesthetists and intensivists, Cook supports the general assumption that “tracheal intubation is an aerosol-generating procedure and…there is robust evidence of an association between tracheal intubation and transmission of infection…” [1]. Crucially, his research group goes on to conduct research, the results of which refute the designation of tracheal intubation/extubation/facemask ventilation as aerosol-generating procedures [2]. His group's results and conclusions are supported by Sperna Weiland et al. [3] in vitro, but contradicted by Dhillon et al. in vivo [3, 4]. Resolving this dilemma has important implications for preventing nosocomial transmission and for facilitating patient throughput [4-6]. Brown et al.'s UK [2] and Dhillon et al.'s Australian [4] quantitative studies used similar methodology to assess aerosol generation within different theatre ventilation systems during airway manoeuvres performed on COVID-19-negative patients undergoing elective surgery. We have compared and contrasted their methods and main findings in Table 1. 33 (extubation) 1310 (volitional) Sperna Weiland et al. (The Netherlands) used in-vitro nebulisation in six types of single-patient hospital rooms with varying air exchange rates, to simulate aerosol-generating procedures. They found considerable variation in the 99% aerosol removal time, between 7 min (standard and negative pressure operating theatres) and 307 min (hospital ward single room), inversely correlated with the air exchange rate (i.e. the greater the air exchange rate, the quicker the aerosol removal). Placing aerosol purification units in the (ward) room with the lowest air exchange rate reduced the 99% aerosol removal time from 307 to 47 min. How should practising anaesthetists interpret data from these three studies? Firstly, there are methodological shortcomings. Secondly, both tracheal intubation and extubation produce (potentially infective) aerosolised particles to a greater/lesser degree, particularly if the patient coughs. Thirdly, therefore, airway procedures involving COVID-19-positive patients should be undertaken by the most experienced anaesthetists available, (ideally) wearing full PPE, including respirators, in the best ventilated environment available, (ideally) a laminar flow ventilation theatre with > 200 room volume air changes per hour, that is, current protocols remain apposite, but could be improved by reinforcing the importance of room ventilation [7]. Fourthly, because the studies confirm that coughing is an aerosol-generating procedure, healthcare workers should continue to wear surgical facemasks at all other times and consider wearing respirators in hospital areas that are poorly ventilated, socially connected and/or contain coughing patients, in order to reduce horizontal transmission until effective vaccination becomes available. Finally, more specific research is needed. As consensus opinion on this issue is urgently required, could we invite Dhillon et al. to repeat Brown et al.'s study to confirm or refute their findings? We also invite Dhillon et al. to comment on why they think their results and conclusions differ from those of Brown et al. and Sperna Weiland et al., and to elaborate on methodological shortcomings in all three studies which might enable future research to provide more definitive answers regarding the absolute and relative risks to anaesthetists from aerosol-generating procedures in different ventilation environments, and how these might be ameliorated further.