key: cord-0697332-9eegpday
authors: Coughlan, Mark F.; Sawhney, Mandeep S.; Pleskow, Douglas K.; Sheil, Conor J.; Qiu, Le; Perelman, Lev T.; Khan, Umar; Bilal, Mohammad; Feuerstein, Joseph D.; Zhang, Xuejun; Glyavina, Maria; Zakharov, Yuri N.; Berzin, Tyler M.; Zhang, Lei; Itzkan, Irving
title: Measuring droplets expelled during endoscopy to investigate COVID-19 transmission risk
date: 2021-07-16
journal: Gastroenterology
DOI: 10.1053/j.gastro.2021.07.013
sha: 62c896a95b21eca75abf7a9b3f0780a7093a0999
doc_id: 697332
cord_uid: 9eegpday

nan

SARS-CoV-2 infection spreads primarily through droplets and aerosols 1 . Various procedures in healthcare, including upper endoscopy, have been categorized as aerosol generating procedures (AGPs) 2 . However, it is unclear if these procedures also produce significant quantities of larger droplets, which pose a greater transmission risk 3 . It is also unclear whether colonoscopies cause an additional risk for healthcare workers, since fecal-oral transmission has been identified as a possible transmission mechanism 2 . The ability to detect and measure droplets is critical for the evaluation of procedure risk, but most available methods lack portability, or cannot distinguish solid particles from liquid droplets, which pose a much higher risk of SARS-CoV-2 transmission 1 . To investigate the droplet generating risk posed by endoscopy procedures, we developed a robust and portable optical instrument capable of distinguishing liquid droplets from solid particles, while also measuring the size and quantity of fast flying droplets in the clinical setting.

The system was designed to image the angular dependent light scattering patterns produced by droplets in the close to forward direction. Mie theory 4 shows that this scattering pattern can be exploited to determine droplet size 5 . The optical layout of the system is shown in Figs. 1A and 1B. Droplets crossed an expanded red laser beam after entering through an aperture in the 3D printed case (Fig. 1C) . One camera imaged the angular dependent light scattering patterns, while a second camera was used to spatially visualize the droplets and co-register them with their scattering patterns. The larger fan maintained an air flow, while the smaller fan cooled the cameras. The entire system was constructed on an 8"x10" optical breadboard and provided a measurement zone of 5x12mm.

J o u r n a l P r e -p r o o f

The system was used to measure droplets produced during ten upper endoscopies and ten colonoscopies. The study was performed according to the Beth Israel Deaconess Medical Center IRB guidance. Consecutive procedures were measured in a single room over two days. Measurements were taken during three time periods for each patient, with one period corresponding to the procedure duration and the other two periods corresponding to controls. During the preprocedure control, the patient and staff were present in the procedure room, but the endoscope had not been inserted into the patient. For the post-procedure control, the endoscope had been removed from the patient, but both staff and patient were still present. The positioning of the device for the upper endoscopy procedures is illustrated in Figs. 1D and 1E. Measurements were taken near the rectum for colonoscopy procedures. Best estimates for the device positioning are given in Supplementary Table 1 .

The data was analyzed by initially extracting the scattering events that occurred during the procedure and controls. A scattering event occurs when either a liquid droplet or solid particle crosses the beam, with typical scattering patterns for each type shown in Figs. 1F and 1G, respectively. Figure 1H shows the average number of scattering events per unit area per minute for all procedures and controls. Figure 1I shows the results when only droplets were considered, while The number of scattering events, which includes both droplets and particles, was considerably higher for both controls, as compared to the procedures. This was not unexpected, since there is much more activity in the procedure room before J o u r n a l P r e -p r o o f and after the procedure. However, in terms of droplets that pose a transmission risk, significantly more were observed in the procedures, as compared to the controls. More droplets were measured during colonoscopy procedures compared to upper endoscopy procedures (4.0•10 -2 mm -2 and 2.8•10 -2 mm -2 , respectively). When adjusted for procedure duration, more droplets per unit time were produced during the upper endoscopies compared to the colonoscopies (3.6•10 -3 mm -2 ·min -1 and 1.9•10 -3 mm -2 ·min -1 , respectively). However, neither of these differences were statistically significant. A similar size distribution is seen for both procedures, with a slightly larger spread observed for the colonoscopy procedures.

It is possible to estimate the total number of droplets produced during upper endoscopies using spreading angles obtained from cough studies 6 . Given the average measurement distance, we determined that approximately 500 liquid droplets could be produced during upper endoscopy. Converting the droplets to volume, and using the average viral load observed by Wölfel and colleagues 7 , it is possible that upper endoscopy procedures produce approximately 6,500 viral copies per procedure.

Our results have several important clinical implications. First, the positioning of our device shows that these droplets can reach nearby healthcare workers.

Second, while the risk of COVID-19 transmission during upper endoscopy has been well recognized, our findings suggest that transmission via droplets during colonoscopy is also possible. Third, we found marked variation among droplets produced by patients, with one patient from the colonoscopy group and one patient from the upper endoscopy group accounting for approximately 50% of the total droplets produced. This upper endoscopy patient was not wearing a procedural oxygen mask (POM), but six patients were. On average, the unmasked patients J o u r n a l P r e -p r o o f produced 2.75 times more droplets than the masked patients. Given that the POM mask almost certainly reduces the emission cone, this difference is probably even more significant. We also observed low numbers of droplets for some patients in both the masked and unmasked group, similar to other studies 8 . For the colonoscopy patient that produced the most droplets, the droplets were spread throughout the procedure. Conversely, for the upper endoscopy patient, all droplets came in a single 33 second period and did not correspond to coughing or endoscope insertion/removal. While other endoscope manipulations may have caused the increased expulsion, it appears that the periods of higher risk may be difficult to identify. To minimize droplet exposure, we suggest only essential personnel remain close to the endoscope operator during the procedure. Table 1 . Patient and room information for ten upper endoscopies (EGD) and ten colonoscopies (Colon). During some endoscopy procedures (PROC), the patient wore a procedural oxygen mask (POM). Angle (θ) and distance (L) notation are illustrated in Fig. 1E . Angles are defined relative to the oral axis for upper endoscopies, with positive angles towards the nose of the patient. Angles are defined relative to the anal canal axis for colonoscopies, with positive angles towards the patient front. The mean ± standard error duration of the upper endoscopies and colonoscopies was 7.9 ±1.0 minutes and 21.0 ±1.6 minutes, respectively. The mean duration of the pre-procedure control and post-procedure control was 7.7 ±0.6 minutes and 9.3 ±0.9 minutes, respectively. 

The method is based on several important facts regarding the scattering of light by spherical droplets in the close to forward direction. Firstly, such scattering can be accurately described using an exact solution for the scattering of electromagnetic plane waves by a dielectric spherical scatterer introduced by Mie 4 . The Mie solution

shows that for water droplets with diameters of 5 μm and above in air, there is a significant scattering peak in the close to forward direction, which could be strong Fig. S1B . Figure S1C shows the averaged scattering pattern (red dotted line) for the droplet shown in Fig. 1G , which is compared to the best fit theoretical curve (blue solid line). This theoretical curve was produced using Mie scattering theory for a 41 µm water droplet in air, with a constant background term added. The system was optimized to measure droplets larger than aerosols, since viral load is proportional to the droplet volume. For example, a 50 µm droplet has a volume 1,000 times larger than a 5 µm aerosol, and therefore carries an approximately 1,000 times higher viral load. 

Author names in bold designate shared co-first authorship