key: cord-354290-o5i4a7nl
authors: Li, Jie; Fink, James B.; Ehrmann, Stephan
title: Author's Reply on High-Flow Nasal Cannula for COVID-19 Patients: Low Risk of Bio-Aerosol Dispersion
date: 2020-08-28
journal: Eur Respir J
DOI: 10.1183/13993003.03136-2020
sha: 
doc_id: 354290
cord_uid: o5i4a7nl

We appreciate the comments of Elshof et al.'s on our article “High-flow nasal cannula for COVID-19 patients: low risk of bio-aerosol dispersion” [1] and agree that further research is warranted to reduce risk of virus transmission from infected patients. The presented in vitro data [2] from a light detection of smoke dispersion distance and velocity model suggesting that high-flow nasal cannula (HFNC) generates larger dispersion distance than nonbreather mask and venturi mask is in contrast to reports from Hui et al., using a similar model [3]. Presumably because the smoke used by Elshof et al. is larger (0.3–2.5 µm) [2] than that used by Hui et al. (≤1 µm) [3], the larger particles dispersing differently. It should be noted that smoke in both models represents only a small fraction of the range of bioaerosols generated by patients during breathing, speaking, coughing or sneezing [4]. Using the same size airway model, the authors observed that the dispersion distance decreased from 71 cm to 25 cm by changing the nasal cannula size from small to large when HFNC flow was set at 30 L·min(−1), however, when HFNC flow was set at 60 L·min(−1), the medium size nasal cannula generated shorter distance than both small and large nasal cannulas. This raises the role of proper fit of prong to nares and highlights the limitations of modelization. Regardless of the sizes of nasal cannula, the dispersion distance was higher with 60 L·min(−1) than 30 L·min(−1), which is inline with Hui et al. results [3] and may be expected, as higher velocity of the gas may carry exhaled smoke to a further distance. However, this effect of total flow did not occure when testing the the venturi mask. Strangely, the venturi mask with large open holes and total gas flow of 40 L·min(−1) generated shorter dispersion distance than normal breathing. These inconsistencies are difficult to interpret without comprehensive peer reviewe of extensive methods and results. Whether smoke imaging models truly reflect the natural features of the transportation and dispersion of bioaerosols generated by patients has not been established and results from these studies should be interpreted cautiously.

We appreciate the comments of Elshof et al"s on our article "High-flow nasal cannula for COVID-19 patients: low risk of bio-aerosol dispersion" 1 and agree that further research is warranted to reduce risk of virus transmission from infected patients. The presented in vitro data 2 from a light detection of smoke dispersion distance and velocity model suggesting that high-flow nasal cannula (HFNC) generates larger dispersion distance than nonbreather mask and venturi mask is in contrast to reports from Hui et al, using a similar model 3 . Presumably because the smoke used by Elshof et al is larger (0.3-2.5 µm) 2 than that used by Hui et al (≤ 1 µm), 3 the larger particles dispersing differently. It should be noted that smoke in both models represents only a small fraction of the range of bioaerosols generated by patients during breathing, speaking, coughing or sneezing. 4 Using the same size airway model, the authors observed that the dispersion distance decreased from 71 cm to 25 cm by changing the nasal cannula size from small to large when HFNC flow was set at 30 L/min, however, when HFNC flow was set at 60 L/min, the medium size nasal cannula generated shorter distance than both small and large nasal cannulas. This raises the role of proper fit of prong to nares and highlights the limitations of modelization. Regardless of the sizes of nasal cannula, the dispersion distance was higher with 60 L/min than 30 L/min, which is inline with Hui et al results 3 and may be expected, as higher velocity of the gas may carry exhaled smoke to a further distance. However, this effect of total flow did not occure when testing the the venturi mask. Strangely, the venturi mask with large open holes and total gas flow of 40 L/min generated shorter dispersion distance than normal breathing. These inconsistencies are difficult to interpret without comprehensive peer reviewe of extensive methods and results.

Whether smoke imaging models truly reflect the natural features of the transportation and dispersion of bioaerosols generated by patients has not been established and results from these studies should be interpreted cautiously.

In a recent clinical study of aerosol particle concentrations and SARS-CoV-2 virus detection in the vicinity of patients with COVID-19, aerosol particle size and concentrations was measured before and after HFNC was applied to patients. No difference was observed between conventional nasal cannula applied prior to HFNC and HFNC. More importantly, no SARS-CoV-2 virus was detected in the room air with sampling cassette placed at 30 cm away from patients" airway for an hour. 5 It should also be noted that oxygen masks including venturi mask, nonbreather mask, simple mask, and aerosol mask, do not enable placement of a filter, except for some oxygen masks with special design. 6 Bioaerosols generated by patients might be exhaled via the holes or the one-way valve on the masks and the high gas flow from the masks help carry those bioaerosols to a further distance. In contrast, patients using HFNC can wear a surgical mask over HFNC, in order to reduce the dispersion of bioaerosols generated by patients. 6, 7 In all, compared to conventional oxygen devices, HFNC has been proven to improve oxygenation and reduce intubation rate in hypoxemic patients. 8 Abandoning HFNC to use other oxygen devices for the uncertain risks of virus transmission is unnecessary and ill advised. Special caution taken to protect personnel during "aerosol generating procedures" is more important than avoidance of "aerosol dispersing procedures". 4 Studying the production of aerosols by breathing support devices using lab models (e.g. smoke dispersion) is interesting but has important limitations because they are just simulations. What is really important, and still lacking in the literature, is a real life study assessing the actual virus cargo within the patient"s generated aerosols and, more important, how infective is such a viral cargo, which would probably depend on the physical and chemical characteristics of the aerosol particles.

High-flow nasal cannula for COVID-19 patients: low risk of bio-aerosol dispersion

High-flow nasal cannula for COVID-19 patients: low risk of bio-aerosol dispersion

Exhaled air dispersion during high-flow nasal cannula therapy versus CPAP via different masks

Coughs and sneezes: Their role in transmission of respiratory viral infections, including SARS-CoV-2

Placing mask on COVID-19 patients during highflow nasal cannula therapy reduces aerosol particle dispersion

Practical strategies to reduce nosocomial transmission to healthcare professionals providing respiratory care to patients with COVID-19

Preliminary findings on control of dispersion of aerosols and droplets during high-velocity nasal insufflation therapy using a simple surgical mask: Implications for the high-flow nasal cannula

Year in review 2019: High-flow nasal cannula (HFNC) oxygen therapy for adult patients

JL conceived of the idea and drafted the manuscript. JBF and SE provided critical revision on the manuscript. All authors reviewed and revised the manuscript and approved the final draft.

Not applicable.

Not applicable.

Not applicable.

Not applicable. 
