Author's Reply on High-Flow Nasal Cannula for COVID-19 Patients: Low Risk of Bio-Aerosol Dispersion

Li, Jie; Fink, James B.; Ehrmann, Stephan

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.