Reducing aerosol dispersion by high flow therapy in COVID-19: High resolution computational fluid dynamics simulations of particle behavior during high velocity nasal insufflation with a simple surgical mask

Abstract

Objective: All respiratory care represents some risk of becoming an aerosol-generating procedure (AGP) during COVID-19 patient management. Personal protective equipment (PPE) and environmental control/engineering is advised. High velocity nasal insufflation (HVNI) and high flow nasal cannula (HFNC) deliver high flow oxygen (HFO) therapy, established as a competent means of supporting oxygenation for acute respiratory distress patients, including that precipitated by COVID-19. Although unlikely to present a disproportionate particle dispersal risk, AGP from HFO continues to be a concern. Previously, we published a preliminary model. Here, we present a subsequent highresolution simulation (higher complexity/reliability) to provide a more accurate and precise particle characterization on the effect of surgical masks on patients during HVNI, low-flow oxygen therapy (LFO2), and tidal breathing.

Methods: This in silico modeling study of HVNI, LFO2, and tidal breathing presents ANSYS fluent computational fluid dynamics simulations that evaluate the effect of Type I surgical mask use over patient face on particle/droplet behavior.

Results: This in silico modeling simulation study of HVNI (40 L min-1) with a simulated surgical mask suggests 88.8% capture of exhaled particulate mass in the mask, compared to 77.4% in LFO2 (6 L min-1) capture, with particle distribution escaping to the room (> 1 m from face) lower for HVNI+Mask versus LFO2+Mask (8.23% vs 17.2%). The overwhelming proportion of particulate escape was associated with mask-fit designed model gaps. Particle dispersion was associated with lower velocity.

Conclusions: These simulations suggest employing a surgical mask over the HVNI interface may be useful in reduction of particulate mass distribution associated with AGPs.

Keywords: aerosol‐generating procedures; exhalation; high flow nasal cannula; high flow oxygen; high velocity nasal insufflation; low flow oxygen; masks; particle dispersion/transmission; patient simulation; prevention/control.

Conflict of interest statement

In accordance with ICMJE guidelines at the time of this study: Reid Prichard and Wayne Strasser declare no conflict of interest. Charles W. Atwood has received fees from Vapotherm, Inc for clinical research consultation. Jessica S. Whittle has received fees for clinical research consultation and speaker honorarium from Vapotherm Inc. Scott Leonard, Leonithas I. Volakis, Ronald J. DeBellis, and George C. Dungan are employees of Vapotherm Inc. No payments were received in preparation of this manuscript.

© 2020 The Authors. JACEP Open published by Wiley Periodicals LLC on behalf of the American College of Emergency Physicians.

Figures

FIGURE 1

Model of the room simulation 3D surfaces

FIGURE 2

(Left) Image of head with surgical mask. (Right) The skin to mask designed gap locations (blue) are symmetric on each side of mask, representative of a “poorlyfitted” or “worst‐case” mask‐fit on a patient. Both images depicted are for the room simulation

FIGURE 3

Visual of the high‐resolution mesh geometry implemented for the room simulation

FIGURE 4

The airflow streamlines for ventilation during the room simulation

FIGURE 5

Velocity contours for all test cases during the room simulation. Images provide the velocity of the gas flows (denoted as m s−1) tested settings both with (left) and without (right) a surgical mask: HVNI at 40 L min−1 (top), low flow at 6 L min−1 (middle), no therapy (bottom). Images are representative of the cross‐section at the sagittal plane

FIGURE 6

Loss of gas flows across the isosurfaces for tested cases during the room simulation. Images provide the exit locations of the gas flows (velocity of 0.5 m s−1 across the isosurfaces) during the tested settings with a surgical mask: HVNI at 40 L min−1 (top), low flow at 6 L min−1 (middle), no therapy (bottom)

FIGURE 7

Relative pressure (Pa) contours within the region of the mask and upper airway for the tested cases during the room simulation. HVNI at 40 L min−1 (top), low flow at 6 L min−1 (middle), no therapy (bottom). Images are representative of the cross‐section at the sagittal plane

FIGURE 8

ANSYS results for the percentage of (top) total particle mass disposition, (middle) particle mass disposition for particles ≤5 µm, and (bottom) particle mass disposition for particles > 5 µm for all tested cases with a surgical mask. For particles > 5 µm during HVNI, the mask captures 9.75% more particles than LFO2, and 11.32% more particles than no therapy (tidal breathing). For particles ≤5 µm during HVNI, the mask captures 130.72% more particles than LFO2, and 223.45% more particles than no therapy (tidal breathing)

FIGURE A1

Rossin‐Rammler diameter distribution of the particles in the room simulation