UMEM Educational Pearls

 

There is currently a high, and appropriate, concern regarding the aerosolization of viral particles during various methods of respiratory support. While studies are limited, here is some of the currently available data (mostly-simulated) on the approximate maximum distances of particle spread:

Nasal Cannula 5LPM:¹ 1 ft 4.5 in

Non-Rebreather Mask, 6-12LPM: 4 in, minimal change with increasing flows¹

High Flow Nasal Cannula

• Simulation:² 30 LPM = 5.6 in / 60 LPM = 8.1 in
• Actual volunteers:³
  • Use of HFNC decreased aerosol dispersion during “violent exhalation” through nares
  • No difference in aerosol dispersion w/normal breathing using HFNC until 60lpm
  • Max spread = 14.4 ft without HFNC (violent exhalation) and 6.2 ft with HFNC (violent exhalation); aerosols airborne for max of 43 seconds

CPAP (20 cmH2O) provided by oronasal mask with good fit (leak from exhaust port):² 11.5 in

Bilevel positive airway pressure w/ oronasal mask (IPAP 10-18/EPAP 4): max dispersal:⁴ 1 ft 7.7 in

Bilevel positive airway pressure with full facemask⁵ (IPAP 18 / EPAP 5): 2 ft 8 in

Bilevel positive airway pressure with helmet:⁴

• IPAP 20 / EPAP 10 = 9 in
• Using helmet w/ air cushion = negligible dispersal

Utility of Surgical Mask:⁶

• No therapy:                 31% of exhaled particles travel, some >3.3 ft
• No therapy + mask:    5% of exhaled particles leak, some >3.3 ft
• 6LPM O2 + mask:       6.9% of exhaled particles leak, some >3.3 ft
• High Velocity Nasal Insufflation (40LPM) + mask: 15.9% of exhaled particles leak, some >3.3 ft

 

Bottom Line: 

In vivo data from actual patients is lacking, however there is potentially lower risk of aerosol spread with HFNC than regular nasal cannula, perhaps due to higher likelihood of a tighter nare/nasal cannula interface. Nonrebreather mask performs well indirectly with the shortest dispersal distance. Noninvasive positive pressure ventilation with an oronasal mask and good seal has a relatively short dispersal distance, and a surgical mask over respiratory support interventions actively decreases amount, if not distance, of particle spread. Use of appropriate PPE and negative pressure rooms, if available, remains key.

 

References

1. Hui DS, Chan MT, Chow B. Aerosol dispersion during various respiratory therapies: a risk assessment model of nosocomial infection to health care workers. Hong Kong Med J 2014; 20: Suppl. 4, 9–13.
2. Hui DS, Chow BK, Lo T, et al. Exhaled air dispersion during high-flow nasal cannula therapy versus CPAP via different masks. Eur Respir J 2019;53(4): 1802339. doi: 10.1183/13993003.02339-2018.
3. Roberts S, Kabaliuk N, Spence C, et al. Nasal high-flow therapy and dispersion of nasal aerosols in an experimental setting. J Crit Care 2015; 30(4):842.
4. Hui, DS, Hall, SD, Chan, MT et al. Noninvasive positive-pressure ventilation: An experimental model to assess air and particle dispersion. Chest 2006; 130: 730–40.
5. Hui DS, Chow BK, Lo T, et al. Exhaled air dispersion during noninvasive ventilation via helmets and a total facemask. Chest 2015; 147: 1336–1343
6. Leonard S, Atwood CW, Walsh BK, et al. Preliminary findings of control of dispersion of aerosols and droplets during high velocity nasal insufflation therapy using a simple surgical mask: Implications for high flow nasal cannula. Chest 2020. Epub ahead of print. doi: 10.1016/j.chest.2020.03.043.