The Pre-brief
sal bridge were widely used before COVID. With the concern for aerosolization of infectious particles, completely sealed NIPPV masks were used with an exhalation port in line with the circuit proximal to a viral filter. This location of the exhalation port proximal to the filter decreased the amount of aerosolized particles released into the air.
Any tubing placed distal to the exhalation port should increase dead space. Various masks have been tested to evaluate their effect on dead space.
Exhalation Ports Over the Bridge of Nose
NIPPV masks with exhalation ports over the nasal bridge actually wash out some anatomic dead space. They can decrease the dead space from a baseline 32% to 28.5%. None of the exhaled gas passes through a viral filter, therefore aerosolized pathogens will be disbursed.
Exhalation Port on Circuit
Masks with exhalation ports on the NIPPV circuit increase dead space by varying degrees. These can increase total dead space from a baseline of around 32% to 40%. However, since a large portion of exhaled gas passes through a viral filter, less pathogens will be aerosolized.
The Debrief
- When choosing a mask for NIPPV, the risk of aerosolization of pathogens must be weighed against the benefit of dead space washout.
- Masks and circuits with exhalation ports proximal to the viral filter add mechanical dead space. However, they decrease the aerosolization of viral particles
- Masks with exhalation ports over the bridge of the nose washout anatomic dead space. This dead space washout comes at the expense of lack of filtration of exhaled gas.
References
- Saatci E, Miller DM, Stell IM, Lee KC, Moxham J. Dynamic dead space in face masks used with noninvasive ventilators: a lung model study. Eur Respir J. 2004 Jan;23(1):129-35. doi: 10.1183/09031936.03.00039503. PMID: 14738244.
