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Protective face masks add significant dead space
PP-PDPI, 07 Jun 2021 06:32:43


Question What is the effect of protective face masks on pulmonary gas exchange? Is there an objective reason that explains the frequently reported breathing limitations and impairments in well-being caused by masks?

Methods A lung simulator with intubation head was used to create breathing through different masks, taped and untaped mounting, mouth open and closed, quiet breathing and moderate exercise. Measurements within the trachea included capnography, pneumotachography and oximetry. From these measurements, indices of the effectiveness of carbon dioxide (CO2) elimination and oxygenation were derived and compared with non-mask reference values.

Results All measurements with masks showed significant increase in end-tidal CO2 (etCO2, +17.4 mmHg on average), even more pronounced in the moderate exercise setting (+25.9 mmHg on average). The extent of the increase of etCO2 varied with different mask types and with the mouth open or closed. The Bohr dead space nearly doubled when wearing a protective face mask. Intratracheal oxygen concentration decreased marginally when wearing protective masks (−0.6 vol% on average). The inspiratory and expiratory resistance increased marginally, on average by 0.4 mbar (L s−1)−1 and 0.3 mbar (L s−1)−1, respectively.

Answer Currently available protective face masks only marginally affect inspiratory oxygen concentration and resistance to breathing. However, they significantly increase dead space ventilation. The increasing end-tidal CO2 concentration could explain the reported symptoms among mask wearers especially in those with pre-existing lung disease.

The accumulation of CO2 is related to the size of the dead space. Simple measures to minimise the dead space would be tight fitting masks and preference for nasal breathing.

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