Discussion
During this SARS CoV 2 pandemic, global healthcare systems have quickly
realized the importance of protecting front-line workers with effective
PPE. To reduce the risk of transmission to staff, N95 respirators (or
the equivalent FFP2 standard) are typically worn by medical and
paramedical staff in high-risk environments for prolonged periods of
time without removal. The effect of this prolonged use on
cardio-respiratory variables such as heart rate and gas-exchange are
poorly defined. Our findings suggest that gas exchange is influenced by
prolonged wearing of the N95 respirator, as demonstrated by a decline in
plasma pH, PvO2, and a concurrent increase in
EtCO2. There were no significant changes in
PvCO2 or bicarbonate levels. Heart rate, as well as
oxygen saturation determined by pulse oximetry, were unaffected.
These findings add to previous work evaluating the impact of prolonged
N95 respirator use on blood gases and physiology. Overall, available
data suggest that changes in blood gases and other physiological
parameters caused by N95 respirators during physical activity are small
even during very heavy exercise9. In 2004, during the
severe acute respiratory syndrome (SARS) outbreak in Taiwan, Kao et al.
investigated the physiological impact of wearing an N95 respirator
during hemodialysis on 39 patients with end-stage renal disease. 70% of
participants had a reduction in the partial pressure of oxygen in
arterial blood (PaO2), and 19% developed various
degrees of hypoxemia8. The physiological effect of N95
respirators among healthcare professionals was investigated by Rebmann
et al., who demonstrated a small but significant increase in
CO2 levels among ten intensive care unit nurses who used
N95 respirators for 12-hour shifts10. Although
CO2 concentration differences were not statistically
significant in our study, there was an upward trend after a 4-hour
shift. A lack of power due to the small sample size may explain the
variance between these findings and those of the present study.
The physiological impact of N95 respirator use during physical effort
has been previously evaluated during exercise using a
treadmill11,12 with findings supporting mild increases
in CO2 levels but no impact on respiratory rate, tidal
volume, or breathing pattern. In the current study, we did not
demonstrate a change in HR or pulse oximetry, possibly implying the
physiological effects in healthy individuals are small and unlikely to
lead to any adverse consequences. However, the changes in pH,
PvO2, and EtCO2 suggest a degree of
hypoventilation and/or CO2-rebreathing that may be
provoked by wearing N95 respirators for prolonged, uninterrupted
periods. The effect of these changes during longer shifts or where
additional tape is used to seal the mask further and in those workers
with pre-existing cardio-respiratory disease has not been defined.
Several studies have suggested mechanistic links between chronic
intermittent hypoxemia and increased production of reactive oxygen
species, endothelial dysfunction, systemic inflammation, and pulmonary
hypertension13,14, which may lead to longer-term
consequences in susceptible individuals.
Strengths of this study include the use of N95 respirators by healthcare
professionals in a typical setting of a 4-hour ED shift and the use of
invasive blood gas assessment as well as non-invasive indices such as
EtCO2 and pulse-oximetry for comparison. Limitations
include the use of venous rather than arterial blood gas measurements to
make it more comfortable for the study participants and provide an
approximation to arterial values. Venous pH and pCO2correlate well with arterial values (although they are not
interchangeable) but are unsuitable for determining patient
oxygenation15,16. The study also did not capture
self-reported symptoms, so correlations with physiological changes could
not be explored. Of course, a control group without N95 masks was not
possible due to safety concerns. Future studies should include
healthcare workers wearing these respirators for longer shifts,
including those in ‘Corona units’ where additional precautions such as
hoods and tape may increase the seal on the face. Also, the effect of
prolonged use of these respirators in smokers and those with
pre-existing cardiorespiratory disease or obesity is not known and is
relevant as these PPE devices will be widely used for the foreseeable
future in hospitals.