Physiologic measurements
Subjects, at the beginning of their ED shift, were instructed to avoid any mask use for at least 15 minutes, whilst isolated away from the clinical area, to ensure baseline measurements were obtained under normal breathing. Participants were then asked to wear the N95 respirator continuously and begin their ED shift. All subjects were provided N95 respirators (Kimberly-Clark, Irving, Texas, US) for this study. Physiological measurements, including oxygen saturation and heart rate (HR), were captured at baseline and each hour up to 4 hours using an O2 saturation oximeter (Welech allyn- Vital Signs Monitor 6000, Skaneateles Falls, NY, USA). A 2CC of venous blood sample was drawn at baseline and at 4-hours (before the N95 mask was removed), using a syringe washed with heparin. The sample was immediately analyzed using a gas analyzer machine (Siemens RAPIDPoint 500, Siemens Healthcare Limited, UK). pH level, partial pressure of venous oxygen (PvO2), partial pressure of venous carbon dioxide (PvCO2), and bicarbonate (HCO3) level were recorded. End‐tidal carbon dioxide (EtCO2) level was measured through non-invasive nasal prongs (Microstream CapmoLine, Philips, UK) representing the EtCO2 levels at 4-hours. After ensuring the first EtCO2 level and a typical capnography waveform, participants were asked to remove the respirator while the EtCO2 levels were continuously monitored. The lowest EtCO2 level at room air and the time required to reach it were recorded.