Effect of Combined Oxygen Therapy and Use of a Surgical Mask or N95 Mask on Inspired Oxygen Fraction and Expired Carbon Dioxide Fraction.

Combined use of a surgical mask and oxygen mask might decrease the inspired oxygen concentration and increase the risk of hypercapnia. We investigated the fraction of inspired oxygen (FiO₂) and end-tidal carbon dioxide (ETCO₂) under different combinations of masks and oxygen flows. Five healthy volunteers were administered oxygen using the following methods: oxygen mask alone (O group), oxygen mask over a surgical mask (S group), and oxygen mask over an N95 mask (N group). FiO₂ and ETCO₂ were measured at oxygen flow rates of 0, 5, and 8 L/min under each mask condition. At oxygen flow rates of 5 and 8 L/min, FiO₂ was lower in the order of N group (0.32 at 5 L/min, 0.36 at 8 L/min), S group (0.45 at 5 L/min, 0.52 at 8 L/min), and O group (0.61 at 5 L/min, 0.73 at 8 L/min). ETCO₂ was higher in the order of N, S, and O groups. In conclusion, wearing the oxygen mask over the surgical mask or N95 mask reduces FiO₂ and increases ETCO₂ in healthy volunteers. Since patients who have emerged from general anaesthesia are more likely to have worse respiratory conditions, they need close observation to avoid hypoxemia and hypercapnia.