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 (FiO) and end-tidal carbon dioxide (ETCO) 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). FiO and ETCO 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, FiO 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). ETCO 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 FiO and increases ETCO 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.