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Pre‐oxygenation in pregnancy: the effect of fresh gas flow rates within a circle breathing system *

We have investigated the effect of oxygen flow rate on pre‐oxygenation in pregnant patients at term using a circle system. Twenty patients presenting for elective Caesarean section maintained tidal volume breathing through a standard circle system for 3 min. Subjects were pre‐oxygenated using oxygen flow rates of 5 l.min−1, 10 l.min−1 and 15 l.min−1 presented in random order. The mean (SD) fractional end‐tidal oxygen at the end of 3 min was 0.86 (0.07) for 5 l.min−1, 0.92 (0.05) for 10 l.min−1and 0.90 (0.09) for 15 l.min−1 (p < 0.001). Entrainment of air occurred in 22% of pre‐oxygenation sessions. Oxygen flow rates of 10 l.min−1 or above provide optimal pre‐oxygenation using a circle system in term parturients. In our study, entrainment of air occurred in a surprisingly high percentage of cases.