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High-flow humidified nasal preoxygenation in pregnant women: a prospective observational study. BACKGROUND: Obstetric airway guidelines recommend preoxygenation in preparation for general anaesthesia to achieve end-tidal oxygen concentrations (etO2) of ≥90%, and mention the potential use of high-flow humidified nasal oxygen (HFNO). We investigated the new HFNO technique (Optiflow™) in term pregnant women. METHODS: Seventy-three term participants underwent a 3 min HFNO protocol (30 L min-1 for 30 s, and then 50 L min-1 for 150 s). The etO2 was assessed for the first four breaths after simulated preoxygenation. The primary outcome was the proportion who achieved etO2 ≥90% for the first expired breath. The secondary outcomes were the proportion who achieved etO2 ≥80%; fetal heart rate before and after the test; the association of body mass index (BMI) to etO2 achieved with preoxygenation; and comfort levels of, and preference for, nasal and face mask preoxygenation. RESULTS: The proportion with first expired breath etO2 ≥90% was 60% [95% confidence interval (CI): 54-66%] and etO2 ≥80% was 84% (95% CI: 80-88%). There was no change in fetal heart rate from before to after the test (P=0.34). There was a negative association of BMI to etO2 achieved with preoxygenation (Pearson correlation: -0.26; P=0.027). There was no difference in comfort scores between nasal cannula and face mask (P=0.40). Forty-one (56%; 95% CI: 35-47%) women preferred the nasal cannula to the face mask. CONCLUSIONS: HFNO using this protocol is inadequate to preoxygenate term pregnant women. Despite encouraging results in the literature reporting results in non-pregnant women, further work is required before justifying its use in pregnant women. CLINICAL TRIAL REGISTRATION: ACTRN 12616000531415p.