Combining oxygen facemask with apnoeic oxygenation using high-flow-nasal-oxygen (HFNO) for preoxygenation in the operating room has not been studied against standard oxygen facemask alone. We hypothesized that facemask-alone would be associated with lower levels of lowest end-tidal oxygen (EtO2) within 2 min after intubation in comparison with facemask combined with HFNO.

In an international prospective before–after multicentre study, we included adult patients intubated in the operating room from September 2022 to December 2022. In the before period, preoxygenation was performed with facemask-alone, which was removed during laryngoscopy. In the after period, facemask combined with HFNO was used for preoxygenation and HFNO for apnoeic oxygenation during laryngoscopy. HFNO was maintained throughout intubation. The primary outcome was the lowest EtO2 within 2 min after intubation. The secondary outcome was SpO2 ≤ 95% within 2 min after intubation. Subgroup analyses were performed in patients without and with obesity. This study was registered 10 August 2022 with ClinicalTrials.gov, number NCT05495841.

A total of 450 intubations were evaluated, 233 with facemask-alone and 217 with facemask combined with HFNO. In all patients, the lowest EtO2 within 2 min after intubation was significantly lower with facemask-alone than with facemask combined with HFNO, 89 (85–92)% vs 91 (88–93)%, respectively (mean difference − 2.20(− 3.21 to − 1.18), p   <  0.001). In patients with obesity, similar results were found [87(82–91)% vs 90(88–92)%, p  = 0.004]; as in patients without obesity [90(86–92)% vs 91(89–93)%, p  = 0.001)]. SpO2 ≤ 95% was more frequent with facemask-alone (14/232, 6%) than with facemask combined with HFNO (2/215, 1%, p  = 0.004). No severe adverse events were recorded.

Combining facemask with HFNO for preoxygenation and apnoeic oxygenation was associated with increased levels of lowest EtO2 within 2 min after intubation and less desaturation.