• Daniel Stolady, Marianna Laviola, Arani Pillai, and Jonathan G Hardman.
• Anaesthesia Queen Elizabeth Hospital King's Lynn NHS Foundation Hospital, Kings Lynn, UK. Electronic address: DanielGareth.Stolady@qehkl.nhs.uk.
• Br J Anaesth. 2021 Apr 1; 126 (4): 889-895.

BackgroundStudies of pulmonary denitrogenation (pre-oxygenation) in obstetric populations have shown high flow nasal oxygen therapy (HFNO) is inferior to facemask techniques. HFNO achieves median end-tidal oxygen fraction (FE'O2) of 0.87 after 3 min. As HFNO prolongs safe apnoea times through apnoeic oxygenation, we postulated that HFNO would still extend safe apnoeic times despite the lower FE'O2 after pre-oxygenation.MethodsThe Interdisciplinary Collaboration in Systems Medicine simulation suite, a highly integrated, high-fidelity model of the human respiratory and cardiovascular systems, was used to study the effect of varying FE'O2 (60%, 70%, 80%, and 90%) on the duration of safe apnoea times using HFNO and facemask techniques (with the airway open and obstructed). The study population consisted of validated models of pregnant women in active labour and not in labour with BMI of 24, 35, 40, 45, and 50 kg m-2.ResultsHFNO provided longer safe apnoeic times in all models, with all FE'O2 values. Labour and increased BMI reduced this effect, in particular a BMI of 50 kg m-2 reduced the improvement in apnoea time to 1.8-8.5 min (depending on the FE'O2), compared with an improvement of more than 60 min in the subject with BMI 24 kg m-2.ConclusionsDespite generating lower FE'O2, HFNO provides longer safe apnoea times in pregnant subjects in labour. Care should be taken when used in patients with BMI ≥50 kg m-2 as the extension of the safe apnoea time is limited.Copyright © 2021 British Journal of Anaesthesia. Published by Elsevier Ltd. All rights reserved.

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