Effect of variable pre-oxygenation endpoints on safe apnoea time using high flow nasal oxygen for women in labour: a modelling investigation

Abstract

Background: Studies 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.

Methods: The 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.

Results: HFNO 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.

Conclusions: Despite 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.

Keywords: apnoea; computer simulation; high-flow nasal oxygenation; obesity in pregnancy; obstetrics.

Fig 1

Oxygen saturation during apnoea (SaO₂) in the virtual subject BMI35 kg m⁻² in labour, during HFNO, closed glottis, and open airway with FiO₂ 21%. Closed glottis simulates failure of HFNO whereas FiO₂ 21% with end-tidal oxygen fraction (FE′O₂) of 90% simulates facemask technique. HFNO, high flow nasal oxygen therapy.

Fig 2

Gain in safe apnoea times, calculated as difference in min to reach SaO₂ 90% between HFNO with end-tidal oxygen fraction (FE′O₂) of 60%, 70%, 80%, 90%, and facemask with FE′O₂ of 90%. HFNO, high flow nasal oxygen therapy.