Maternal morbidity and mortality from airway complications during general anaesthesia for pregnant people having caesarean section remains a significant issue globally. Large databases demonstrate approximately 10 times that of the non-pregnant population. It is likely that a better preoxygenation technique may lead to a reduction in maternal morbidity and mortality from airway complications. High flow humidified nasal oxygen (HFNO) is a new technique that has been studied by the investigators in two previous trials in pregnant people: Firstly, “High Flow Humidified Nasal Preoxygenation in Pregnant Women” (HINOP1) which showed that HFNO was feasible and acceptable in 60 per cent of the cohort. Followed by “High flow humidified nasal oxygen versus face mask oxygen for preoxygenation of pregnant women” (HINOP2) which showed that pregnant people achieved better oxygen levels with the new high flow nasal oxygen.
The third proposed study, HINOP3 will focus on the specific group of high risk pregnant people – those with a high body mass index (BMI) as these people are at particularly high risk of airway complications during general anaesthesia for caesarean section.
It is hoped that this study will successfully demonstrate the advantages of high flow humidified nasal oxygen in pregnant people with high body mass. This could then pave the way for widespread international use of this technique in pregnant people, and an extension of its application for apnoeic oxygenation, the desired result of which would be improving the safety of tracheal intubation in pregnant people with high body mass who are at high risk of oxygen desaturation.
Professor Alicia Dennis and Dr Patrick Tan
The Royal Women’s Hospital, Melbourne.
The project was awarded $A83,985 funding through the ANZCA research grants program for 2023.
We carried out this study to find better ways to keep pregnant patients safe during general anaesthesia. The project asked the question:
“Can high flow nasal oxygen fill the lungs with as much oxygen as a standard anaesthetic face mask in pregnant patients with a high body mass index (BMI) before a general anaesthetic?”
Before a patient goes to sleep under a general anaesthetic, extra oxygen is given to fill the lungs. This is called “preoxygenation”. This creates an oxygen “reserve” that the body can draw on while normal breathing is paused at the start of the anaesthetic. The larger this oxygen reserve, the longer the patient can stay at safe oxygen levels. This step is especially important in pregnant patients. During pregnancy, changes in the body mean that oxygen levels can drop more quickly once anaesthesia begins. At the same time, managing the airway can be more challenging, which increases the risk to both the patient and the baby.
These risks are even higher in pregnant patients with a high body mass index (BMI over 30 kg/m²). Because this group is more likely to experience airway difficulties under anaesthetic. It is important to know whether newer oxygen techniques can keep them safer. High flow nasal oxygen is a modern method of delivering oxygen through soft nasal tubes. It can be used instead of the traditional face mask before a general anaesthetic and can even continue delivering oxygen during the first moments after anaesthesia begins. It may help maintain safe oxygen levels for longer.
In this study, we compared high-flow nasal oxygen with the standard face mask to see which method was better at filling the lungs with oxygen in this high-risk group of pregnant patients. The results may help guide safer anaesthetic care for pregnant patients with higher BMI.
The study found that high flow nasal oxygen produced higher lung oxygen levels than the standard face mask in pregnant patients with a high body mass. Importantly, it achieved the recommended clinical target for lung oxygen in two-and-a-half times more patients than current practice.
These outcomes directly address the problem we aimed to solve: pregnant patients with higher BMI lose oxygen more quickly and are at greater risk if difficulties occur during the start of a general anaesthetic. By showing that high flow nasal oxygen creates a larger and more reliable oxygen reserve, the study provides evidence that this technique may keep these patients safer during the critical moments when their breathing is paused and their airway is being managed.
These oxygen delivery methods have not previously been studied in this high-risk group of pregnant patients with high body mass. Our findings show that high flow nasal oxygen may offer a safer and more effective way to increase lung oxygen reserves before a general anaesthetic. This has the potential to improve patient safety during the most vulnerable part of anaesthesia—when breathing pauses and the airway is being managed. Evidence supporting a superior oxygenation technique could be highly impactful because airway related complications are a source of preventable harm. Our results may therefore contribute to future updates of international airway management guidelines and encourage broader adoption of high flow nasal oxygen in obstetric anaesthesia. In practical terms, this could lead to routine use of high flow nasal oxygen as part of standard airway preparation for pregnant patients undergoing general anaesthesia.
Presentations
Abstract accepted for presentation at:
- ANZCA Annual Scientific Meeting 2026 – Gilbert Brown Prize Session
- Obstetric Anaesthetists’ Association (OAA) Annual Scientific Meeting 2026 – Liverpool, UK
“High flow nasal oxygen in pregnant women” – PhD Completion Seminar (February 2026) – P Tan
“Time to Switch the Optiflow. High flow nasal oxygen in obstetric anaesthesia” – ANZCA VIC Winter Meeting (July 2023) – P Tan Full manuscript drafted - publication submission planned mid-2026