This topic was selected because an evidence synthesis beats a textbook on it: the trials disagree, or the guidance has moved recently.
THE EVIDENCE
What each source contributes, and how strong it is
Study design first — a cohort and a randomised trial do not carry the same weight.
Design
Year
Journal
What it found
Meta-analysis
2023
Anesthesia and analgesia
In obese patients high-flow nasal cannula lengthened safe apnoea time and was more
Meta-analysis
2022
Medicine
High-flow nasal oxygen achieved a higher PaO2 and about 131 seconds more safe apnoea
Review
2026
critical care & pain medicine
HFNO prolongs safe apnoea time and decreases desaturation incidence compared to
Randomised trial
2023
British journal of anaesthesia
Median apnoea time in morbidly obese patients reached 18 minutes with either
Review
2026
British journal of anaesthesia
High-flow nasal oxygen does not clear carbon dioxide
Randomised trial
2019
EClinicalMedicine
Compared with non-invasive ventilation, preoxygenating obese patients with high-flow
6 resolved citations behind this deck; every point above traces to one of them.
WHERE THE GUIDANCE SITS
What the guidelines and pooled evidence say
2 findings, each on the slide that follows.
Anesthesia and analgesia 2023
In obese patients high-flow nasal cannula lengthened safe apnoea time and was more
In obese patients high-flow nasal cannula lengthened safe apnoea time and was more comfortable
Medicine 2022
High-flow nasal oxygen achieved a higher PaO2 and about 131 seconds more safe apnoea
High-flow nasal oxygen achieved a higher PaO2 and about 131 seconds more safe apnoea time than facemask, without reducing desaturation during…
WHERE THE GUIDANCE SITS
In obese patients high-flow nasal cannula lengthened safe apnoea time and was more
Meta-analysis · Anesthesia and analgesia
In obese patients high-flow nasal cannula lengthened safe apnoea time and was more comfortable, but did not reduce the odds of desaturating below 92% — it buys time rather than removing the risk.
High-Flow Nasal Cannula for Apneic Oxygenation in Obese Patients for Elective Surgery: A Systematic Review and Meta-Analysis, Anesthesia and analgesia 2023 · PMID 36469483
THE NUMBERS
It lengthens apnoea time without removing the risk
Obese patients, six trials pooled.
The desaturation difference did not reach significance — it buys time, it does not remove the problem.
High-Flow Nasal Cannula for Apneic Oxygenation in Obese Patients for Elective Surgery: A Systematic Review and Meta-Analysis, Anesthesia and analgesia 2023 · PMID 36469483
WHERE THE GUIDANCE SITS
High-flow nasal oxygen achieved a higher PaO2 and about 131 seconds more safe apnoea
Meta-analysis · Medicine
High-flow nasal oxygen achieved a higher PaO2 and about 131 seconds more safe apnoea time than facemask, without reducing desaturation during intubation.
A Comparison of High-Flow Nasal Cannula and Standard Facemask as Pre-Oxygenation Technique for General Anesthesia: A PRISMA-compliant Systemic Review and Meta-Analysis, Medicine 2022 · PMID 35451383
WHAT THE TRIALS FOUND
Where randomised evidence moved the question
2 findings, each on the slide that follows.
British journal of anaesthesia 2023
Median apnoea time in morbidly obese patients reached 18 minutes with either
Median apnoea time in morbidly obese patients reached 18 minutes with either high-flow nasal or low-flow facemask oxygen
EClinicalMedicine 2019
Compared with non-invasive ventilation, preoxygenating obese patients with high-flow
Compared with non-invasive ventilation, preoxygenating obese patients with high-flow nasal cannula gave a lower end-tidal oxygen and more…
WHAT THE TRIALS FOUND
Median apnoea time in morbidly obese patients reached 18 minutes with either
Randomised trial · British journal of anaesthesia
Median apnoea time in morbidly obese patients reached 18 minutes with either high-flow nasal or low-flow facemask oxygen, but desaturation below 92% was less likely with high flow (hazard ratio 0.27).
Apnoeic Oxygenation in Morbid Obesity: A Randomised Controlled Trial Comparing Facemask and High-Flow Nasal Oxygen Delivery, British journal of anaesthesia 2023 · PMID 35027169
WHAT THE TRIALS FOUND
Compared with non-invasive ventilation, preoxygenating obese patients with high-flow
Randomised trial · EClinicalMedicine
Compared with non-invasive ventilation, preoxygenating obese patients with high-flow nasal cannula gave a lower end-tidal oxygen and more desaturation below 95% (30% versus 12%), although patients found it far more comfortable.
High-Flow Nasal Cannulae Versus Non-Invasive Ventilation for Preoxygenation of Obese Patients: The PREOPTIPOP Randomized Trial, EClinicalMedicine 2019 · PMID 31528849
THE NUMBERS
Against non-invasive ventilation, high flow came off worse
PREOPTIPOP, 100 obese patients.
Better tolerated, but worse preoxygenation — comfort and effectiveness point in opposite directions here.
High-Flow Nasal Cannulae Versus Non-Invasive Ventilation for Preoxygenation of Obese Patients: The PREOPTIPOP Randomized Trial, EClinicalMedicine 2019 · PMID 31528849
IN PRACTICE
What the cohorts and reviews add
2 findings, each on the slide that follows.
critical care & pain medicine 2026
HFNO prolongs safe apnoea time and decreases desaturation incidence compared to
HFNO prolongs safe apnoea time and decreases desaturation incidence compared to conventional oxygen therapy, especially in high-risk patients like…
British journal of anaesthesia 2026
High-flow nasal oxygen does not clear carbon dioxide
High-flow nasal oxygen does not clear carbon dioxide: arterial CO2 accumulates across a wide range of flow rates, so treat it as apnoeic oxygenation…
IN PRACTICE
HFNO prolongs safe apnoea time and decreases desaturation incidence compared to
Review · critical care & pain medicine
HFNO prolongs safe apnoea time and decreases desaturation incidence compared to conventional oxygen therapy, especially in high-risk patients like obese or cardiothoracic surgical patients.
High-Flow Nasal Oxygen in the Perioperative Setting: A Narrative Review, Anaesthesia, critical care & pain medicine 2026 · PMID 41903885
IN PRACTICE
High-flow nasal oxygen does not clear carbon dioxide
Review · British journal of anaesthesia
High-flow nasal oxygen does not clear carbon dioxide: arterial CO2 accumulates across a wide range of flow rates, so treat it as apnoeic oxygenation and monitor CO2 rather than assuming ventilation.
High-Flow Nasal Oxygenation, Not Ventilation: Time to Stop Holding Our Breath, British journal of anaesthesia 2026 · PMID 41402219
THE NUMBERS
High-flow nasal oxygen extended safe apnoea time
Morbidly obese patients, safe apnoea time to SpO2 95%.
About 76 seconds more, a 40% longer margin before desaturation, in exactly the patients who have least to spare.
In a network meta-analysis of 52 studies and 3914 patients, high-flow nasal oxygen with the patient head-up was the technique most likely to prolong safe apnoea time, by a mean of 291 seconds (95% credible interval 138 to 456) against a supine facemask and 203 seconds against a head-up facemask.
THRIVE, Anaesthesia 2015
Difficult airways where the apnoeic window is the whole problem
In 25 patients with difficult airways undergoing hypopharyngeal or laryngotracheal surgery, transnasal humidified rapid-insufflation ventilatory exchange gave a median apnoea time of 14 minutes (range 5 to 65) with no patient desaturating below 90%.
HFNC for apnoeic oxygenation in obese patients meta-analysis, Anesth Analg 2023
Obese patients for elective surgery, with realistic expectations
Across six studies in 351 obese patients, high-flow nasal cannula gave a significantly longer safe apnoea time than facemask preoxygenation (mean difference 124 seconds) and lower odds of discomfort (OR 0.13), but there was no difference in the odds of desaturation below 92% (OR 0.49, 95% CI 0.15 to 1.63) on low-certainty evidence.
CONTRAINDICATIONS
When it is not
THRIVE, Anaesthesia 2015
Treating apnoeic oxygenation as ventilation
During THRIVE, end-tidal carbon dioxide rose at 0.15 kPa per minute and post-apnoea levels reached a mean of 7.8 kPa with a maximum of 15.3 kPa. Oxygenation buys time; it does not clear carbon dioxide, and the clock is running the whole time.
PREOXI trial, noninvasive ventilation for preoxygenation, N Engl J Med 2024
Choosing it over noninvasive ventilation in the hypoxaemic critically ill
In the PREOXI trial of 1301 critically ill adults, preoxygenation with noninvasive ventilation produced hypoxaemia in 9.1% versus 18.5% with an oxygen mask, and cardiac arrest in 0.2% versus 1.1%, with no increase in aspiration. In that population the randomised evidence sits with positive pressure.
HFNC for apnoeic oxygenation in obese patients meta-analysis, Anesth Analg 2023
Reading longer apnoea time as prevented desaturation
In obese elective surgical patients, high-flow nasal cannula did not significantly change the odds of desaturation below 92%, the lowest arterial oxygen content, or the lowest peripheral saturation before intubation, despite extending the safe apnoea window.
In the same network meta-analysis that ranked high-flow nasal oxygen first for safe apnoea time, arterial desaturation was less likely to occur when a facemask with pressure support was used than with the other techniques studied. The strategy that maximises the apnoeic window is not automatically the one that minimises every risk.
The technique that ranked highest for prolonged safe apnoea time was high-flow nasal oxygen with the patient in the head-up position, and it held that rank in the subgroup of studies that used no apnoeic oxygenation at all.
THRIVE, Anaesthesia 2015
It only works through an open airway
In the original THRIVE series, upper airway patency was maintained with jaw thrust throughout the apnoeic period, and the mechanism proposed was flow-dependent deadspace flushing with continuous positive airway pressure — all of which requires a patent passage from nose to trachea.
Start the flow before the patient lies down
Get the cannula on and running while they are still sitting up and talking, and do not let anyone swap it for a facemask at induction. Treating the two as sequential steps throws away the part of the technique that was doing the work.
IN THE ROOM
What this changes about the next case
Colour is the strength of the evidence behind each step, not the urgency.
1
1
In obese patients high-flow nasal cannula lengthened safe apnoea time and was more comfortable, but did not reduce the odds of desaturating below 92% — it buys time rather than removing the risk.
2
2
High-flow nasal oxygen achieved a higher PaO2 and about 131 seconds more safe apnoea time than facemask, without reducing desaturation during intubation.
3
3
Median apnoea time in morbidly obese patients reached 18 minutes with either high-flow nasal or low-flow facemask oxygen, but desaturation below 92% was less likely with high flow (hazard ratio 0.27).
4
4
Compared with non-invasive ventilation, preoxygenating obese patients with high-flow nasal cannula gave a lower end-tidal oxygen and more desaturation below 95% (30% versus 12%), although patients found it far more comfortable.
5
5
HFNO prolongs safe apnoea time and decreases desaturation incidence compared to conventional oxygen therapy, especially in high-risk patients like obese or cardiothoracic surgical patients.
The oral-boards stem on the next slide puts these into one scenario.
KEY TAKEAWAYS
What to carry into the next case
In obese patients high-flow nasal cannula lengthened safe apnoea time and was more
Anesthesia and analgesia 2023
High-flow nasal oxygen achieved a higher PaO2 and about 131 seconds more safe apnoea
Medicine 2022
HFNO prolongs safe apnoea time and decreases desaturation incidence compared to
critical care & pain medicine 2026
Median apnoea time in morbidly obese patients reached 18 minutes with either
British journal of anaesthesia 2023
In obese patients high-flow nasal cannula lengthened safe apnoea time and was more
Questions I'll ask you in the room
Walk me through your setup for this before we start.
What are you watching on the monitor that would tell you this is going wrong?
What is your first move if it does?
What would you want ready in the room before induction?
Oral boards stem
A 47-year-old with a BMI of 45 and a short neck is scheduled for laparoscopic cholecystectomy. You anticipate a short safe apnoea time and the resident asks how best to preoxygenate.
Board questions
1. A colleague states that high-flow nasal oxygen "ventilates" an apnoeic patient. In one or two sentences, say what high-flow nasal oxygen does and does not do to arterial CO2 during apnoea, and what that means for how you monitor the patient.
Show answer
It does not clear carbon dioxide. Arterial CO2 accumulates across a wide range of flow rates, so treat high-flow nasal oxygen as apnoeic oxygenation and monitor CO2 rather than assuming the patient is being ventilated.
Call it apnoeic oxygenation, not ventilation - the oxygen buys you time while the CO2 keeps climbing.
High-Flow Nasal Oxygenation, Not Ventilation: Time to Stop Holding Our Breath, British journal of anaesthesia 2026 · PMID 41402219
2. A systematic review and meta-analysis compared high-flow nasal cannula with standard preoxygenation in obese patients having elective surgery. Name the two outcomes that favoured high-flow nasal cannula, name the outcome that did not improve, and state in one sentence what that pattern means for how you use the technique.
Show answer
Safe apnoea time was longer and patients found it more comfortable, but the odds of desaturating below 92% were not reduced. In this pooled analysis high-flow nasal cannula buys time rather than removing the risk of desaturation.
The extra apnoeic time is real, but in this meta-analysis it did not translate into fewer desaturations below 92% - plan as though the patient can still desaturate.
High-Flow Nasal Cannula for Apneic Oxygenation in Obese Patients for Elective Surgery: A Systematic Review and Meta-Analysis, Anesthesia and analgesia 2023 · PMID 36469483
3. A randomised trial compared high-flow nasal cannula with non-invasive ventilation for preoxygenating obese patients. Describe how the two techniques compared on end-tidal oxygen, on desaturation below 95%, and on patient comfort, and say which technique the oxygenation endpoints favoured.
Show answer
High-flow nasal cannula gave a lower end-tidal oxygen and more desaturation below 95% than non-invasive ventilation (30% versus 12%), although patients found high-flow far more comfortable. The oxygenation endpoints favoured non-invasive ventilation in this trial.
Comfort and oxygenation can point in opposite directions - in this trial the more comfortable technique was the one that desaturated more often.
High-Flow Nasal Cannulae Versus Non-Invasive Ventilation for Preoxygenation of Obese Patients: The PREOPTIPOP Randomized Trial, EClinicalMedicine 2019 · PMID 31528849
4. A meta-analysis reports that high-flow nasal oxygen achieved a higher PaO2 and about 131 seconds more safe apnoea time than facemask preoxygenation. A resident concludes that high-flow nasal oxygen therefore prevents desaturation during intubation. State whether the cited result supports that conclusion, and explain what the data did and did not show.
Show answer
It does not support it. The pooled data showed a higher PaO2 and about 131 seconds more safe apnoea time, but no reduction in desaturation during intubation - the oxygenation and time endpoints improved while the desaturation endpoint did not.
A better surrogate - PaO2, seconds of apnoea - is not the same as fewer desaturations, and here the two came apart.
A Comparison of High-Flow Nasal Cannula and Standard Facemask as Pre-Oxygenation Technique for General Anesthesia: A PRISMA-compliant Systemic Review and Meta-Analysis, Medicine 2022 · PMID 35451383
The bottom lineIn obese patients high-flow nasal cannula lengthened safe apnoea time and was more
Sources
[1] High-Flow Nasal Cannula for Apneic Oxygenation in Obese Patients for Elective Surgery: A Systematic Review and Meta-Analysis, Anesthesia and analgesia 2023 · PMID 36469483 open
[2] A Comparison of High-Flow Nasal Cannula and Standard Facemask as Pre-Oxygenation Technique for General Anesthesia: A PRISMA-compliant Systemic Review and Meta-Analysis, Medicine 2022 · PMID 35451383 open
[3] High-Flow Nasal Oxygen in the Perioperative Setting: A Narrative Review, Anaesthesia, critical care & pain medicine 2026 · PMID 41903885 open
[4] Apnoeic Oxygenation in Morbid Obesity: A Randomised Controlled Trial Comparing Facemask and High-Flow Nasal Oxygen Delivery, British journal of anaesthesia 2023 · PMID 35027169 open
[5] High-Flow Nasal Oxygenation, Not Ventilation: Time to Stop Holding Our Breath, British journal of anaesthesia 2026 · PMID 41402219 open
[6] High-Flow Nasal Cannulae Versus Non-Invasive Ventilation for Preoxygenation of Obese Patients: The PREOPTIPOP Randomized Trial, EClinicalMedicine 2019 · PMID 31528849 open