The study, led by Huabing Wang and colleagues at the People’s Hospital of Zhengzhou University and published in Frontiers in Anesthesiology, enrolled sixty obese men aged 18 to 65 who were scheduled for elective, non-thoracic abdominal surgery. Half were current smokers, defined as people consuming at least ten cigarettes a day for more than two years who had continued smoking until at least fourteen days before their operation. The other half had never smoked. Crucially, the two groups were well matched at baseline: their ages, body mass indices, neck circumferences, airway measurements, sleep apnoea risk scores, haemoglobin levels and other characteristics showed no statistically significant differences, meaning smoking status stood out as the key variable separating them.

Every participant underwent the same carefully standardised preparation. Positioned supine with the head elevated thirty degrees, each man received five minutes of pre-oxygenation through a high-flow nasal cannula delivering pure oxygen at sixty litres per minute, a rate previous research has identified as optimal. Arterial blood gases were sampled before and after this pre-oxygenation phase. Anaesthesia was then induced intravenously with midazolam, sufentanil, propofol and rocuronium, and ninety seconds after the muscle relaxant took effect, the oxygen supply was deliberately switched off. From that moment, researchers measured how long each patient could tolerate apnoea, the absence of breathing, before his peripheral oxygen saturation fell from full to 92 percent, the threshold the team defined as the limit of safe apnoeic oxygenation.

The result was striking. Never smokers maintained safe oxygen levels for a median of 373.5 seconds, more than six minutes, while smokers desaturated at a median of just 262.5 seconds, under four and a half minutes. The interquartile ranges of 106.25 and 138.5 seconds respectively underscored how much variability existed within each group, yet the difference between them reached statistical significance with a P value of 0.012. In practical terms, smoking erased roughly a third of the buffer that protects an unconscious, non-breathing patient from hypoxaemia, the dangerously low blood oxygen that can injure the brain and heart within minutes.

The physiology behind this gap is a story of shrinking reserves. Obesity itself is already hostile to oxygen storage: fat deposited in the chest wall and abdomen compresses the lungs, reducing functional residual capacity, the volume of oxygen-rich air left in the lungs at the end of a normal breath. At the same time, heavier bodies consume more oxygen per minute. When anaesthesia stops breathing, the alveolar oxygen stores in an obese patient are depleted rapidly, and arterial oxygen partial pressure plunges. High-flow nasal oxygen was developed precisely to counteract this vulnerability. By delivering a constant, high-concentration flow of gas, it flushes carbon dioxide out of the nasopharyngeal dead space, reduces rebreathing, generates a modest positive pressure that helps keep alveoli open, and allows oxygen to passively diffuse into the lungs even while the patient is apnoeic, a phenomenon known as apnoeic oxygenation.

Smoking undermines these mechanisms at every level. Chronic exposure to cigarette smoke damages the architecture of the alveolar walls, shrinking the surface area available for gas exchange and impairing diffusion of oxygen into the blood. It also increases airway resistance and the risk of perioperative bronchospasm, and it diminishes what the authors describe as effective pulmonary reserve. In the trial, these structural disadvantages meant that even after identical five-minute pre-oxygenation, the smokers entered their apnoeic window with a shallower cushion and burned through it faster. Interestingly, arterial blood gas measurements taken immediately after pre-oxygenation showed no significant differences between the groups, with arterial oxygen partial pressures rising to comparable high levels in both. This suggests the deficit in smokers is less about loading oxygen into the lungs and more about how efficiently that loaded oxygen can be exchanged across damaged lung tissue once breathing ceases.

The study also surfaces a subtle monitoring problem that has implications far beyond this experiment. Standard pulse oximeters estimate oxygen saturation by shining two wavelengths of light through the skin and distinguishing oxygenated haemoglobin from deoxygenated haemoglobin. But carboxyhaemoglobin, the carbon monoxide-bound form that is chronically elevated in smokers, absorbs light in a way that these two-wavelength devices cannot separate from oxyhaemoglobin. When carboxyhaemoglobin exceeds about five percent, a level commonly reached in active smokers, the displayed saturation overestimates the true fraction of oxygen-carrying haemoglobin by roughly the amount of the excess. In other words, the smokers in this study may have been even closer to genuine hypoxaemia than their monitors indicated, and their true safe apnoea times may have been shorter than the 262.5 seconds recorded. The authors flag this caveat explicitly, urging clinicians to interpret pulse oximetry readings in smokers with considerable caution.

Not every finding favoured the non-smokers. During twenty-four hours of postoperative follow-up, six of the thirty never smokers, twenty percent, experienced nausea or vomiting, compared with just one smoker, 3.3 percent, a difference that was statistically significant at P equals 0.044. The researchers suggest several possible explanations: long-term nicotine exposure may induce adaptive changes in the central nervous system that raise the vomiting reflex threshold and alter neurotransmitter signalling, including serotonin pathways involved in postoperative nausea and vomiting. Nicotine also affects gastrointestinal motility and gastric emptying, which could reduce reflux and nausea. The authors are careful to note that the specific targets of nicotine were not explored in this trial and that the underlying mechanisms warrant future investigation. No patients in either group experienced dizziness or nasal bleeding, and haemodynamics and blood gas values remained similar between groups throughout the perioperative period.

The trial has limitations worth weighing. It was conducted at a single university-affiliated hospital between September 2024 and May 2025, included a modest sample of sixty men, and relied on self-reported smoking status, although the strict enrollment criteria of at least ten cigarettes daily for over two years created a clearly defined contrast. Restricting the study to men limits generalisability to women, though the authors note this choice minimised sex-related confounding. Exclusion criteria also removed patients with asthma, chronic obstructive pulmonary disease and other lung diseases, meaning the results apply to obese smokers without diagnosed respiratory illness. Still, the study was adequately powered by pre-experimental calculation, registered at ClinicalTrials.gov under NCT06562660, and its core result fits squarely within established respiratory physiology.

The clinical message is unambiguous. As obesity rates climb and many patients arrive in the operating room as active smokers, anaesthesiologists can no longer assume that high-flow nasal pre-oxygenation buys everyone the same margin of safety. For obese smokers, the authors recommend heightened vigilance, proactive and possibly extended pre-oxygenation, more rigorous oxygenation assessment that looks beyond pulse oximetry, and faster intervention when saturation begins to fall. Above all, the findings add a compelling, quantifiable argument for smoking cessation before surgery: quitting does not merely lower long-term risks of cancer and heart disease, it directly extends the minutes of grace that keep a patient safe between the last breath and the first breath of the machine.

Subject of Research: The effect of smoking on safe apnoea duration during high-flow nasal oxygen pre-oxygenation in obese men undergoing anaesthesia

Article Title: Perioperative challenges: does smoking compromise high-flow nasal oxygen efficacy in obese Men? A randomised controlled trial

Article References: Wang, H., Zhang, C., Li, L., Tian, Y., & Chen, C. (2026). Perioperative challenges: does smoking compromise high-flow nasal oxygen efficacy in obese Men? A randomised controlled trial. Frontiers in Anesthesiology, 5, Article 1915310. https://doi.org/10.3389/fanes.2026.1915310

Image Credits: AI Generated

DOI: 10.3389/fanes.2026.1915310

Keywords: anaesthesia, high-flow nasal oxygen, apnoeic oxygenation, obesity, smoking, hypoxaemia, pre-oxygenation, pulse oximetry, carboxyhaemoglobin, postoperative nausea and vomiting, randomised controlled trial, perioperative medicine