The fraction of inspired oxygen (FiO₂) represents the proportion of oxygen breather in among the other atomic and chemical components of regular inspiration (e.g., nitrogen, xenon, and other less contributory components). The ideal intraoperative FiO₂ is unclear, with both extremes—too little and too much oxygen representation—presenting the potential for harmful consequences, including pulmonary complications.¹
In a retrospective cohort study published in 2024, high FiO₂ (>0.6 or 60%) was found to be an independent risk factor for increased risk of pulmonary complications within the first seven postoperative days (after propensity score matching, OR: 1.393; 95% confidence interval [CI] 1.077-1.804; P = 0.012).² At normal barometric pressure, high FiO₂ for prolonged periods of time greater than or equal to 24h was associated with increased risk of low pressure oxygen poisoning leading to pulmonary toxicity.
Additional concerns include congestion of the respiratory tree, pulmonary edema, and atelectasis. Given this timeframe, management of these risks is particularly important for patients who are transferred to the intensive care unit after surgery, intubated, and sedated for further monitoring.³ Prolonged exposure to elevated FiO₂ can lead to a syndrome known as oxygen toxicity.
This syndrome refers to a constellation of symptoms resulting from prolonged exposure to increased oxygen concentration. While symptoms and manifestations vary, effects are widespread, involving alterations to the central nervous system (e.g., headache, cold shivering, fatigue, vision changes, nausea, and seizures), lungs (e.g., dyspnea, fever, and pulmonary edema), and the eyes (e.g., retinopathy of prematurity in premature babies, retinal edema, and cataract formation when exposure is chronic).⁴
Low or insufficient FiO₂ can lead to numerous postoperative complications as well. Prolonged hypoxemia or even acute hypoxemia in susceptible patients can lead to respiratory arrest. Other potential complications include myocardial ischemia, dysrhythmias, and brain dysfunction, which may manifest in the immediate or delayed postoperative period.⁵ Even further, prolonged, or acute exposure to low FiO₂ in the susceptible individual can lead to type I respiratory failure, defined by a partial pressure of oxygen <60 mmHg. Type I respiratory failure has numerous potential etiologies, but low FiO₂ is a predominant risk factor.⁶
In conclusion, the ideal perioperative FiO₂ likely varies by case as well as by physician preference and patient risk factors. Excessive FiO₂ as well as insufficient FiO₂ increase patient risk. Insufficient FiO₂ can lead to type I respiratory failure, defined as insufficient (<60 mmHg) circulating oxygen to meet metabolic demands, leading to organ—including pulmonary—dysfunction. Meanwhile, elevated FiO₂ leads to increased risk of atelectasis, pulmonary edema, and swelling of the respiratory tree—a collection of symptoms which comprise oxygen toxicity.
References
1. Fuentes S, Chowdhury YS. Fraction of Inspired Oxygen. In: StatPearls. StatPearls Publishing; 2025. Accessed December 3, 2025. http://www.ncbi.nlm.nih.gov/books/NBK560867/
2. Wang T, Zhao W, Ma L, et al. Higher fraction of inspired oxygen during anesthesia increase the risk of postoperative pulmonary complications in patients undergoing non-cardiothoracic surgery: a retrospective cohort study. Front Physiol. 2024;15:1471454. doi:10.3389/fphys.2024.1471454
3. Mach WJ, Thimmesch AR, Pierce JT, Pierce JD. Consequences of hyperoxia and the toxicity of oxygen in the lung. Nurs Res Pract. 2011;2011:260482. doi:10.1155/2011/260482
4. Cooper JS, Phuyal P, Shah N. Oxygen Toxicity. In: StatPearls. StatPearls Publishing; 2025. Accessed December 3, 2025. http://www.ncbi.nlm.nih.gov/books/NBK430743/
5. Sun Z, Sessler DI, Dalton JE, et al. Postoperative Hypoxemia Is Common and Persistent: A Prospective Blinded Observational Study. Anesth Analg. 2015;121(3):709-715. doi:10.1213/ANE.0000000000000836
6. Mirabile VS, Shebl E, Sankari A, Burns B. Respiratory Failure in Adults. In: StatPearls. StatPearls Publishing; 2025. Accessed December 3, 2025. http://www.ncbi.nlm.nih.gov/books/NBK526127/
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