Preoxygenation: Technique and Physiological Basis

Physiological Basis: Functional Residual Capacity and Denitrogenation

At the moment of anaesthetic induction, the oxygen reserve available to sustain aerobic metabolism during apnoea resides predominantly within the functional residual capacity (FRC) – the volume of gas remaining in the lungs at the end of a normal passive expiration. In healthy adults in the sitting position, FRC is approximately 30–35 ml/kg, and this lung volume represents the principal oxygen reservoir available during apnoea. In the breathing of room air, the FRC is composed predominantly of nitrogen (approximately 79%), which is physiologically inert and contributes nothing to oxygen delivery. The process of preoxygenation involves progressive replacement of this nitrogen with oxygen – a process termed denitrogenation – so that the entire gas volume of the FRC becomes available as an oxygen reserve.

​

Without preoxygenation, the time from onset of apnoea to critical oxygen desaturation in a healthy adult breathing room air is very short, typically less than two minutes. Effective preoxygenation substantially extends this safe apnoea time. The adequacy of preoxygenation is most reliably assessed by measuring the fraction of expired oxygen (FEO’2): a value of 0.85 or greater indicates adequate denitrogenation of the FRC. An equivalent target is an end-tidal oxygen (EtO2) concentration of approximately 0.90. When end-tidal gas monitoring is unavailable, the minimum recommended duration of preoxygenation is 3 minutes of tidal volume breathing of 100% oxygen, or alternatively eight vital capacity breaths, which achieves comparable denitrogenation more rapidly.

​

Factors That Reduce Preoxygenation Efficacy

Several physiological and pathological factors reduce FRC and thereby limit the oxygen reserve achievable through preoxygenation. Body position is one of the most important modifiable determinants of FRC: the supine position reduces FRC compared with sitting, whilst the head-up position increases it. Age, pregnancy, obesity, and underlying lung disease all independently reduce FRC. In patients with acute respiratory distress syndrome (ARDS), ventilated lung may represent only 40% of total lung capacity, profoundly limiting the available oxygen reserve. Conditions that increase oxygen consumption – including fever, sepsis, agitation, and pain – accelerate the rate of desaturation during apnoea and reduce safe apnoea time independently of oxygen reserves. These factors explain why critically ill patients, those with obesity, and pregnant women are at particular risk of rapid desaturation during airway management and why preoxygenation technique must be tailored to individual risk.

​

Standard Facemask Technique

The traditional technique of preoxygenation employs a tight-fitting facemask delivering oxygen at a fraction of inspired oxygen (FiO2) of 1.0 via an anaesthetic machine circuit. Using a semi-closed circuit (such as a Mapleson C circuit) requires a high fresh gas flow of 15 L/min to prevent rebreathing of expired carbon dioxide. When a closed anaesthetic machine circuit is used, FiO2 of 1.0 can be delivered directly. Self-inflating bag-valve-mask resuscitators are not suitable for preoxygenation because their integrated valves generate negative pressure during spontaneous breathing, impeding effective gas delivery and preventing reliable high FiO2 administration. Tight-fitting facemask preoxygenation for 3 minutes of tidal volume breathing, or eight vital capacity breaths, achieves adequate denitrogenation in most patients breathing at normal respiratory rates. Preoxygenation with positive airway pressure (such as CPAP at 5–10 cmH2O via facemask) is recommended in critically ill patients with moderate-to-severe hypoxaemia, as it recruits collapsed alveoli and further augments the functional oxygen reserve before induction.

​

High-Flow Nasal Oxygen

High-flow nasal oxygen (HFNO) has become an increasingly important technique for preoxygenation and apnoeic oxygenation. HFNO delivers warmed, humidified oxygen at flow rates typically between 30 and 95 L/min via soft nasal cannulae. Its preoxygenation efficacy derives from two mechanisms. First, at high flows it delivers an FiO2 approaching 1.0, particularly when the patient breathes with the mouth closed. Second, HFNO generates a flow-dependent positive end-expiratory pressure (PEEP) effect of approximately 0.5–1 cmH2O per 10 L/min of flow with closed-mouth breathing, which increases end-expiratory lung volume and thereby augments FRC and the total oxygen reserve accumulated during preoxygenation. A key additional advantage of HFNO is that it can be continued seamlessly through the apnoeic period following induction, maintaining passive oxygenation by mass flow of oxygen down the trachea even in the absence of respiratory effort.

​

A prospective randomised controlled trial comparing HFNO at flow rates of 45, 70, and 95 L/min during preoxygenation found no significant difference in safe apnoea time between rates, with median apnoea times of 472, 523, and 483 seconds respectively (P=0.59). Patient discomfort was lowest at 45 L/min, suggesting this flow rate provides adequate preoxygenation efficacy with greater tolerability. In lower-FRC states – such as obesity or the flat-supine position – the flow-related PEEP effect of HFNO may confer proportionally greater FRC augmentation compared with higher-FRC states. In higher-risk patients, including those with predicted difficult airways, anticipated rapid desaturation, or where positive pressure ventilation is to be avoided, HFNO at 60 L/min or more, continued through the apnoeic period, is a recommended strategy.

​

Position and Preoxygenation

Patient position substantially modulates preoxygenation efficacy through its direct effect on FRC. The head-up position (20–30°) increases end-expiratory lung volume and FRC compared with the supine position, increasing the oxygen reserve accumulated during preoxygenation and reducing the risk of atelectasis. The combination of HFNO and the head-up position is the highest-ranked preoxygenation strategy in terms of safe apnoea time, based on a systematic review and network meta-analysis of 52 randomised controlled trials enrolling 3,914 patients. HFNO in the head-up position prolonged safe apnoea time by a mean of 291 seconds compared with facemask preoxygenation in the supine position, and by 203 seconds compared with facemask preoxygenation in the head-up position. In obstetric patients, where pregnancy reduces FRC, increases oxygen consumption, and elevates aspiration risk, a 45° head-up position combined with HFNO maintained oxygen saturation above 92% at 18 minutes in 88% of patients, compared with 62% with conventional facemask oxygenation. Hypoxaemia complicates up to 17% of rapid sequence inductions for Caesarean delivery, making optimisation of preoxygenation especially important in this group.

​

A pragmatic, risk-stratified approach to preoxygenation recommends head-up positioning at 20–30° with positive airway pressure for all patients, targeting EtO2 of 0.90 or 3–5 minutes of preoxygenation. For low-risk patients, facemask with PEEP with or without pressure support, or HFNO at 30 L/min or above continued through apnoea, is appropriate. For higher-risk patients, HFNO at 60 L/min or more, continued through the apnoeic period, is the preferred strategy.

​

Conclusion

Preoxygenation is an essential preparatory step before every anaesthetic induction and tracheal intubation. Its physiological basis rests on the denitrogenation of the FRC – the principal intrapulmonary oxygen reservoir – thereby maximising safe apnoea time. Adequacy is confirmed by achieving an FEO’2 of 0.85 or EtO2 of 0.90, or after at least 3 minutes of tidal breathing with 100% oxygen when monitoring is unavailable. HFNO in the head-up position represents the best evidenced strategy for maximising safe apnoea time across patient populations, and continuation of HFNO through the apnoeic period provides ongoing oxygenation support. Recognition of individual factors that reduce FRC and accelerate desaturation – including obesity, pregnancy, critical illness, and high metabolic demand – is fundamental to tailoring preoxygenation strategy to each patient.