Preoxygenation and apneic oxygenation for airway management for anesthesia

INTRODUCTION — Preoxygenation refers to measures that increase oxygen reserves prior to the onset of apnea, typically during an attempt at airway management. Apneic oxygenation refers to delivery of oxygen in the absence of significant tidal volume, which can occur during an attempt at airway management or during procedures on the airway. Whereas preoxygenation and apneic oxygenation are often thought of as distinct entities, high flow nasal oxygen (HFNO) may be used for both.

This topic will discuss the rationale for preoxygenation, techniques for preoxygenation and apneic oxygenation, and patient monitoring during these techniques. Other aspects of airway management for anesthesia and for emergency medicine and intensive care are discussed separately in numerous other UpToDate topics. (See "Airway management for induction of general anesthesia" and "Rapid sequence intubation in adults for emergency medicine and critical care".)

Use of HFNO for procedures that require airway management without an airway device in place (eg, laryngeal surgery) is discussed separately. (See "Anesthesia for laryngeal surgery", section on 'High flow nasal oxygen' and "Anesthesia for head and neck surgery", section on 'High flow nasal oxygen'.)

PREOXYGENATION — All patients should be preoxygenated prior to induction of general anesthesia. Preoxygenation involves replacing nitrogen in the alveolus with oxygen [1]. Thus, the term "denitrogenation" is often used interchangeably with "preoxygenation". In this topic we will use "preoxygenation," which is more prevalent in the literature.

PHYSIOLOGIC BASIS FOR PREOXYGENATION AND APNEIC OXYGENATION — The goal of preoxygenation is to maximize the amount of oxygen in the lung, primarily in the functional residual capacity (FRC), and therefore to delay the onset of hypoxia during apnea. This involves "washing out" the nitrogen that is the primary gas in alveoli when breathing room air. It is crucial to deliver 100 percent oxygen during preoxygenation to attain an end tidal oxygen (EtO₂) concentration of approaching 90 percent. The alveolus achieves nearly complete denitrogenation at 90 percent EtO₂ assuming there is five percent CO₂ and five percent water vapor in the alveolus [2]. During preoxygenation, meaningful prolongation of the time to desaturation does not occur until the EtO₂ reaches 90 percent. This is shown in a figure (figure 1). In practice it can be difficult to achieve 90 percent exhaled oxygen fraction due to dead space (resulting in higher CO₂ concentration) or mask leaks (due to residual air). The measured exhaled gas fractions will only be accurate in the context of adequate ventilation through the circuit [3].

When a patient is breathing room air, the amount of oxygen in the blood exceeds the amount in the lungs. As the patient breathes 100 percent oxygen during preoxygenation, the amount of oxygen in the FRC increases rapidly and far exceeds the amount in the blood (table 1 and figure 2). The FRC is critical to the utility of preoxygenation; oxygen in the FRC is the reservoir of oxygen used to maintain saturation during apnea when no new oxygen is supplied.

Factors that affect the speed of preoxygenation, highest EtO₂ achieved (efficacy), and time to desaturation during apnea include the following (table 2):

●Maximum FiO₂ – It is crucial to deliver 100 percent oxygen to the lung during preoxygenation. Effective preoxygenation depends on fresh gas flow ≥10 L/minute, significantly exceeding minute ventilation and peak inspiratory flow to eliminate rebreathing. When fresh oxygen gas flow falls below the peak inspiratory flow, inspired oxygen may be diluted with entrained room air. Inadequate oxygen flow and use of fraction of inspired oxygen (FiO₂) <1.0 have been shown to reduce the speed and efficacy of preoxygenation [4,5]. In a computational modeling study, increasing the FiO₂ during apneic oxygenation from 0.9 to 1.0 more than doubled the time to desaturation compared with increasing the FiO₂ from 0.21 to 0.9 (figure 1) [2].

The type of circuit and the fresh gas flow used affect efficacy of preoxygenation. As examples, in volunteer studies of simulated preoxygenation with 100 percent oxygen at 5 L/minute, Mapleson A circuits and circle systems were more likely to achieve EtO₂ approaching 90 percent, whereas with oxygen at 10 L/minute, the three types of circuits performed similarly (figure 3) [6,7]. Self-inflating bags are not as effective for preoxygenation as either Mapleson circuits or circle systems [8-10].

●Airway patency – During apneic oxygenation, it is essential to maintain an open airway to allow constant delivery of oxygen into the trachea. After the onset of apnea, oxygen is consumed from the alveoli at approximately 250 mL per minute and carbon dioxide is excreted into the alveoli at approximately 20 mL per minute [11]. The greater alveolar oxygen absorption creates a negative pressure gradient relative to the airways; oxygen delivered to the trachea diffuses down the pressure gradient into the alveoli. This process depends on having an open airway and constant delivery of oxygen into the trachea.

●Effects of FRC – The rate of nitrogen washout is governed by a time constant (T), which is proportional to the FRC and inversely proportional to alveolar ventilation (V_A). Efficacy can be limited by insufficient time devoted to preoxygenation. A lower time constant speeds denitrogenation.

T ∝ FRC/V_A

From this formula, the larger the FRC, the longer the time constant, and thus the longer time required for full preoxygenation. Increasing alveolar ventilation reduces the time needed for preoxygenation. Clinical implications are as follows:

•Increasing FRC slows preoxygenation (time constant greater) but for any duration of preoxygenation, increased FRC yields a longer time until desaturation, which is the primary goal for preoxygenation [4].

•In patients with smaller than average FRC (eg, obesity, late term pregnancy, ascites, children, older age) and the supine position, denitrogenation will be faster (time constant smaller), though the reservoir of oxygen available during apnea will be less and desaturation will occur more quickly (figure 4).

Positioning the patient in the head up position during preoxygenation may increase FRC, and may therefore prolong the time to desaturation [12]. In a meta-analysis of six randomized trials (227 patients) that compared a head up versus supine positioning for preoxygenation for intubation for anesthesia, head up positioning increased the safe apnea time (mean difference 62 seconds, 95% CI 43-81 seconds) [13]. Safe apnea time was variably defined in the included studies, as the time from onset of apnea to desaturation to 90, 92, 93, or 95 percent.

●Breathing pattern during preoxygenation – Most studies have found that preoxygenation with eight vital capacity breaths over the course of one minute is as fast and effective and delays desaturation as well as three minutes of tidal volume breathing (TVB), and that both techniques are superior to preoxygenation with four deep breaths over 30 seconds [14-19]. (See 'Tight fitting face mask with passive delivery' below.)

●Oxygen consumption – Patients with higher oxygen consumption (eg, pregnancy, children, sepsis) will desaturate more quickly after preoxygenation.

●Shunt physiology – Shunt physiology limits the possibility of increasing blood oxygenation even with effective preoxygenation. Patients with reduced FRC below the closing volume of the lung may exhibit shunt physiology. To some extent, this can be overcome with efforts to increase FRC (eg, head up positioning, positive pressure ventilation).

Critically ill patients may have shunt physiology that may not be fully reversed during preoxygenation. Common examples are pneumonia, pulmonary edema, and acute respiratory distress syndrome (ARDS). (See "Acute respiratory distress syndrome: Epidemiology, pathophysiology, pathology, and etiology in adults".)

CHOOSING A PREOXYGENATION TECHNIQUE — Preoxygenation can be performed with a facemask, with positive airway pressure techniques (eg, noninvasive ventilation [NIV], continuous positive airway pressure [CPAP], positive end expiratory pressure [PEEP], bilevel positive airway pressure [BIPAP], pressure support ventilation by face mask), with high flow nasal oxygen (HFNO), or combinations thereof. The technique chosen should be individualized, based on patient and clinical factors, and availability of resources and equipment.

Patients without risk factors for hypoxia during intubation — For patients without risk factors for rapid desaturation or prolonged attempts at intubation, we suggest using routine facemask preoxygenation without positive pressure techniques. Face mask preoxygenation is technically the most straightforward option, is reliably effective in healthy patients, and has been well-studied. (See 'Tight fitting face mask with passive delivery' below.)

Patients with reduced FRC — For patients with obesity, pregnancy, an abdominal mass or ascites, we suggest using noninvasive positive pressure techniques for preoxygenation with head up positioning. Noninvasive ventilation may open atelectatic airways, and both techniques may increase functional residual capacity (FRC) [20]. (See 'Tight fitting face mask with active delivery' below.)

Patients with predicted difficulty with airway management — For patients with predicted difficulty with airway management, we suggest using apneic oxygenation in addition to preoxygenation, as additional time may be required to manage and control the airway. We use HFNO where available, and other options (eg, low flow nasal cannulae, oropharyngeal catheters) if HFNO cannot be used. (See 'High flow nasal oxygen' below and 'Other options for apneic oxygenation' below.)

Critically ill hypoxemic patients — For critically ill hypoxemic patients, we suggest using positive pressure techniques (ie, NIV, CPAP, BiPAP, pressure support facemask ventilation with a ventilator or anesthesia machine) provided that the patient is cooperative and a sufficient seal can be obtained. Many of these patients are already receiving NIV support. Preoxygenation with noninvasive ventilation may maintain higher oxygen saturation during intubation in hypoxemic patients, compared with standard face mask preoxygenation, and was endorsed by the 2018 Difficult Airway Society Guidelines for intubation of the critically ill patient [21]. Such patients may desaturate more quickly during apnea, poorly tolerate the physiologic effects of hypoxia, and require advanced techniques for recruitment of alveoli after airway management to restore oxygenation. (See 'Efficacy and prolongation of time to desaturation' below and 'Tight fitting face mask with active delivery' below.)

For patients who are not cooperative, optimal preoxygenation may not be possible, and this should be documented. One alternative is to administer a low dose induction agent to sedate the patient adequately to allow preoxygenation. The patient's risk of aspiration must be considered when contemplating this approach.

Use of HFNO alone is of limited utility for preoxygenation in the critically ill population. Combining HFNO (50 L/min or higher) with delivery of oxygen via a mouthpiece may be useful for preoxygenation in a patient who is already receiving HFNO for respiratory compromise [22]. Use of HFNO for preoxygenation in critically ill patients is discussed separately. (See "Heated and humidified high-flow nasal oxygen in adults: Practical considerations and potential applications", section on 'Intubation support'.)

DECIDING TO USE APNEIC OXYGENATION FOR INTUBATION — Positive pressure ventilation by mask is routinely used for oxygenation between induction of anesthesia and laryngoscopy, after which apnea occurs. We suggest the additional use of apneic oxygenation during intubation for patients at high risk of hypoxia during intubation. This may include patients with the following:

●Obesity (See "Anesthesia for the patient with obesity", section on 'Preoxygenation and apneic oxygenation'.)

●Pregnancy (See "Airway management for the pregnant patient", section on 'Preoxygenation and apneic oxygenation'.)

●Intrinsic lung disease

●Predicted difficulty with airway management (See "Management of the difficult airway for general anesthesia in adults", section on 'Patient preparation'.)

●Diminished physiologic reserve (ie, potentially intolerant of even transient desaturation)

●Patients who undergo rapid sequence induction and intubation (RSII), in whom face mask ventilation may be particularly problematic (See "Rapid sequence induction and intubation (RSII) for anesthesia", section on 'Preoxygenation'.)

The degree (ie, incremental benefit) to which apneic oxygenation delays desaturation varies and is not easily predicted. Some studies have found that apneic oxygenation was noninferior to routine approaches, without demonstrating strong clinically relevant benefit [23-29]. Nonetheless, while apneic oxygenation may not delay desaturation during intubation in all patients, it is a low risk, relatively straightforward technique and serves as an additional safeguard in patients at high risk of hypoxia during intubation.

Apneic oxygenation will not be equally efficacious in all patients. Obesity, intrinsic lung diseases such as pulmonary fibrosis or chronic obstructive pulmonary disease (COPD), or pregnancy may limit efficacy and shorten time to desaturation [30], however it can be difficult to predict in advance which patients are at risk for rapid desaturation during apneic oxygenation. Patients who require supplemental oxygen at baseline and have impaired gas exchange at the alveolar level or with shunt physiology may be the most susceptible to failure of apneic oxygenation.

Use of apneic oxygenation in the intensive care unit and emergency department is discussed separately. (See "Rapid sequence intubation in adults for emergency medicine and critical care", section on 'Adjunct strategies to maximize preoxygenation'.)

TECHNIQUES FOR DELIVERING OXYGEN — Oxygen can be administered with various types of face masks and nasal cannulae (high flow and standard low flow), and less commonly, with other devices.

Tight fitting face mask with passive delivery

●Breathing techniques – The most common method of preoxygenation is tidal breathing with a tight-fitting mask with a fraction of inspired oxygen (FiO₂) of 1.0 over a period of at least three minutes and up to five minutes. Eight rapid vital capacity breaths over one minute may be as effective and is useful when time is of the essence (eg, emergency intubation), though it requires a cooperative patient [7,16]. Preoxygenation should optimally be continued until the end tidal oxygen (EtO₂) is >90 percent when this measurement is available.

●Maximizing FiO₂ – Common reasons for failing to achieve an FiO₂ close to 1.0 are leaks around the face mask [31], rebreathing exhaled gases, and using breathing equipment incapable of delivering FiO₂ of 1.0 [8,9,32].

•Obtaining mask seal – Entrainment of air around the face mask may be reduced by applying the mask tightly. An adequate seal is indicated by appropriate movement of the reservoir bag, a normal capnogram and end tidal CO₂, and the expected FiO₂.

A seal may be challenging in patients who are edentulous, who have facial hair, or who have a nasogastric tube in place. Patients in whom a seal cannot be achieved or who don't tolerate a tight mask seal can breathe through a mouthpiece connected to the breathing circuit, with their nose closed.

•Reducing rebreathing – Oxygen flow should be ≥10 L/minute to exceed peak inspiratory flow and avoid rebreathing of exhaled gases.

•Choosing equipment – During anesthesia, the anesthesia machine breathing circuit is typically used and can provide high oxygen flow, positive pressure, and monitoring for EtO_2. Outside of anesthesia, the masks commonly used for preoxygenation are nonrebreathing masks and the self-inflating bag-valve-mask (BVM) devices. Neither reliably provide an FiO₂ of close to 1.0 [8,9]. BVM devices vary with respect to efficacy for preoxygenation. Those with one way exhalation valves may be more effective than those without [32,33].

When using self-inflating bags, a potential problem is undetected disconnect from the oxygen source, since the bag will continue to self-inflate with air.

Mapleson circuits may provide better performance than self-inflating bags [10] and provide visual confirmation of ventilation (ie, bag deflating and reflating with respiration).

Tight fitting face mask with active delivery

Techniques — Options for preoxygenation with active delivery (positive pressure) include noninvasive ventilation (NIV; with continuous positive airway pressure [CPAP], bilevel positive airway pressure [BPAP], pressure support facemask ventilation with a ventilator or anesthesia machine). All of these methods require a ventilator, CPAP machine, or an anesthesia machine. NIV typically provides positive end expiratory pressure (PEEP) to increase the functional residual capacity (FRC). In the operating room, PEEP or CPAP can also be provided with the anesthesia machine adjustable pressure-limiting (APL) valve with a tight face mask seal.

For patients receiving BPAP and CPAP in the intensive care unit, it is reasonable to continue the NIV mode and turn the FiO₂ to 1.0 for preoxygenation prior to intubation. This strategy uses equipment already in place and avoids disrupting the therapeutic effect of positive pressure ventilation that would occur when transitioning to a face mask.

Efficacy and prolongation of time to desaturation — NIV may increase functional residual capacity, reduce atelectasis and intrapulmonary shunting, improve the efficacy of preoxygenation, and prolong time to desaturation. NIV may be most beneficial for patients with reduced FRC (eg, with obesity, pregnancy, abdominal mass or ascites), and for critically ill patients.

●In a meta-analysis of small randomized trials of patients who underwent elective surgery, preoxygenation with NIV prolonged time to desaturation (7 studies; mean difference 96.3 seconds, 95% CI 29.4–163.1 seconds) and increased the likelihood of achieving fraction of expired oxygen (FeO₂)>90 percent at 3 minutes (2 studies; odds ratio 3.0, 95% CI 1.5–6.0), compared with routine facemask preoxygenation [20].

●In one trial, 50 patients with body mass index (BMI) ≥40 kg/m² who underwent bariatric surgery were randomly assigned to preoxygenation for three minutes either in the ramped position with conventional face mask ventilation or in reverse Trendelenberg position with pressure support ventilation by mask [34]. In patients who were preoxygenated with NIV, time to desaturation to 92 percent was longer (258 versus 217 seconds) and the percentage of patients who achieved FEO₂ of 0.9 at 3 minutes was greater (88 versus 54 percent), compared with conventional face mask ventilation.

●Preoxygenation with NIV may be beneficial in critically ill patients. In a randomized trial of 57 hypoxemic patients who required intubation for respiratory failure in the intensive care unit (ICU), preoxygenation with NIV achieved higher oxygen saturation at the end of preoxygenation, during intubation, and after intubation, compared with preoxygenation with a nonrebreather BVM technique [35]. In another randomized trial of critically ill patients with respiratory failure, the risk of hypoxemia during intubation was similar after preoxygenation with NIV versus high flow nasal oxygen for preoxygenation and apneic oxygenation, and occurred in approximately one quarter of patients [36]. Patients with baseline severe hypoxemia appeared to benefit more from NIV.

High flow nasal oxygen — High flow nasal oxygen (HFNO) is a system used to deliver heated and humidified oxygen at high flow (up to 70 L/minute), which matches or exceeds peak inspiratory flow. (See "Heated and humidified high-flow nasal oxygen in adults: Practical considerations and potential applications".)

During airway management for anesthesia, HFNO is usually used primarily for apneic oxygenation during intubation, in which case it is typically used for preoxygenation as well. If HFNO is primarily being used for apneic oxygenation, another option is to place the HFNO catheter after preoxygenating by facemask. This allows confirmation of the ability to ventilate with positive pressure by mask.

Technique — HFNO is frequently used for preoxygenation (sometimes in conjunction with supplemental oxygen by facemask) and then transitioned to apneic oxygenation. Awake patients may not tolerate very high flows, even when the gas is humidified. Thus, for preoxygenation the initial oxygen flow rate is usually set to 30 L/minute. The patient is asked to take tidal volume breaths via nasal breathing, with the mouth closed [37-39] for three minutes. After induction of anesthesia, the flow rate is increased to 70 L/minute and maintained until the endotracheal tube is in place. Airway patency must be maintained via jaw thrust and an oro- or nasopharyngeal airway if necessary, while HFNO is used.

Optimal duration of effective oxygenation depends on maximal preoxygenation and delivery of 100 percent oxygen. In a computational modeling study, increasing the FiO₂ during apneic oxygenation from 0.9 to 1.0 more than doubled the time to desaturation compared with increasing the FiO₂ from 0.21 to 0.9 (figure 1) [2].

Advantages and limitations

Potential advantages to using HFNO for preoxygenation include the following:

●HFNO may be better tolerated by the patient than a tight-fitting mask. One study found slightly increased patient comfort and ease of use of HFNO compared with traditional methods for preoxygenation [40].

●HFNO does not depend on achieving a good mask seal and frees up the anesthesia clinician's hands for other tasks, potentially permitting a longer duration of preoxygenation.

●Some features of HFNO promote rapid preoxygenation and increased oxygen stores. It reduces physiologic dead space by creating turbulence in the upper airway [41], rapidly clearing this space of nitrogen. HFNO also increases airway pressure and lung volume during spontaneous ventilation [38,39,42], but possibly not during apnea [43,44].

●Preoxygenation with HFNO offers a seamless transition to apneic oxygenation.

However, there are limitations to using HFNO for preoxygenation:

●Even with the patient's mouth closed, air can be entrained and can reduce the effective FiO₂. It may be possible to counteract this effect by administering oxygen via a mouthpiece during HFNO. In one study, 79 adult surgical patients were randomly assigned to preoxygenation with conventional face mask, HFNO, or HFNO along with high flow oxygen delivered via a mouthpiece [22]. Time to desaturation after induction of anesthesia was prolonged in patients preoxygenated with both HFNO and oxygen by mouthpiece, and was similar in the other two groups.

●The FEO₂ cannot be measured when using HFNO without removing the device; FEO₂ >0.9 is used as a goal for preoxygenation with other methods. In most cases, preoxygenation with HFNO continues for three minutes, or longer if required based on oxygen saturation.

●While possible, simultaneous application of HFNO and face mask ventilation can result in leaks during mask ventilation, so the operator may be unable to confirm the ability to successfully ventilate by mask prior to attempting airway management.

Efficacy and prolongation of time to desaturation

•Preoxygenation – Whether the use of HFNO improves the efficacy of preoxygenation or prolongs the time to desaturation compared with standard face mask preoxygenation is unclear, and studies have found conflicting results. A meta-analysis of randomized trials that compared face mask preoxygenation with HFNO found higher partial pressure of arterial oxygen (PaO₂) at the end of preoxygenation with HFNO [45]. By contrast, other trials have found no difference in oxygen saturation in patients with predicted airway difficulty [46] or worse EtO₂ at the end of preoxygenation in pregnant patients [47,48] with the use of HFNO.

There is not compelling evidence to support the use of HFNO rather than spontaneous ventilation with a facemask via an anesthesia machine breathing circuit for preoxygenation prior to routine intubation or rapid sequence induction and intubation. Most studies of the peri-intubation use of HFNO have assessed outcomes with the use of HFNO for both preoxygenation and apneic oxygenation.

•Apneic oxygenation – Apneic oxygenation after preoxygenation can significantly prolong desaturation; in some studies peripheral oxygen saturation (SpO₂) has been maintained at over 90 percent for 100 minutes [49,50].

However, the literature on the benefits of apneic oxygenation with HFNO is conflicting and inconclusive. This may reflect the difficulty of designing a study that applies consistent methodology to a large enough population to yield a sufficient number of "can't intubate, can't ventilate" patients. Some studies have found prolonged time to desaturation or reduced incidence of desaturation with the use of HFNO [51-56], whereas others have found no benefit during rapid sequence induction [57] or compared with facemask ventilation during conventional induction [58].

In one randomized trial including 100 surgical patients with obesity (PREOPTIPOP), preoxygenation and apneic oxygenation with HFNO resulted in increased incidence of desaturation to <95 percent and lower nadir oxygen saturation, compared with preoxygenation with NIV without subsequent apneic oxygenation [59]. This finding is supported by modelling using the Interdisciplinary Collaboration in Systems Medicine simulation suite (a model based on the Nottingham Physiologic Simulator), which predicted lower EtO₂ with HFNO versus noninvasive positive pressure ventilation (NIPPV) [60].

HFNO should not be considered a method for ventilation but purely as a strategy to maintain oxygenation in an apneic patient. When used for prolonged apneic oxygenation, CO₂ should be monitored, and there should be a plan to actively ventilate when clinically indicated. (See 'Monitoring during preoxygenation and apneic oxygenation' below.)

When HFNO was initially used for apneic oxygenation during anesthesia, the term transnasal humidified rapid insufflation ventilatory exchange (THRIVE) was used. However, HFNO has not consistently been shown to slow the rate of rise of CO₂ compared with low flow apneic oxygenation techniques. Some studies have reported reduced rate of rise of CO₂ during apneic oxygenation with HFNO [61-66], while others have found no difference, compared with no apneic oxygenation [67-70].

Other options for apneic oxygenation

Standard nasal cannulae — Standard nasal cannulae are readily available and may be used for apneic oxygenation. For apneic oxygenation the patient is usually preoxygenated with a face mask simultaneously with nasal oxygen, which is continued during intubation after the face mask is removed. Flow rates are limited through standard cannulae, and at higher flow rates dry oxygen can desiccate the airway and may be uncomfortable for awake patients [71]. Oxygen flow is often started at approximately 5 L/minute and turned up to as high as 15 L/minute after induction. Several small trials have found that the use of standard nasal cannula oxygen delivered at up to 5 L/minute following facemask preoxygenation delayed the onset of desaturation [72-74]. (See 'Deciding to use apneic oxygenation for intubation' above.)

Other catheters — A variety of alternative approaches to apneic oxygenation in patients under general anesthesia have been described in the literature. These include a 10Fr nasal catheter (5 L/minute flow) [74], 8Fr nasal catheter [75], an adapted nasopharyngeal airway (12/L min flow [76]), and a 6.0 RAE endotracheal tube placed in the oropharynx [77]. These approaches are most likely to be used for oxygenation during airway procedures whereas high flow nasal cannula is preferred for apneic oxygenation as part of intubation. Some devices that have been used in children are shown in a figure (picture 1).

A simulation study concluded that the placement of the oxygenation source may have a role in efficacy and found that a location near the base of the tongue (catheter or tube) may be preferred during apneic oxygenation for procedures [78], however this has not yet been studied clinically.

MONITORING DURING PREOXYGENATION AND APNEIC OXYGENATION — Various modalities have been used to assess oxygenation and ventilation and related parameters during preoxygenation and apneic oxygenation.

Pulse oximetry — Pulse oximetry is a standard monitor used during anesthesia and airway management. Oxygen desaturation is an endpoint that determines whether to intervene during airway management. However, peripheral oxygen saturation (SpO₂) may be a misleading guide to the efficacy of preoxygenation. An SpO₂ of 100 percent may occur well before the lungs are adequately denitrogenated. Conversely, failure of SpO₂ to increase substantially during preoxygenation does not necessarily indicate failure of the technique; patients with substantial pulmonary shunting may achieve excellent pulmonary oxygen reservoirs while remaining hypoxemic [79].

End tidal oxygen — Whenever possible, end tidal oxygen (EtO₂) should be monitored during preoxygenation. It is a simple, noninvasive method to assess alveolar concentration of oxygen; EtO₂ >90 percent is a goal for preoxygenation. When preoxygenation is performed based only on time (three minutes), EtO₂ is often not achieved, both in the operating room and in emergency department settings [80,81]. (See 'Physiologic basis for preoxygenation and apneic oxygenation' above.)

Importantly, EtO₂ can be considered an accurate endpoint for preoxygenation only if it is accompanied by a normal capnogram and appropriate movement of the reservoir bag during respiration. Absent these findings, the EtO₂ may reflect sampling of fresh gas flow, rather than true EtO₂ [3].

Transcutaneous carbon dioxide — Transcutaneous carbon dioxide (TCCO₂) monitoring may be useful during apneic oxygenation when end tidal CO₂ cannot be measured (eg, during high flow nasal oxygen), and has been advocated as a standard of care during tubeless laryngeal surgery [82]. The technology and its limitations are discussed separately. (See "Venous blood gases and other alternatives to arterial blood gases", section on 'Transcutaneous carbon dioxide'.)

In one observational study of healthy surgical patients who had apneic oxygenation with high flow nasal oxygen (HFNO) for 20 to 45 minutes, monitored with both TCCO₂ and arterial blood gases, there was good correlation between TCCO₂ and arterial partial pressure of carbon dioxide (PaCO₂) at PaCO₂ <75 mmHg [83]. At higher levels, TCCO₂ overestimated PaCO₂. For all patients apnea was discontinued for acidosis, with a mean pH of 7.12 and mean PaCO₂ of 84 mmHg.

Some newer TTCO₂ systems combine carbon dioxide monitoring with reflectance oximetry probe. In one study, the reflectance probe detected oxygen desaturation more quickly than a standard fingertip pulse oximeter [84].

COMPLICATIONS — Complications of preoxygenation and apneic oxygenation are few and are outweighed by the benefit of avoiding hypoxemia. Potential adverse effects include absorption atelectasis, gastric insufflation, oxygen toxicity, and in the case of apneic oxygenation, hypercarbia.

●Absorption atelectasis – When breathing room air, alveoli contain high concentration of nitrogen, which is poorly soluble in blood and remains largely in the alveoli, helping to stent them open. Once the nitrogen has been replaced with oxygen, if apnea or hypoventilation occurs, oxygen in the alveoli may be absorbed faster than it is delivered, causing alveoli to collapse. This is referred to as absorption atelectasis.

Using a fraction of inspired oxygen (FiO₂) lower than 1.0 for preoxygenation and apneic oxygenation may reduce the incidence of absorption atelectasis but would also reduce the time to desaturation during intubation. (See 'Physiologic basis for preoxygenation and apneic oxygenation' above.)

Atelectasis can be prevented or reversed with use of positive end expiratory pressure (PEEP) or recruitment maneuvers after intubation. These techniques and other components of lung protective ventilation are discussed separately. (See "Mechanical ventilation during anesthesia in adults", section on 'Lung protective ventilation during anesthesia'.)

●Gastric insufflation – Use of positive airway pressure techniques during preoxygenation could theoretically insufflate air into the stomach and increase the risk of regurgitation and aspiration. There is no evidence that this occurs when using continuous positive airway pressure (CPAP) or high flow nasal oxygen (HFNO) for preoxygenation [85-87]. HFNO during anesthetic induction may lead to a small increase in gastric volume [88] and may be exacerbated by airway obstruction. The increase in gastric volume associated with HFNO may be less than that produced by facemask positive pressure ventilation at 15 cm H₂O [89]. There are no reports of gastric aspiration or cardiovascular compromise associated with use of HFNO during anesthetic induction.

●Oxygen toxicity – Oxygen toxicity can occur after administration of supplemental oxygen; there is not a well-defined FiO₂ or duration of exposure below which oxygen toxicity does not occur. Nonetheless, the risk of oxygen toxicity is likely very small given the short duration of administration of 100 percent oxygen for preoxygenation. Oxygen toxicity is discussed separately. (See "Adverse effects of supplemental oxygen", section on 'Lung parenchymal injury'.)

●Hypoxemia and hypercarbia – These potential complications are the primary concerns with apneic oxygenation. Hypoxemia is readily detected by pulse oximetry, though a fall in SpO₂ suggests that arterial saturation has been dropping for some time. (See 'Pulse oximetry' above.)

Oxygenation can be maintained for prolonged periods with apneic oxygenation, however without ventilation there is no removal of CO₂. Respiratory acidosis develops, and accumulation of alveolar CO₂ dilutes the alveolar oxygen concentration. The arterial partial pressure of carbon dioxide (PaCO₂) rises approximately 8 to 16 mmHg per minute during the first minute of apnea, with a subsequent rise of approximately 3 mmHg per minute during continued apnea, ultimately causing hypercarbia and acidosis [11]. The rise in CO₂ during apnea was demonstrated in a study of healthy surgical patients who were intubated, paralyzed, and provided apneic oxygenation through the endotracheal tube attached to 100 percent oxygen via the anesthesia circuit [49]. The longest duration of apnea was 53 minutes, with apnea terminated for acidemia, with a pH of 6.72 and partial pressure of carbon dioxide of 250 mmHg. At that time oxygen saturation was still 98 percent.

Lack of ventilation during HFNO and monitoring CO₂ during apneic ventilation are discussed above. (See 'Efficacy and prolongation of time to desaturation' above and 'Transcutaneous carbon dioxide' above.)

SOCIETY GUIDELINE LINKS — Links to society and government-sponsored guidelines from selected countries and regions around the world are provided separately. (See "Society guideline links: Airway management in adults".)

SUMMARY AND RECOMMENDATIONS

●Physiologic basis

•The goal of preoxygenation is to maximize the amount of oxygen in the lung, primarily in the functional residual capacity (FRC), to delay the onset of hypoxia when apnea occurs during airway management.

•Factors that affect the speed of preoxygenation, highest end tidal oxygen (EtO₂) achieved, and the rate of desaturation during apnea include the fraction of inspired oxygen (FiO₂), airway patency, the breathing pattern during preoxygenation, oxygen consumption, and any shunt physiology.

•Increased FRC slows preoxygenation, but results in a longer time until desaturation during apnea.

•During apneic oxygenation, oxygen diffuses from the trachea into alveoli down the pressure gradient created by absorption of oxygen from the alveoli. This process depends on maintaining an open airway. (See 'Physiologic basis for preoxygenation and apneic oxygenation' above.)

●Choosing a preoxygenation technique – All patients require preoxygenation prior to airway management for anesthesia.

•Patients without risk factors for hypoxia – For patients without risk factors for hypoxia, we suggest using routine preoxygenation (with tidal breathing with a tight fitting face mask with FiO₂ 1.0 for three to five minutes, optimally to an EtO₂ >90 percent) rather than high flow nasal oxygen (HFNO) or positive pressure delivery (Grade 2C). If time is limited, eight vital capacity breaths may be as effective as three minutes of tidal breathing. (See 'Tight fitting face mask with passive delivery' above and 'Patients without risk factors for hypoxia during intubation' above.)

•Patients with predicted difficulty with airway management – For patients without other risk factors for hypoxia but with predicted difficulty with airway management, we suggest using both preoxygenation and apneic oxygenation, rather than preoxygenation alone (Grade 2C), as additional time may be required to manage and control the airway. Where available, we use HFNO for apneic oxygenation, and other options (eg, low flow nasal cannulae, oropharyngeal catheters) if HFNO is not available. (See 'Patients with predicted difficulty with airway management' above and 'High flow nasal oxygen' above and 'Other options for apneic oxygenation' above.)

•Patients with reduced FRC – For patients with reduced FRC due to obesity, pregnancy, an abdominal mass or ascites, we suggest using active delivery with noninvasive ventilation techniques (NIV; eg, continuous positive airway pressure [CPAP], bilevel positive airway pressure [BPAP], pressure support facemask ventilation with a ventilator or anesthesia machine) for preoxygenation, along with head up positioning rather than routine facemask preoxygenation or HFNO (Grade 2C). NIV may reduce atelectasis and both techniques increase functional residual capacity, which may improve the efficacy of preoxygenation, and prolong time to desaturation. (See 'Patients with reduced FRC' above and 'Tight fitting face mask with active delivery' above.)

•Critically ill hypoxemic patients – For critically ill hypoxemic patients, we suggest using NIV (eg, CPAP, BiPAP, pressure support facemask ventilation with a ventilator or anesthesia machine) for preoxygenation rather than routine facemask preoxygenation or HFNO, provided that the patient is cooperative and a sufficient seal can be obtained (Grade 2C). (See 'Critically ill hypoxemic patients' above and 'Tight fitting face mask with active delivery' above.)

●Monitoring

•Pulse oximetry is an endpoint used to determine whether to intervene during airway management, but should not be used to assess effective preoxygenation. Peripheral oxygen saturation (SpO₂) of 100 percent may occur well before the lungs are denitrogenated and may never be achieved even after effective preoxygenation in patients with intrapulmonary shunting. (See 'Pulse oximetry' above.)

•EtO₂ should be monitored during preoxygenation whenever possible, with a goal of >90 percent. (See 'End tidal oxygen' above.)

•Transcutaneous carbon dioxide (TCCO₂) monitoring may be useful during apneic oxygenation when end tidal CO₂ cannot be measured (eg, during high flow nasal oxygen). (See 'Transcutaneous carbon dioxide' above.)

●Complications – Potential complications of preoxygenation and apneic oxygenation are outweighed by the benefit of avoiding hypoxemia. Gastric insufflation and oxygen toxicity are theoretical problems. (See 'Complications' above.)

•Absorption atelectasis refers to collapse of alveoli when nitrogen has been replaced with oxygen during preoxygenation, and the oxygen is absorbed into the blood. Atelectasis can be reversed with use of positive end expiratory pressure (PEEP) or recruitment maneuvers after intubation.

•Hypoxemia and hypercarbia are potential problems during apneic oxygenation. HFNO does not reliably remove carbon dioxide and can cause respiratory acidosis.

ACKNOWLEDGMENT — The UpToDate editorial staff acknowledges Anil Patel, FRCA and Michael Gilhooly, FRCA who contributed to earlier versions of this topic review.