Airway/review article
Preoxygenation and Prevention of Desaturation During Emergency Airway Management

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Patients requiring emergency airway management are at great risk of hypoxemic hypoxia because of primary lung pathology, high metabolic demands, anemia, insufficient respiratory drive, and inability to protect their airway against aspiration. Tracheal intubation is often required before the complete information needed to assess the risk of periprocedural hypoxia is acquired, such as an arterial blood gas level, hemoglobin value, or even a chest radiograph. This article reviews preoxygenation and peri-intubation oxygenation techniques to minimize the risk of critical hypoxia and introduces a risk-stratification approach to emergency tracheal intubation. Techniques reviewed include positioning, preoxygenation and denitrogenation, positive end expiratory pressure devices, and passive apneic oxygenation.

Introduction

Maintaining hemoglobin saturation during airway management is critical to patient safety. Desaturation to below 70% puts patients at risk for dysrhythmia, hemodynamic decompensation, hypoxic brain injury, and death.1, 2 The challenge for emergency physicians is to secure a tracheal tube rapidly without critical hypoxia or aspiration. In patients without pulmonary pathology, adequate hemoglobin, or low metabolic demands and an initial pulse oximetry reading of 100% on room air, there is a low risk of desaturation after adequate preoxygenation. Conversely, in a septic patient with multilobar pneumonia who is already hypoxemic (oxygen saturation ≤90%) despite 100% oxygen at high flow, there is an immediate risk of critical tissue hypoxia during tracheal intubation.

This article reviews preoxygenation and peri-intubation oxygenation techniques to minimize the risk of hypoxemia during emergency tracheal intubation of adult patients. It introduces a risk-stratification approach based on initial pulse oximetry level in response to oxygen administration and provides recommendations about specific techniques based on periprocedural risk. Techniques reviewed include positioning, preoxygenation and denitrogenation, use of positive pressure devices to increase mean airway pressure, and passive apneic oxygenation during tracheal intubation efforts.

Section snippets

What is the Rationale for Providing Preoxygenation Before Tracheal Intubation?

Preoxygenation allows a safety buffer during periods of hypoventilation and apnea. It extends the duration of safe apnea, defined as the time until a patient reaches a saturation level of 88% to 90%, to allow for placement of a definitive airway. When patients desaturate below this level, their status is on the steep portion of the oxyhemoglobin dissociation curve and can decrease to critical levels of oxygen saturation (<70%) within moments (Figure 1).3

The standard anesthesia induction of

What is the Best Source of High FiO2 for Preoxygenation?

The duration of safe apnea times in most of the preoxygenation literature is predicated on anesthesia circuits that are capable of delivering 90% to 100% FiO2 when used with a well-fitting mask. However, the usual source of oxygen during ED preoxygenation is a facemask with an oxygen reservoir. This device is erroneously referred to as the nonrebreather mask despite an absence of 1-way valves covering all of its ports. True nonrebreather masks set at 15 L/minute for patients with normal

For What Period of Time Should the Patient Receive Preoxygenation?

Ideally, patients should continue to receive preoxygenation until they denitrogenate the functional residual capacity of their lungs sufficiently to achieve greater than 90% end-tidal oxygen level.14 Although the mass spectrometers in many EDs allow the measurement of end-tidal oxygen levels, in practice this is rarely performed. Instead, expediency often demands an empiric timing of preoxygenation.

Three minutes' worth of tidal-volume breathing (the patient's normal respiratory pattern) with a

Can Increasing Mean Airway Pressure Augment Preoxygenation?

Mean airway pressure may be increased during preoxygenation through the use of techniques such as noninvasive positive-pressure ventilation. If patients have not achieved a saturation greater than 93% to 95% before tracheal intubation, they have a higher likelihood of desaturation during their apneic and tracheal intubation periods.2, 16, 18, 22, 23 If patients do not achieve this saturation level after 3 minutes of tidal-volume breathing with a high FiO2 source, it is likely that they are

In What Position Should the Patient Receive Preoxygenation?

Supine positioning is not ideal to achieve optimal preoxygenation. When one is placed flat, it is more difficult to take full breaths and more of the posterior lung becomes prone to atelectatic collapse,3 which reduces the reservoir of oxygen contained within the lungs and therefore reduces safe apnea time.

Lane et al42 performed a randomized controlled trial of patients preoxygenated in a 20-degree head-up position versus a control group that was left supine. After 3 minutes of preoxygenation,

How Long Will it Take for the Patient to Desaturate After Preoxygenation?

Although breathing at a high FiO2 level will slightly increase the bloodstream stores of oxygen, the primary benefit of preoxygenation is the creation of a reservoir of oxygen in the alveoli. When a patient is breathing room air, 450 mL of oxygen is present in the lungs; this amount increases to 3,000 mL when a patient breathes 100% oxygen for a sufficient time to replace the alveolar nitrogen. A patient breathing room air will have a total oxygen reservoir in the lungs and bloodstream of

Can Apneic Oxygenation Extend the Duration of Safe Apnea?

Alveoli will continue to take up oxygen even without diaphragmatic movements or lung expansion. In an apneic patient, approximately 250 mL/minute of oxygen will move from the alveoli into the bloodstream. Conversely, only 8 to 20 mL/minute of carbon dioxide moves into the alveoli during apnea, with the remainder being buffered in the bloodstream.53 The difference in oxygen and carbon dioxide movement across the alveolar membrane is due to the significant differences in gas solubility in the

When and How Should We Provide Manual Ventilations During the Apneic Period?

Practitioners should not initiate laryngoscopy before full muscle relaxation to maximize laryngeal exposure and to avoid triggering the patient's gag reflex and active vomiting just before apnea. Ventilation provides 2 potential benefits during the onset phase of muscle relaxation: ventilation and increased oxygenation through alveolar distention and reduction in shunting.

The first benefit is minimal in most clinical scenarios. On average, PaCO2 increases 8 to 16 mm Hg in the first minute of

What Positioning and Maneuvers Should the Patient Receive During the Apneic Period?

Apneic oxygenation requires a patent airway for oxygen to reach the hypopharynx and be entrained into the trachea; once the patient is sedated and paralyzed, it is imperative to keep the posterior pharyngeal structures and tongue from occluding the passage of gas. Head elevation, chin lift, and jaw thrust will accomplish this in most patients; a jaw thrust alone should be used if there is risk for cervical spine injury. In some patients, a nasal trumpet or oral airway may also be required.

Does the Choice of Paralytic Agent Affect Preoxygenation?

The choice of paralytic agent may influence the time to desaturation during airway management. In a study of operative patients, the time to desaturation to 95% was 242 seconds in patients receiving succinylcholine versus 378 seconds in a group given rocuronium.95 Similarly, in obese patients undergoing surgery, the succinylcholine group desaturated to 92% in 283 seconds versus 329 seconds in the rocuronium group.96 When used at a dose of greater than or equal to 1.2 mg/kg, rocuronium provides

Risk Stratification and Conclusions

Patients requiring emergency airway management can be risk stratified into 3 groups, according to pulse oximetry after initial application of high-flow oxygen. The recommended techniques to use for patients in each group are shown in Table 2, and a logistic flow of preoxygenation steps is shown in Figure 3.

Head-elevated positioning is simple and easy to apply in all patients; in the patient immobilized for cervical spine injury, it is beneficial to tilt the foot of the bed downward.

Patients at

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    Supervising editors: Gregory W. Hendey, MD; Donald M. Yealy, MD

    Funding and support: By Annals policy, all authors are required to disclose any and all commercial, financial, and other relationships in any way related to the subject of this article as per ICMJE conflict of interest guidelines (see www.icmje.org). The authors have stated that no such relationships exist.

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    Publication date: Available online November 2, 2011.

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