Preoxygenation: Physiologic Basis, Benefits, and Risks

Abstract and Introduction

Abstract

Preoxygenation before anesthetic induction and tracheal intubation is a widely accepted maneuver, designed to increase the body oxygen stores and thereby delay the onset of arterial hemoglobin desaturation during apnea. Because difficulties with ventilation and intubation are unpredictable, the need for preoxygenation is desirable in all patients. During emergence from anesthesia, residual effects of anesthetics and inadequate reversal of neuromuscular blockade can lead to hypoventilation, hypoxemia, and loss of airway patency. In accordance, routine preoxygenation before the tracheal extubation has also been recommended. The objective of this article is to discuss the physiologic basis, clinical benefits, and potential concerns about the use of preoxygenation. The effectiveness of preoxygenation is assessed by its efficacy and efficiency. Indices of efficacy include increases in the fraction of alveolar oxygen, increases in arterial oxygen tension, and decreases in the fraction of alveolar nitrogen. End points of maximal preoxygenation (efficacy) are an end-tidal oxygen concentration of 90% or an end-tidal nitrogen concentration of 5%. Efficiency of preoxygenation is reflected in the rate of decline in oxyhemoglobin desaturation during apnea. All investigations have demonstrated that maximal preoxygenation markedly delays arterial hemoglobin desaturation during apnea. This advantage may be blunted in high-risk patients. Various maneuvers have been introduced to extend the effect of preoxygenation. These include elevation of the head, apneic diffusion oxygenation, continuous positive airway pressure (CPAP) and/or positive end-expiratory pressure (PEEP), bilevel positive airway pressure, and transnasal humidified rapid insufflation ventilatory exchange. The benefit of apneic diffusion oxygenation is dependent on achieving maximal preoxygenation, maintaining airway patency, and the existence of a high functional residual capacity to body weight ratio. Potential risks of preoxygenation include delayed detection of esophageal intubation, absorption atelectasis, production of reactive oxygen species, and undesirable hemodynamic effects. Because the duration of preoxygenation is short, the hemodynamic effects and the accumulation of reactive oxygen species are insufficient to negate its benefits. Absorption atelectasis is a consequence of preoxygenation. Two approaches have been proposed to reduce the absorption atelectasis during preoxygenation: a modest decrease in the fraction of inspired oxygen to 0.8, and the use of recruitment maneuvers, such as CPAP, PEEP, and/or a vital capacity maneuver (all of which are commonly performed during the administration of anesthesia). Although a slight decrease in the fraction of inspired oxygen reduces atelectasis, it does so at the expense of a reduction in the protection afforded during apnea.

Introduction

The ability of preoxygenation, using a high fraction of inspired oxygen (FIO₂) before anesthetic induction and tracheal intubation, to delay the onset of apnea-induced arterial oxyhemoglobin desaturation has been appreciated for many years.^[1–3] For patients at risk for aspiration, during rapid sequence induction/intubation where manual ventilation is undesirable, preoxygenation has become an integral component.^[4–7] Preoxygenation is also important, when difficulty with ventilation or tracheal intubation is anticipated and when the patient has limited oxygen (O₂) reserves.^[8,9] In 2003, guidelines from the American Society of Anesthesiologists Task Force on the Management of the Difficult Airway included "face mask preoxygenation before initiating management of the difficult airway."^[10] Because the "cannot intubate, cannot ventilate" situation is unpredictable, the need for preoxygenation is desirable in all patients.^[8,11] In 2015, guidelines developed by Difficult Airway Society in the United Kingdom for the management of unanticipated difficult intubation included the statement that all patients should be preoxygenated before the induction of general anesthesia.^[12]

Residual effects of anesthetics or inadequate reversal of muscle relaxants can complicate emergence from anesthesia. These effects can lead to decreased functional activity of the pharyngeal muscles, upper airway obstruction, inability to cough effectively, a 5-fold increase in the risk of aspiration, and attenuation of the hypoxic drive by the peripheral chemoreceptors.^[13,14] Hypoventilation, hypoxemia, and loss of airway patency may follow these changes. Preoxygenation can also minimize neostigmine-induced cardiac arrhythmias.^[15] In accordance, "routine" preoxygenation before the reversal of neuromuscular blockade and tracheal extubation has been recommended, given the potential for airway and ventilation problems.^[16] Guidelines for the management of tracheal extubation proposed in 2012 by the Difficult Airway Society in the United Kingdom include the statement that it is vital to preoxygenate before extubation because of various perioperative anatomical and physiologic changes that may compromise gas exchange.^[17] Preoxygenation has also been recommended before any interruption of ventilation, such as during open tracheobronchial suctioning.^[16]

The current review describes the physiologic basis and clinical benefits of preoxygenation. Special considerations for preoxygenation in high-risk patient populations are discussed. Over the years, concerns have been expressed in the literature regarding potential undesirable effects of preoxygenation. These effects include delayed diagnosis of esophageal intubation, tendency to cause absorption atelectasis, production of reactive oxygen species, and adverse hemodynamic changes. We describe these effects and discuss whether they justify modifying preoxygenation in selected clinical situations.

Table 1. Body O ₂ Stores (in mL) During Room Air and 100% O ₂ Breathing
Body Store	Room Air	100 % O₂
Lungs	450	3000
Blood	850	950
Dissolved in tissue fluids	50	100
Combined with myoglobin	200	200
Total	1550	4250

Adapted from Nunn.¹⁸

Table 2. Stages of Preoxygenation
Stage	Description	Determinant of t	Recommendation
1	Washout of anesthesia circuit by O₂ flow	Size of circuit/O₂ flow rate	Washout of circuit by high O₂ flow before placing face mask
2	Washout of FRC by VA	FRC/VA	Use of O₂ flow rate that eliminates rebreathing

Abbreviations: FRC, functional residual capacity; t, time required for flow through a container (volume) to equal its capacity; VA, alveolar ventilation. Adapted from Baraka and Salem.¹⁶

Table 3. Factors Affecting the Efficacy and Efficiency of Preoxygenation
Efficacy
Inspired oxygen concentration
Presence of leak
Anesthetic system used
Level of FGF
Type of breathing (tidal volume or deep breathing)
Duration of breathing
VA/FRC ratio
Efficiency
Oxygen volume in lungs
Alveolar oxygen tension
FRC
Systemic oxygen supply versus demand balance
Arterial oxygen content
Cardiac output
Whole body oxygen consumption

Abbreviations: FGF, fresh gas flow; FRC, functional residual capacity; VA, alveolar ventilation.
Adapted from Baraka and Salem.¹⁶

Table 4. Techniques of Preoxygenation
Tidal volume breathing
One vital capacity breath followed by tidal volume breathing
Single tidal capacity breath
Four deep breaths (4 inspiratory capacity breaths)
Eight deep breaths (8 inspiratory capacity breaths)
Extended deep breathing (12–16 inspiratory capacity breaths)