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http://www.medscape.com/viewpublication/2510Anesthesia & Analgesia
Preoxygenation: Physiologic Basis, Benefits, and Potential Risks
Usharani Nimmagadda, MD; M. Ramez Salem, MD; George J. Crystal, PhD
Usharani Nimmagadda, MD*†, M. Ramez Salem, MD*† and George J. Crystal, PhD†
*Department of Anesthesiology, Advocate Illinois Masonic Medical Center, Chicago, Illinois; and †Department of Anesthesiology, University of Illinois College of Medicine, Illinois.
Address correspondence to Usharani Nimmagadda, MD, Department of Anesthesiology, Advocate Illinois Masonic Medical Center, 836 West Wellington Ave, Chicago, IL 60657. Address e-mail to
Anesth Analg. 2017;124(2):507-517.
Abstract and Introduction
http://www.medscape.com/viewarticle/875398_2Preoxygenation: Physiologic Basis, Efficacy, and Efficiency
http://www.medscape.com/viewarticle/875398_3Preoxygenation for High-risk Patient Populations
http://www.medscape.com/viewarticle/875398_4Technique of Preoxygenation
http://www.medscape.com/viewarticle/875398_5Techniques to Enhance Preoxygenation
http://www.medscape.com/viewarticle/875398_6Potential Risks of Preoxygenation
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.
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.
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.
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
Preoxygenation: Physiologic Basis, Efficacy, and Efficiency
Preoxygenation increases the body O2 stores, the main increase occurring in the functional residual capacity. The size of the increases in O2 volume in the various body tissues is difficult to assess with precision, but assuming that the partition coefficient for gases approximates the gas-water coefficients, the estimated increases are appreciable (
Variation in the volume of O2 stored in the functional residual capacity (□), blood (▴), tissue (^), and whole body (▪) with the duration of preoxygenation. Published with permission from Campbell and Beatty.19
Preoxygenation increases FAO2 and decreases FAN2 (Figure 2).
Comparison of mean end-tidal O2 and N2 concentration obtained at 30-second intervals during 5-minute period of spontaneous tidal volume oxygenation using the circle absorber and NasOral systems in 20 volunteers. Data are mean ± SD. Published with permission from Nimmagadda et al.31
Many factors affect efficacy and efficiency (
Bearded patients, edentulous patients, elderly patients with sunken cheeks, use of the wrong size face mask, improper use of head straps, and the presence of gastric tubes are common factors causing air entrainment and a lower FIO2. The absence of a normal capnographic tracing, and a lower than expected end-tidal carbon dioxide concentration (EtCO2) and EtO2 should alert the anesthesiologist to the presence of leaks in the anesthetic circuit.
Increasing the FGF from 5 to 10 L does not increase appreciably the FIO2 during tidal volume breathing, although it does so during deep breathing.
All investigations have demonstrated that preoxygenation markedly delays arterial oxyhemoglobin desaturation during apnea.
Arterial oxyhemoglobin saturation (Sao2) versus time of apnea in an obese adult, a 10-kg child with low functional residual capacity and high ventilation, and a moderately ill adult compared with a healthy adult. FAO2 indicates fractional alveolar oxygen concentration; VE, expired volume. Published with permission from Benumo
Preoxygenation for High-risk Patient Populations
Rapid sequence induction/intubation is commonly performed in pregnant women who are given general anesthesia and preoxygenation is essential in these patients. Maximal preoxygenation can be achieved more rapidly in pregnant than in nonpregnant women because of a higher alveolar ventilation and a lower functional residual capacity.
Morbidly Obese Patients
Studies have demonstrated that, following preoxygenation with tidal volume breathing for 3 minutes, the time required for SaO2 to fall to 90% during apnea is markedly reduced in morbidly obese patients (BMI > 40 kg/m2) compared with nonobese patients.
Some anesthesiologists may prefer awake fiberoptic intubation rather than rapid sequence induction/intubation in morbidly and super morbidly obese patients (BMI > 50 kg/m2), especially when they have associated problems.
Studies have demonstrated that maximal preoxygenation (EtO2 = 90%) can be accomplished in children faster than in adults.
Aging is associated with significant structural and physiologic changes in the respiratory system.
Patients With Pulmonary Disease
Both efficacy and efficiency can be adversely affected by pulmonary disease. Significant pulmonary disease is associated with a decreased functional residual capacity, appreciable ventilation—perfusion mismatch, and an increased VO2, which can reduce the margin of safety. Anesthesia has been shown to cause further impairment to gas exchange in patients with chronic obstructive pulmonary disease.
Patients at High Altitude
High altitude does not alter the concentration of inspired O2 (21%), but the reduced barometric pressure produces a decrease in the partial pressure of alveolar and arterial PO2.
Technique of Preoxygenation
To provide effective preoxygenation, a methodical approach is necessary. The importance of preoxygenation with a tight-fitting mask should be explained to the patient beforehand. Once preoxygenation is initiated, EtO2 and FIO2 values should be monitored closely. If the EtO2 value does not increase as expected, the anesthesia provider may have to hold the mask with both hands and/or replace the mask with a better-fitting one. Whenever possible, the induction should not start until the EtO2 value approximates or exceeds 90%
Techniques to Enhance Preoxygenation
Apneic Diffusion Oxygenation
Preoxygenation followed by "apneic diffusion oxygenation" is an effective maneuver for prolonging the safe duration of apnea.
The benefit of apneic diffusion oxygenation is dependent on achieving maximal preoxygenation before apnea, maintaining airway patency, and the existence of a high functional residual capacity to body weight ratio. Fraioli et al
The time (duration of apnea) required to reach 50% Sao2 with an open airway exposed to various ambient O2 fractions. Published with permission from McNamara and Hardman.76
Apneic diffusion oxygenation can be achieved by maximal face mask preoxygenation followed by O2insufflation up to 15 L/min through a nasopharyngeal or an oropharyngeal cannula or through a needle inserted in the cricothyroid membrane. In healthy patients with an unobstructed airway, this technique can provide at least 10 minutes of adequate oxygenation. The clinical applications include patients who are difficult to intubate or ventilate and patients with limited oxygen reserves. The technique can also be used during bronchoscopy and can provide adequate time for short glottic surgical procedures unimpeded by the presence of a tracheal tube or the patient's respiratory excursions. Although oxygenation can be maintained for longer periods, a limiting factor of apneic oxygenation is the progressive rise of PaCO2 during apnea.
Continuous Positive Airway Pressure and Positive End-expiratory Pressure
Use of continuous positive airway pressure (CPAP) during preoxygenation of obese patients did not delay the onset of desaturation, because the functional residual capacity returned to the pre-CPAP level when the patient was induced and the mask was removed.
Noninvasive Bilevel Positive Airway Pressure
BiPAP (bilevel positive airway pressure; inspiratory positive airway pressure and expiratory positive airway pressure) combines the benefits of pressure support ventilation and CPAP and keeps the lungs open during the entire respiratory cycle. BiPAP has been used during preoxygenation to decrease intrapulmonary shunting and to increase the margin of safety during apnea in morbidly obese patients.
Transnasal Humidified Rapid Insufflation Ventilatory Exchange
Transnasal humidified rapid insufflation ventilatory exchange (THRIVE) is a new technique that is available for use in critically ill patients and in patients with difficult airways. The technique combines the benefits of apneic oxygenation and CPAP with a reduction in CO2 levels through gaseous mixing and flushing of the dead space (Figure 5).
The OptiFlow high-flow humidified O2 delivery system. The O2 humidification unit (A) received O2 from a standard O2 regulator and delivers humidified O2 to a custom-built transnasal O2 cannula (B and C) like a standard nasal O2 cannula (D). Published with permission from Patel and Nouraei.82
Potential Risks of Preoxygenation
Delayed Diagnosis of Esophageal Intubation
Although an unrecognized esophageal intubation ultimately results in severe hypoxemia, minutes may elapse before this occurs. Preoxygenation extends the time period before hypoxemia ensues and, thus, delays the detection of a misplaced endotracheal tube when SpO2 is being used as an indicator. Cases attributing a delayed diagnosis of esophageal intubation to preoxygenation
Atelectasis occurs in 75% to 90% of healthy individuals undergoing general anesthesia,
Techniques that have been proposed to decrease the extent of absorption atelectasis following preoxygenation are (1) decreasing the concentration of FIO2 and (2) various recruitment maneuvers. Studies using computer modeling, as well as those involving actual measurements in patients using computerized tomography (CT), have demonstrated that decreasing the value of FIO2 can have a profound effect on the extent of atelectasis.
Recruitment maneuvers are commonly performed in patients under general anesthesia, but they have particular value in conjunction with preoxygenation. These maneuvers include CPAP, PEEP, and/or reexpansion maneuver. A CT study found that the combined use of CPAP (6 cm H2O) during 5 minutes of preoxygenation with face mask while breathing spontaneously, and PEEP (6 cm H2O) during mask ventilation for additional 5 minutes during induction of anesthesia, prevented the marked increase in atelectasis that was evident in a control group.
Production of Reactive Oxygen Species
Oxygen is a paramagnetic atom containing 2 unpaired electrons in its outer shell that usually exists in the form of dioxygen (O2). In biological tissues, the dioxygen molecule can be accidentally or deliberately split, producing reactive oxygen species, which include superoxide anion, hydroxyl radical, and hydrogen peroxide.
The cardiovascular responses during preoxygenation have received limited attention and have not been well characterized. But there have been many studies, both in humans and animal models, assessing the steady state cardiovascular responses during high O2 breathing, which may provide insight into the hemodynamic changes during preoxygenation. However, the changes in PaO2 during preoxygenation are dynamic and brief, and, furthermore, they have been demonstrated to vary in different patient populations. Thus, caution should be exercised in extrapolating the experimental findings described below to a given patient undergoing preoxygenation.
Several studies in normal male subjects have demonstrated that breathing 100% O2 causes a modest decrease in heart rate accompanied by a parallel decrease in cardiac output. Systemic vascular resistance and arterial blood pressure increase.
A number of physiologic studies have assessed the effect of inhalation of 100% O2 in the human coronary circulation.
It is well established that inhalation of high O2 can also reduce cerebral blood flow because of vasoconstriction.
Studies in animal models have demonstrated that hyperoxia causes vasoconstriction and a decrease in blood flow in peripheral vascular beds, including the kidney, gastrointestinal tract, and hindlimb.
The literature provides overwhelming evidence that preoxygenation, whether instituted before induction or to emergence from anesthesia, delays the onset of hypoxemia during apnea. On that basis, preoxygenation should be performed in all patients given general anesthesia. Preoxygenation should also be performed whenever there is an anticipated interruption of O2 delivery, such as during open tracheobronchial suctioning, and before and during awake fiber-optic intubation, especially in high-risk patients, such as the supermorbidly obese. The technique should be performed correctly, with monitoring of EtO2. Because the advantage of preoxygenation may be blunted in high-risk patients, various maneuvers are available to prolong its effectiveness. The clinician should be familiar with these maneuvers. Absorption atelectasis during preoxygenation can be readily minimized, and thus it should not be a deterrent to the routine use of the technique.
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