Nimmagadda Usharani, Salem M Ramez, Crystal George J
From the *Department of Anesthesiology, Advocate Illinois Masonic Medical Center, Chicago, Illinois; and †Department of Anesthesiology, University of Illinois College of Medicine, Illinois.
Anesth Analg. 2017 Feb;124(2):507-517. doi: 10.1213/ANE.0000000000001589.
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.
麻醉诱导和气管插管前的预充氧是一种广泛接受的操作，旨在增加机体氧储备，从而延缓呼吸暂停期间动脉血红蛋白去饱和的发生。由于通气和插管困难不可预测，所有患者都需要进行预充氧。在麻醉苏醒期，麻醉药的残留效应和神经肌肉阻滞逆转不充分可导致通气不足、低氧血症和气道通畅性丧失。因此，也建议在气管拔管前常规进行预充氧。本文的目的是讨论预充氧的生理基础、临床益处以及使用时的潜在问题。预充氧的有效性通过其效能和效率来评估。效能指标包括肺泡氧分数增加、动脉血氧张力增加和肺泡氮分数降低。最大预充氧（效能）的终点是呼气末氧浓度达到90%或呼气末氮浓度达到5%。预充氧的效率反映在呼吸暂停期间氧合血红蛋白去饱和的下降速率上。所有研究都表明，最大预充氧可显著延缓呼吸暂停期间动脉血红蛋白去饱和。在高危患者中，这一优势可能会减弱。已引入各种操作来延长预充氧的效果。这些操作包括抬高头部、呼吸暂停扩散充氧、持续气道正压通气（CPAP）和/或呼气末正压通气（PEEP）、双水平气道正压通气以及经鼻湿化快速充气通气交换。呼吸暂停扩散充氧的益处取决于实现最大预充氧、保持气道通畅以及高功能残气量与体重比值的存在。预充氧的潜在风险包括食管插管的延迟检测、吸收性肺不张、活性氧的产生以及不良血流动力学效应。由于预充氧的持续时间较短，血流动力学效应和活性氧的积累不足以抵消其益处。吸收性肺不张是预充氧的一个后果。已提出两种方法来减少预充氧期间的吸收性肺不张：将吸入氧分数适度降至0.8，以及使用肺复张手法，如CPAP、PEEP和/或肺活量手法（所有这些手法在麻醉给药期间通常都会进行）。虽然吸入氧分数略有降低可减少肺不张，但这样做是以降低呼吸暂停期间提供的保护为代价的。
