Preoxygenation: Physiologic Basis, Benefits, and Potential Risks

Usharani Nimmagadda, MD; M. Ramez Salem, MD; George J. Crystal, PhD

Disclosures

Anesth Analg. 2017;124(2):507-517. 

In This Article

Techniques to Enhance Preoxygenation

Apneic Diffusion Oxygenation

Preoxygenation followed by "apneic diffusion oxygenation" is an effective maneuver for prolonging the safe duration of apnea.[16,70–73] The physiologic basis of this maneuver is as follows. During apnea in adults, VO2 averages 230 mL/min, whereas CO2 delivery to the alveoli is only 21 mL/min.[16] The remaining 90% (or more) of CO2 is buffered within body tissues. The result is that lung volume decreases initially by 209 mL/min, which creates a pressure gradient between the upper airway and the alveoli, and provided that the airway is not obstructed, O2 enters the lung via diffusion. Because CO2 cannot be exhaled, PaCO2 rises from 8 to 16 mm Hg in the first minute of apnea, followed by a linear rise of approximately 3 mm Hg/min.[74]

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[75] demonstrated that patients with a low predicted functional residual capacity/body weight ratio (37 ± 9 mL/kg) could not tolerate apneic oxygenation for more than 5 minutes, whereas patients with a high predicted functional residual capacity/body weight ratio (53 ± 8 mL/kg) could tolerate apneic oxygenation for at least 15 minutes. Although PaO2 falls in direct relation to PAO2, SaO2 remains greater than 90% as long as the hemoglobin can be reoxygenated in the lungs.[32,71,75] SaO2 starts to decrease only after the O2 stores in the lungs are depleted, and PaO2 falls below 60 mm Hg. When SaO2 is <80%, the rate of decrease in saturation is approximately 30%/min. In the presence of an airway obstruction, gas volume in the lungs decreases rapidly, and intrathoracic pressure decreases at a rate dependent on lung compliance and VO2. When the airway obstruction is relieved, a rapid flow of O2 into the lungs resumes, and with high FIO2, preoxygenation is restored.[32] Some studies have demonstrated that, with a patent airway, apneic diffusion oxygenation can maintain SaO2 above 90% for up to 100 minutes.[71] When FIO2 is at a high level, a small increment can produce a profoundly disproportionate delay in hemoglobin desaturation; the delay in hemoglobin desaturation achieved by increasing FIO2 from 0.9 to 1.0 was greater than that achieved by increasing FIO2 from 0.21 to 0.9 (Figure 4).[76]

Figure 4.

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 O2 insufflation 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.[74]

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.[78] However, the use of CPAP during preoxygenation followed by mechanical ventilation using positive end-expiratory pressure (PEEP) for 5 minutes before removing the mask and securing the airway, delayed the desaturation time.[78,79]

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.[80] The technique has also been used to reduce postoperative pulmonary dysfunction and to treat patients with respiratory failure from various etiologies.[81]

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).[82] THRIVE is administered through a standard, commercially available, nasal, high-flow oxygen delivery system. Insufflation of O2 up to 70 L/min via a purpose-made nasal cannula is used initially to provide preoxygenation, which can be continued during intravenous induction and neuromuscular blockade until a definitive airway is secured. CPAP of approximately 7 cm H2O splints the upper airways and reduces shunting.[84] The THRIVE technique has been demonstrated to appreciably prolong the safe duration of apnea while avoiding increase in CO2.[83]

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

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