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

Preoxygenation for High-risk Patient Populations

Pregnant Patients

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.[22,41] However, during apnea, pregnant women tend to develop oxyhemoglobin desaturation more rapidly because of a limited O2 volume in their smaller functional residual capacity combined with an increased VO2. The time required for SaO2 to decrease to 95% during apnea was found to be 173 seconds in pregnant women and 243 seconds in nonpregnant women in the supine position.[42] Use of the 45° head-up position results in an increase in desaturation time in nonpregnant women but not in pregnant women. It is possible that the gravid uterus prevents the descent of the diaphragm and does not allow the expected increase in functional residual capacity in the head-up position.[42] In pregnant women, the 4 deep breathing technique is inferior to the 3-minute tidal volume breaths technique and should not be used, except in emergencies.[43] An increased minute ventilation in pregnant women requires that an O2 flow of 10 L/min be used during preoxygenation.[44]

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.[45,46] During apnea following preoxygenation, the average time to reach an SaO2 of 90% in patients with normal body weight was 6 minutes, whereas that in morbidly obese patients was only 2.7 minutes.[47] These findings are particularly concerning because morbid obesity is often complicated by obstructive sleep apnea, which can make mask ventilation and intubation more difficult. Rapid oxyhemoglobin desaturation during apnea in morbidly obese patients was attributed to an increased VO2 and a markedly reduced functional residual capacity. The supine position enhances this decrease in functional residual capacity because of a cephalad displacement of the diaphragm. Placing severely obese patients in the 25° head-up position during preoxygenation has been shown to prolong the time of desaturation by approximately 50 seconds.[48]

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.[49] An advantage of this approach is the maintenance of airway patency during spontaneous breathing until an "unhurried" tracheal intubation can be accomplished. Face mask preoxygenation should precede intubation attempts and should be continued with the placement of a nasal cannula or an O2 catheter in the oropharynx. O2 flow (up to 5 L/min) through the working channel of the scope has the double advantage of insufflating O2 and enhancing laryngeal visualization by preventing fogging and pushing secretions away. It is important to recognize that airway obstruction can hinder the egress of gases from the fiber-optic scope, which, if prolonged, can result in barotrauma. Thus, caution cannot be overemphasized when this approach is utilized. Techniques to enhance preoxygenation, which are described later, are especially important in morbidly and supermorbidly obese patients.

Pediatric Patients

Studies have demonstrated that maximal preoxygenation (EtO2 = 90%) can be accomplished in children faster than in adults.[50,51] With tidal volume breathing, an EtO2 of 90% can be reached within 100 seconds in almost all children, whereas with deep breathing, it can be reached in 30 seconds.[50,51] Nevertheless, because children have a smaller functional residual capacity and a higher VO2 than adults, they are at a greater risk for developing hypoxemia, when there is interruption in O2 delivery, such as during apnea or airway obstruction.[52–54] In a comparison of 3 groups of children who breathed O2 (Flo2 = 1.0) with tidal volume breathing for 1, 2, and 3 minutes before apnea, the time needed for SaO2 to decrease from 100% to 95% and then to 90% during apnea was least in those who breathed O2 for 1 minute and there was no difference between those who breathed O2 for 2 and 3 minutes.[55] Based on these findings, 2 minutes of preoxygenation with tidal volume breathing seems sufficient for a maximum benefit and to allow a safe period of apnea.[55] The benefit of preoxygenation is greater in an older child than that in an infant. For example, in an 8-year-old child, the duration of safe period of apnea can be extended from 0.47 minute without preoxygenation to 5 minutes or longer with preoxygenation.[56] The younger the child, the faster the onset of desaturation.[53,54,57] Most infants reach a SaO2 of 90% in 70 to 90 seconds after the onset of apnea (in spite of preoxygenation),[58] and this period can be even shorter in the presence of upper respiratory tract infection.[59] Pediatric anesthesiologists have expressed concerns about the use of the "adult" version of the rapid sequence induction/intubation technique in children.[60] The concerns include the safe duration of apnea and the potential for cricoid pressure-induced airway obstruction. A modified version of the rapid sequence induction/intubation technique, with emphasis on complete muscular relaxation, gentle manual ventilation using high O2 concentration without cricoid pressure and sufficient anesthetic depth before intubation would seem more appropriate in children.[61]

Elderly Patients

Aging is associated with significant structural and physiologic changes in the respiratory system.[62,63] The changes include weakened respiratory muscles and parenchymal alterations within the lungs accompanied by a decrease in the elastic recoil. Lung volumes are decreased with increased closing volume, resulting in ventilation—perfusion mismatch, a reduced pulmonary reserve, and an impaired O2 uptake at the lung. Even though basal VO2 decreases with aging, the impaired O2 uptake produces a more rapid desaturation during apnea under anesthesia.[63] In elderly patients, tidal volume breathing for 3 minutes or longer has been shown to be more effective than the 4 deep breathing technique.[64,65]

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.[66] Even brief interruption of ventilation, such as during suctioning can result in significant desaturation. However, atelectasis is not a consequence, possibly because chronic hyperinflation of the lungs resists volume decrease and collapse.[67] Maximal preoxygenation, which is essential in these patients, may require as much as 5 minutes or longer with tidal volume breathing.[68]

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.[69] For example, in Flagstaff, AZ, with an altitude of 2100 m (approximately 7000 feet above sea level), PaO2 is reduced from the normal value of 100 mm Hg to approximately 74 mm Hg.[69] As altitude increases, PaO2 decreases exponentially. No studies, to our knowledge, have been performed to assess the effect of high altitude on preoxygenation. This is difficult to predict because of the multiple determinants of preoxygenation and the potential influence of compensatory mechanisms, especially in acclimatized individuals. It is possible that patients at high altitude will require a longer duration of preoxygenation to achieve an acceptable degree of protection, but this remains to be confirmed experimentally.

Comments

3090D553-9492-4563-8681-AD288FA52ACE

processing....