Apnea during ETI is usually well tolerated in patients undergoing elective surgical procedures. The rise in PCO₂ during apnea is nonlinear, but approximates 3–4 mmHg ∙ min⁻¹. The rate of PCO₂ rise will be much greater if the patient has an increased metabolic rate. Fever, tissue injury, and systemic inflammation associated with acute illness can greatly increase metabolic rate and CO₂ production. Therefore, clinicians should anticipate complications of ETI in patients with a high metabolic rate and a disease process that makes them intolerant of hypercapnia. In these patients, continuing ventilation during the intubation process may be beneficial.

NIV can improve gas exchange during ETI by cyclically augmenting the transpulmonary pressure gradient to maintain alveolar ventilation. NIV can maintain a satisfactory level of ventilation, even during deep sedation, as demonstrated in a case series of its application in procedural anesthesia [3]. In patients undergoing intubation during spontaneous breathing, NIV can theoretically decrease the work of breathing and improve ventilation by augmenting tidal volumes.

Traditionally, clinicians perform intubation after induction of anesthesia. After neuromuscular blockade, the lungs recoil to the functional residual capacity (FRC). During apnea, gas exchange continues as mixed venous blood continues to flow to the alveolar capillary bed. Therefore, the FRC is the air reservoir that supplies the circulation with oxygen during apnea. Both the volume of air in the FRC as well as its partial pressure of oxygen determine how quickly arterial desaturation occurs during apnea. The rate of total oxygen consumption in the tissues as well as the magnitude and distribution of pulmonary blood flow also determine the time to desaturation. To lengthen the time before desaturation, clinicians have patients spontaneously breathe high FiO₂ to fill the FRC with pure oxygen. This preoxygenation is effective in patients presenting for anesthesia for elective surgery [4]. However, standard methods of preoxygenation are ineffective due to the deranged physiology of critically ill patients. In critically ill patients, atelectasis commonly reduces FRC and increases the right-to-left shunt fraction. Shock may reduce pulmonary blood flow. Moreover, the high oxygen consumption during acute illness can reduce mixed venous oxygen saturation. All of these factors lead to hypoxia during ETI. Patients with an oxygen saturation less than or equal to 93 % during preoxygenation uniformly desaturate to <90 % during intubation [5].

A number of investigators have shown that NIV may be superior to standard preoxygenation in appropriate scenarios. Through positive pressure, NIV can recruit atelectatic lung in some patients, thereby increasing the size of the FRC and reducing shunt fraction. It can also decrease the work of breathing during spontaneous ventilation. Moreover, by increasing alveolar ventilation, NIV can increase the alveolar oxygen content by CO₂ washout. Finally, NIV can decrease the excessive work of breathing made by critically ill patients. Theoretically, this can raise mixed venous oxygen saturation and improve the effectiveness of preoxygenation. The recruitment and benefit of preoxygenation with NIV can continue even after ETI is completed [6].

Given the success of NIV for preoxygenation, investigators have used NIV during ETI to prevent desaturation. NIV can be a form of apneic oxygenation. If the airway is patent, any high-flow oxygen device can promote the replacement of alveolar gas that flows into the alveolar capillary beds through bulk flow. Therefore, high-flow nasal cannula (HFNC) can be used as an apneic oxygenation method, leading to reduced prevalence of severe hypoxemia during intubation of critically ill patients with mild-to-moderate hypoxemia in a pilot study [7]. However a randomized controlled trial found that HFNC during emergent ETI of patients with severe hypoxemia did not prevent desaturation [8]. It is possible that high-flow devices have limited benefits during ETI because they provide little (if any) lung recruitment and ventilation. NIV during intubation via a nasal interface could theoretically promote lung recruitment through positive end-expiratory pressure (PEEP). The possible benefits of PEEP are especially important because anesthesia and supine positioning cause atelectasis and promote desaturation during ETI.

The ability of NIV to promote alveolar recruitment may be especially important because of denitrogenation. Preoxygenation with high FiO₂ effectively removes inert nitrogen from the alveolar air spaces and blood. Unfortunately, nitrogen normally acts as a pneumatic splint to maintain the patency of unstable lung units. Without nitrogen, the partial pressure of gas is very low in mixed venous blood returning to capillaries. Denitrogenation, therefore, increases the gas pressure gradient from alveoli compared with alveolar capillary blood. After denitrogenation, oxygen will rapidly flow from the alveoli into the capillaries, leading to alveolar instability and atelectasis, a physiological phenomenon also demonstrated on computed tomography imaging [9]. PEEP facilitates alveolar recruitment, which can counteract the adverse effect of denitrogenation on lung recruitment and improve oxygenation. Additionally, nasal NIV during ETI can also aid oxygenation by partially supporting ventilation.

The principle that oxygenation can be supported by nasal NIV throughout the process of intubation has been demonstrated for bronchoscopic [10] and direct laryngoscopic intubation [11].

During deep sedation, airway obstruction commonly occurs at the level of the soft palate. Anesthesia reduces the tone of the pharyngeal muscle dilators, which narrows the anteroposterior diameter of the airway. These effects are most prominent in the supine position. This pattern of airway obstruction in anesthesia is similar to obstructive sleep apnea. In both situations, the application of nasal NIV can relieve the airway obstruction. Nasal NIV can displace the soft palate anteriorly, which partially prevents the leakage of air out the oropharynx. Incremental increases in positive airway pressures lead to a linear increase in airway area at a given airway level. Therefore, nasal NIV can serve as a pneumatic splint for the oropharynx during anesthesia. Pilot data demonstrate that this splinting can aide ETI. Figure 68.1 shows fiber-optic video images from a morbidly obese patient’s hypopharynx. Nasal NIV (continuous positive airway pressure (CPAP) 20 cm H₂O) relieved upper airway obstruction during fiber-optic-guided nasotracheal intubation [12].