Preoxygenation

Introduction

• Peri-intubation or extubation hypoxia has been associated with serious complications, such as cardiac arrest.¹
• Preoxygenation serves to extend the safe apneic time during airway manipulation by increasing the percentage of the functional residual capacity (FRC) that is composed of oxygen.^2,3
• Under normal circumstances, the gaseous composition of the lungs closely mimics the atmosphere, with nitrogen accounting for most of the lung volume (~78%) and oxygen making up a smaller fraction (~21%). Preoxygenation allows oxygen to replace most of the nitrogen, creating a buffer to limit hypoxia during periods of apnea and airway manipulation, such as those seen during intubation and extubation. The terms preoxygenation and denitrogenation are used synonymously.^2,3
• During apnea, the arterial partial pressure of oxygen decreases steadily as gas exchange continues. As long as hemoglobin can obtain oxygen in the lungs, the arterial hemoglobin oxygen saturation (SaO₂) is preserved.^2,3 Once this oxygen store is depleted, SaO₂ falls rapidly at a rate of approximately 30% every minute.²
• Increasing a patient’s FiO₂ prior to periods of apnea increases the time to hemoglobin desaturation (Table 1).

• Preoxygenation is especially important when difficulty with intubation is anticipated.³ Preoxygenation is a crucial step cited in the 2022 American Society of Anesthesiologists Practice Guidelines for Management of the Difficult Airway.⁴ Given that difficulty with intubation/ventilation can be unpredictable, preoxygenation for every patient to an end-tidal oxygen concentration of 90% or higher can be helpful in all patient populations and should be employed wherever possible. Preoxygenation is likewise important in cases where mask ventilation is contraindicated, such as planned rapid sequence intubation (RSI).³

Stages of Preoxygenation

• When using a semi-closed circle absorber circuit, there are two stages of preoxygenation (Table 2).³
  • The first stage involves the washout of the circuit itself. This is achieved through high oxygen flow rates prior to application to the patient.³
  • The second stage refers to the washout of the patient’s FRC. This is achieved through the patient’s alveolar ventilation coupled with oxygen flow rates that eliminate rebreathing.³
• The standard definition of maximal preoxygenation is an end-tidal O₂ concentration of 90% and an end-tidal N₂ concentration of 5%.³

Factors Affecting Preoxygenation

• The factors that affect preoxygenation efficiency and efficacy include FiO₂, duration of preoxygenation, and the alveolar ventilation/FRC ratio (Table 3).³
  • Commonly faced problems include leakage around a face mask, rebreathing of exhaled gases, the use of resuscitation bags that are incapable of delivering a high FiO₂, and patient cooperation.³
    • Common causes of mask leakage include edentulous status, beards, facial tubing (i.e., nasogastric tubes), or improper mask size.³

Techniques of Preoxygenation

• Several preoxygenation techniques exist, with similar efficacy when considering the limitations associated with the chosen technique (Table 4).³
  • Deep breathing techniques can be influenced by flow rate even at higher flows (i.e., going from a flow rate of 5 to 10 L/min), while this makes a negligible difference when using a tidal volume technique. This is because when lower flow rates are utilized with the deep breathing technique, the minute ventilation may exceed the fresh gas flow, increasing the rate of rebreathing.³ This is unlikely to occur during normal tidal volume breathing patterns.

Methods of Preoxygenation

Standard Mask or Non-rebreather

• A standard mask or non-rebreather mask can be used to deliver 100% oxygen with a flow rate of 10 L/min or more.
• They are commonly utilized due to availability, ease of use, and adequacy in most patient populations.
• The clinician can remain hands-free when using a mask with mask straps.
• They enable the monitoring of end-tidal oxygen and carbon dioxide concentrations.
• They can be supplemented with positive end-expiratory pressure or manual ventilation as needed
• They may be inadequate in critically ill patients and other special patient populations.

High Flow Nasal Cannula (HFNC) (Figure 1)

• HFNCs can be used to deliver 100% oxygen at flow rates of up to 70 L/min.
• They allow pharyngeal dead space washout.³
• HFNCs can reduce the work of breathing and improve mucociliary clearance.
• HFNCs are relatively comfortable and can be used in conjunction with a standard face mask.
• There is no positive pressure ventilation capability.
• HFNCs’ use in critically ill patients has been shown to be superior to standard masks in hypoxia outcomes.⁵
• They can be combined with the transnasal humidified rapid insufflation ventilatory exchange (THRIVE) technique.
  • THRIVE combines the benefits of apneic oxygenation with continuous positive airway pressure (CPAP) with a reduction in carbon dioxide levels through gaseous mixing and flushing of the dead space.³ A CPAP of approximately 7 cm H₂O splints the upper airways and reduces shunting.³
• Clinicians should consider using HFNCs in patients with obesity, critically ill patients, or those at the highest risk for difficult intubation.

Noninvasive Positive Pressure Ventilation (CPAP, Bilevel Positive Airway Pressure [BiPAP]) (Figure 2)

• Noninvasive ventilation (NIV) can be used to deliver 100% oxygen.
• It provides higher levels of positive end-expiratory pressure than a standard mask for THRIVE methods and can help with the recruitment of atelectatic lung tissue.
• NIV reduces the patient’s work of breathing.
• NIV use in critically ill patients has been shown to be superior to a standard mask in hypoxia outcomes.⁵
• The FRC expansion benefits can be lost with the removal of the mask.³
• NIV cannot be combined with a standard mask.
• NIV should be considered in patients with obstructive sleep apnea, chronic obstructive pulmonary disease, or pulmonary edema.
• Please see the OA summary on noninvasive ventilation for more details. Link

Apneic Oxygenation (Figure 3)

• Apneic oxygenation involves ongoing exposure to high FiO₂ during periods of apnea. Oxygen is typically administered via a nasal cannula (standard or HFNC) during an intubation attempt.
• It can increase time to significant hypoxia to up to 15 minutes in patients with a high predicted FRC/body weight ratio.³
  • Some studies have shown a lengthening of time to SaO₂ of less than 90% to up to 100 minutes in patients lacking airway obstruction.
• Apneic oxygenation can be particularly helpful in cases of difficult intubation/ventilation, patients with limited oxygen reserves, and patients with high oxygen consumption rates.³
• This method requires either an unobstructed airway or insertion of a needle through cricothyroid membrane, which can be invasive.
  • The nasal cannula technique may create issues with mask seal if combined with the standard mask preoxygenation method, which could compromise the active preoxygenation that precedes the apnea time.

Specific Patient Population Considerations

Pregnant Patients

• RSI is recommended in pregnant patients due to physiological changes associated with pregnancy.
• Pregnant patients can achieve maximal oxygenation more rapidly secondary to high alveolar ventilation and reduced FRC.³
• SaO₂ falls more quickly due to reduced FRC and higher oxygen consumption rates.³

Morbidly Obese Patients

• The time required for SaO₂ to fall to 90% or less during apnea is reduced in morbidly obese patients (2.7 min in morbidly obese patients compared to 6 min in non-obese patients).³ This is likely due to a decreased FRC and increased oxygen consumption rate.³
• These patients often have obstructive sleep apnea which may make other aspects of induction/intubation difficult.

Pediatric Patients

• Pediatric patients may be able to achieve maximal preoxygenation, characterized by an end-tidal oxygen concentration of 90% or higher, faster than adults.
• They have a smaller FRC and higher oxygen consumption rate.³
  • They are prone to rapid desaturation, which can be associated with significant hemodynamic compromise, including bradycardia requiring advanced cardiopulmonary resuscitation. This desaturation time is indirectly correlated with age.³
• Pediatric patients are less responsive to hypoxia improvement maneuvers.
• They are prone to upper respiratory infections, which may shorten the time required for significant desaturation during apnea.
• Preoxygenation may be limited by patient cooperation and agitation state.

Elderly Patients

• Advanced age is associated with weakened respiratory muscles and a decrease in elastic recoil.
• Elderly patients have decreased lung volumes and increased closing volumes, resulting in a ventilation-perfusion mismatch, reduced pulmonary reserve, and impaired O₂ uptake.³
• They also have a reduced oxygen consumption rate.³

Patients With Pulmonary Disease

• Patients with pulmonary disease may require longer preoxygenation times for maximal oxygenation of FRC.
• Pulmonary disease is often associated with decreased FRC, ventilation-perfusion mismatch, and increased metabolic consumption of oxygen rate.³
  • In some cases, depending on disease and severity, this can decrease patient’s time to significant hypoxia during apnea events.
• General anesthesia may worsen impairment of gas exchange seen in chronic obstructive pulmonary disease.³

Patients With Obstructive Sleep Apnea

• Patients with obstructive sleep apnea are independently associated with difficult airway and mask ventilation.
• There is an increased susceptibility to airway collapse and respiratory depression.⁶
  • This is worsened by many medications administered during the perioperative period.
  • These patients may require higher levels of hypoxia and hypercapnia prior to the initiation of their own natural ventilatory response.
• There is an increased difficulty with apneic oxygenation strategies (i.e. nasal cannula, HFNC).³
• There is a higher risk of significant hypoxia during extubation.
• Extubation to CPAP or BiPAP should be considered and continued in the postanesthesia care unit in appropriate patient populations.⁶