Preoxygenation and Apneic Oxygenation



Preoxygenation and Apneic Oxygenation


Brian E. Driver

Robert F. Reardon



INTRODUCTION

Hypoxemia during airway management in critically ill patients is associated with dysrhythmias, hypoxic brain injury, and cardiac arrest. Hypoxemia can often be avoided by meticulous preoxygenation. Preoxygenation is critical because modern airway management most often uses rapid sequence intubation, which renders the patient apneic and paralyzed while laryngoscopy and tube delivery are performed. In most studies of modern airway management, this duration is between 1 and 2 minutes, however, in difficult intubations, this apneic duration can be prolonged. Critical hypoxemia often occurs when clinicians focus primarily on laryngoscopy and tube placement during the apneic period rather than oxygenation, and is particularly common in stressful or difficult intubations where task fixation predominates over situational awareness.

The principles of preoxygenation are simple but are often poorly understood and imprecisely applied. First, clinicians performing airway management must know that the main goal is continued gas exchange. While the ultimate goal is to restore gas exchange with mechanical ventilation after intubation, it is much more important to prevent hypoxemia during the intubation. Second, adequate preoxygenation requires a fraction of inspired oxygen (FiO2) of 1.0 (i.e., 100% oxygen), which can only be achieved with (1) a system that includes a tightly sealed mask (e.g., noninvasive positive pressure ventilation [NIPPV], an anesthesia circuit, a continually held perfectly sealed bag-mask device), or (2) a system with high oxygen flow rates (e.g., nonrebreather mask at flush-rate oxygen >40 L/min, high-flow nasal oxygen [HFNO]).1,2

Optimal preoxygenation increases the safe apnea time, which is defined as the duration from when the patient stops breathing to when the oxygen saturation falls below 90%. Safe apnea time varies from several seconds to several minutes, depending on body habitus, comorbidities, acuity of illness, oxygen consumption, and oxygen reservoir created through preoxygenation efforts (Fig. 17.1). Increasing the safe apnea time not only reduces the risk of hypoxemia, but also provides more time to perform unrushed and methodical laryngoscopy and endotracheal tube placement. Conversely, if preoxygenation is omitted or performed poorly, the oxygen saturation will drop more quickly which causes intubation to feel rushed and frantic. In addition, the concept of safe apnea time implies oxygen saturations in the mid-to-high 90s percent range. Patients who cannot achieve a saturation that high represent the physiologically difficult airway because safe apnea time will be limited or impossible. In critically ill patients, preoxygenation needs to be stratified based on the underlying physiologic abnormalities (Chapter 7, The physiologically difficult airway: hypoxemia).







PRINCIPLES OF PREOXYGENATION

The main goal of preoxygenation is to create an adequate reservoir of oxygen available to resaturate hemoglobin in the pulmonary circulation during apnea. The volume available for this oxygen
reservoir is the volume of gas in the lungs at end-expiration or, in this case, apnea, which is the patient’s functional residual capacity (FRC) and is ˜30 mL/kg in adult patients. Thus, positioning plays an important role in maximizing the size of that reservoir because the FRC is greatest when patients are in the upright position and lowest in the supine position. All patients should be situated in an upright position to maximize the effectiveness of preoxygenation. This position allows full utilization of a patient’s FRC. Those who cannot tolerate the upright position (e.g., those with spinal precautions) should be preoxygenated in a reverse Trendelenburg position. Upright or head-up positioning during preoxygenation is especially important in obese patients, who are prone to rapid desaturation during apnea, and in patients whose abdominal mass further reduces the size of the FRC (Fig. 17.1). Patients with lung pathology and decreased FRC from internal volume loss, rather than external compression, usually require the addition of positive pressure or HFNO for maximal preoxygenation.

The cornerstone of preoxygenation involves replacing mixed alveolar gases—mostly nitrogen—with oxygen. This process is also called denitrogenation. If the FRC is denitrogenated, oxygen becomes the predominant alveolar gas and, if measured, the proportion of exhaled gas that is oxygen (i.e., fraction of expired oxygen [FeO2] or ETO2 if measured breath-by-breath) would be near 90%—it cannot reach 100% because of exhaled CO2 and water vapor. Tidal breathing 100% oxygen for 3 to 5 minutes will adequately denitrogenate patients with healthy lungs. Alternatively, cooperative patients with healthy lungs can be preoxygenated by having them perform eight maximal volume deep breaths (i.e., vital capacity breaths) while breathing 100% oxygen. It is very important to understand the differences in the fraction of inspired oxygen (FIO2) provided by commonly used oxygen delivery systems to ensure the patient is breathing 100% oxygen (FIO2 1.0) (Tables 17.1 and 17.2).















The preoxygenation device used should be left in place until the laryngoscope blade enters the mouth. Patients rapidly lose the pulmonary reservoir of oxygen if they breathe even two to three breaths of room air after preoxygenation.3 At the onset of apnea the preoxygenation device may be removed if performing bag-mask ventilation during the apneic period.

There are widespread misconceptions about the FIO2 supplied by common oxygen delivery devices. Optimal preoxygenation requires delivery of 100% oxygen, but many common oxygen delivery methods do not provide an FIO2 of 1.0 (Table 17.1). Although the oxygen flowing from
the tubing is 100% oxygen, the patient’s inspiratory flow rate exceeds the delivered oxygen flow rate resulting in that 100% oxygen from the tubing being diluted by nitrogen from ambient air, dropping the FIO2 closer to ambient air by a degree proportional to the work of breathing. In other words, the higher the inspiratory flow requirement, the further the FIO2 drops toward ambient air for a fixed oxygen flow rate. As a result, the primary limitations of conventional oxygen delivery methods are the low oxygen flow rates used (≤15 L/min) and the presence of significant mask leaks. Note that the bag-valve mask (BVM) and nonrebreather mask are both listed in Table 17.1 as inadequate for preoxygenation if improper technique or inadequate oxygen flow is used.

However, both the BVM and nonrebreather mask can provide 100% FIO2 with proper technique (Table 17.2), principally by ensuring a tight mask seal and one-way valves are present when using a BVM, and by using very high oxygen flow rates with a nonrebreather mask.

Using a nonrebreather mask with a low flow rate (<30 L/min) results in inadequate preoxygenation because of ambient air entrainment, even in the presence of a full reservoir bag. Patients typically have inspiratory flow rates higher than the commonly set flow rate of 15 L/min, and some patients breathe at a higher minute ventilation than the total oxygen delivered over one minute. This means that even if the patient breathes in every molecule of oxygen delivered from the reservoir during each inhalation phase, each inhalation still contains a substantial amount of ambient air. Thus, the inspired FIO2 decreases as the patient’s minute ventilation and inspiratory flow increase and ambient air makes up the difference.

Using a nonrebreather mask with a high flow rate (≥40 L/min), conversely, can provide a high concentration of oxygen. High flow rates are easily achieved with standard equipment by opening a standard flow meter to the “flush” rate (˜50 L/min). This delivers an FIO2 closer to 1.0. Table 17.2 lists systems and accompanying flow rates that can deliver high FIO2 in the setting of high minute
ventilation and inspiratory flow, and, for some devices, regardless of mask seal. The key is to deliver 100% oxygen at a flow rate well above the patient’s inspiratory needs so that ambient air volume is not required to satisfy the patient’s inspiratory effort. The oxygen flow rate is likely more important than the oxygen delivery device. Although high flow rates can be noisy, they allow delivery of high-concentration oxygen using a nonrebreather face mask or nasal cannula.


OXYGEN DELIVERY DEVICES

Many devices can provide adequate preoxygenation if used properly but can also perform poorly if not used properly. Other devices should never be used for routine preoxygenation. The three most practical and commonly used preoxygenation methods are NIPPV, HFNO, and a nonrebreather mask at flush-rate oxygen. Preoxygenation methods should be stratified for each individual patient based on underlying physiologic rate limiting step, as shown in Figure 17.2. However, some patients are minimally conscious, unconscious, or apneic, which limits the preoxygenation options. A mental framework on how to approach preoxygenation that accounts for these scenarios and can guide the preoxygenation strategy in the context of practial limitations is provided in Fig. 17.3.