The Physiologically Difficult Airway: Hypoxemia

Jarrod M. Mosier

INTRODUCTION: SAFE APNEA

Rapid sequence intubation (RSI) requires an apneic interval from the cessation of spontaneous breathing on induction until intubation and mechanical ventilation resume “breathing” for the patient. This apneic time requirement can vary depending on the patient’s difficulty, the operator’s skill, and unforeseen challenges such as needing a smaller tube size. Safety with airway management in patients undergoing RSI involves a “safe apnea” time sufficiently long to maintain oxygen saturation during laryngoscopy and intubation. This safe apnea time is the entire purpose of preoxygenation, and desaturation during intubation is a major source of airway management-related harm and death,¹^,² which increases with successive attempts.²^,³^,⁴^,⁵ A recent study identified independent risk factors for desaturation in patients enrolled in randomized clinical trials.⁶ They identified the following independent risk factors: hypoxemic respiratory failure as the indication for intubation [Odds Ratio (OR) 2.70], lower oxygen saturation at induction (OR 0.92 per 1% increase [above 95%]), younger age (OR 0.97 per 1-year increase), higher body mass index (OR 1.03 per 1 kg/m² [above 23 kg]), race (OR 4.58 for White vs. Black), and operator experience (OR 2.83 if <100 intubations). Thus, hypoxemia is an important cause of the physiologically difficult airway and is critically important to address for all intubations. Preoxygenation is as important, if not more, for the procedure as any other single variable. The goal with preoxygenation is to provide an adequate safe apnea duration, and apneic oxygenation’s goal is to extend that time with continuous oxygen delivery during apnea. This chapter will explore hypoxemia as it relates to the physiologically difficult airway to inform the airway management strategy, while Chapter 17, Principles of Peri-Intubation Oxygenation will go into further detail for each method of preoxygenation.

PREOXYGENATION

The goal of preoxygenation is to build an oxygen reservoir for the patient to draw upon during apnea and maintain oxygen saturation. Successful preoxygenation is dependent on three components: (i) a volume of gas to work with (functional residual capacity [FRC]), (ii) replacement of that gas with oxygen (denitrogenation), and (iii) availability of that volume to the pulmonary circulation (minimized ventilation/perfusion [V/Q] mismatch and shunt). One or a combination of abnormalities of those necessary requirements limits the goal of achieving time for laryngoscopy, intubation, and initiation of mechanical ventilation. For example, patients with severe Acute Respiratory Distress Syndrome (ARDS) can be easily denitrogenated, but a small volume FRC and low V/Q (high shunt) dramatically reduces the time available for intubation. High oxygen consumption also increases the rate at which that reservoir of oxygen is consumed. This is primarily driven by the patient’s illness and is difficult to manipulate in the peri-intubation period. Preventing desaturation is a critical step for the safety of emergency airway management. “Racing” hypoxemia by attempting a fast intubation as a “forced to act,” while tempting in a stressful situation, is fraught with danger for many patients such as refractory hypoxemia and ARDS. Although you may get lucky and the oxygen saturation would not have dropped critically during the attempt, it could plummet even before intubating conditions are created by the RSI drugs. If any unexpected delay occurs (e.g., challenging, or prolonged laryngoscopy), this may be the tipping point into a bradycardic arrest. The safest option is to optimize the three necessary variables for preoxygenation, and the preoxygenation strategy is often stratified by the rate-limiting step (Fig. 7.1).⁷^,⁸^,⁹

Figure 7.1: Stratified preoxygenation based on rate-limiting step.

Patients lie along a distribution of risk of desaturation (x-axis). A normalized bell curve is shown as a representative example (solid line). The shape of the bell-curve distribution will likely be skewed by the clinical setting. For example, in the ED, many patients are started on noninvasive respiratory support as a first-line therapy, so the distribution of risk may be skewed such that a higher density of incidence has a rate-limiting step of denitrogenation and relatively low risk of desaturation (dashed line). In the ICU, many patients are intubated because of failed noninvasive respiratory support, so that distribution may be skewed with a higher density of incidence of low FRC and high shunt, and a large proportion with refractory hypoxemia (dashed dotted line). Preoxygenation strategies have varying efficacy based on the underlying physiology and risk of desaturation. In patients at low-to-moderate risk of desaturation, usually those with a large FRC and low shunt fraction, denitrogenation is the rate-limiting step. Flush flow oxygen in the upright position with mask ventilation between induction and laryngoscopy if a low risk of aspiration, and apneic oxygenation should allow for a prolonged safe apnea time for RSI. In patients at moderate-to-high risk, the rate-limiting steps are mostly reduced FRC and as severity increases, increasing shunt fraction. More advanced preoxygenation (e.g., NIPPV, HFNO), in the ramped positioning to safely proceed with RSI, although with a shorter safe apnea time. For patients at the highest risk with refractory hypoxemia, a high shunt fraction may make safe apnea impossible. An awake, spontaneous breathing approach is likely the safest approach. HFNO, high-flow nasal oxygen; iNO, inhaled nitric oxide; NIPPV, noninvasive positive pressure ventilation; RSI, rapid sequence intubation; V/Q, ventilation/perfusion.

Denitrogenation

Denitrogenating the FRC and replacing it with oxygen will maximize the potential amount of oxygen available during apnea. Denitrogenation can be accomplished with 3 minutes of tidal breathing or eight vital capacity breaths if the patient is breathing on a closed circuit and 100% FIO₂ (assuming a normal respiratory rate and tidal volume). In most EDs and ICUs, closed circuits are rarely available and most oxygen sources available using conventional oxygen therapy involve loosely fitting reservoir-based nonrebreather masks. Although the oxygen flowing into the reservoir is constant, which increases the FIO₂ and volume available during inspiration, the inspiratory flow rate generated by the patient leads to room air entrainment around the nonrebreather mask, contaminating the inspired volume with ambient air. As the inspiratory flow generated by the patient often increases with the severity of respiratory failure, ambient air entrainment worsens as respiratory effort increases. The result is that ambient room air dilutes the FRC, and the severity of this dilution increases in patients that need to increase FIO₂ the most.

Step 1: Optimize denitrogenation. The best way to compensate for the entrainment of ambient room air in the absence of a closed circuit is to use “flush-flow rate” through a wide-open valve
(see Chapter 17). The more pressurized the hospital’s oxygen system, the higher the flow will be achieved at flush rate. Flush rate can be as high as 90 L/min, which provides enough flow to more closely approximate the inspiratory flow rate of the critically ill patient with a high respiratory demand breathing supplemental oxygen with an open circuit. One important consideration is that any spontaneous breaths after removing the source of oxygen can result in rapid renitrogenation, thus the patient should be fully apneic before removing the oxygen source.

The effect of denitrogenation can be prolonged, theoretically indefinitely, with the application of oxygen continuously to the nasopharynx during apnea. The ability to maintain alveolar oxygenation during apnea, or apneic oxygenation, is partially contingent upon the pressure gradient between the nasopharynx and the alveoli, which is a function of the amount of flow applied and the rate of peripheral oxygen consumption. Thus, apneic oxygenation performed using high-flow nasal oxygen (HFNO) systems is expected to perform better than apneic oxygenation using 10 to 15 L/min from a standard nasal cannula. Apneic oxygenation has been shown to lengthen the period of safe apnea and increase first-attempt success (secondary to more laryngoscopy time), although it does not perform as well in patients with significant V/Q mismatch.

Functional Residual Capacity

As there is no tidal breathing during apnea, by definition, the reservoir of gas available is that which is present in the lungs at end-expiration and is the FRC. The FRC is dependent on height and age. In a healthy adult, the FRC is roughly 25 to 30 mL/kg, resulting in around 2 L in a 70-kg adult. Anything that compresses or fills the alveoli will reduce the FRC. Thoracic or abdominal fat, ascites, or a late-term gravid uterus all compress the alveoli externally, whereas hemorrhage, pneumonia, edema, and ARDS all obliterate airspace internally. In patients undergoing intubation for pneumonia or other causes of hypoxemia, or in patients with ascites, late-term pregnancy, pleural effusions, and obesity, the FRC is often the limiting factor in preoxygenation. In patients with ARDS, the FRC decreases in proportion to the severity of the airspace disease, to as few as 5 to 10 mL/kg in patients with a PaO₂/FiO₂ ratio <100.¹⁰

Step 2: Improve the FRC. This includes easier things to reduce external compression like upright positioning to get the weight of the abdomen off the chest. Moderately difficult interventions include those aimed at reducing compression from the pleural space, such as draining pleural effusions, pneumothoraces, or hemothoraces. Difficult interventions are those that improve FRC by recruiting alveoli or reducing pulmonary edema by using positive end-expiratory pressure (PEEP), HFNO (end-expiratory lung volume, PEEP-like effect), and diuresis. These are more difficult as not all airspace disease is recruitable and not all extravascular lung water is diuresable.

V/Q Mismatch

A fully denitrogenated 70-kg patient with an FRC of 2 L and an oxygen consumption of 250 mL/min should have several minutes of safe apnea (defined as time to desaturate to 90%), which can be illustrated by the following equation¹¹: