• David Misita, MD

I.	INTRODUCTION Relevant Anatomy
II.	TECHNIQUES FOR ANESTHETIZING THE AIRWAY Preparation for Awake Intubation Topical Anesthesia of the Nose, Mouth, Tongue, & Pharynx Topicalization by Use of Local Anesthetic Reservoirs Inhalation of Aerosolized (Atomized) Local Anesthetic
III.	TECHNIQUES FOR BLOCKING INDIVIDUAL NERVES OF THE AIRWAY Blockade of the Glossopharyngeal Nerve Superior Laryngeal Nerve Block Recurrent Laryngeal Nerve Block Blockade of the Palatine Nerves Blockade of the Anterior Ethmoid Nerve Step-by-Step Method for Orotracheal Fiberoptic Intubation Using Topical Anesthesia Only
IV.	SUMMARY

INTRODUCTION

Recent developments in regional anesthesia have resulted in a number of innovative and refined options to practitioners, often allowing regional techniques to be used for patients with presumed difficult airways. However, not every surgery can be performed under regional anesthesia. In addition, even in the hands of the most skilled regional anesthesiologist, blocks are subject to a certain rate of complications or failure.^1–4 In addition, there are many situations in which the anesthesiologist is called on to secure an airway in less than ideal circumstances. Expertise with regional anesthesia of the airway allows intubation in awake patients with suspected difficult intubation, upper airway trauma, or cervical spine fractures. Therefore, it is essential that every regional anesthesiologist be skilled in the administration of general anesthesia and especially in the management of the difficult airway.

In recent years, there have been many advances in difficult airway management. The introduction of the laryngeal mask airway, and later the intubating laryngeal mask airway have changed the American Society of Anesthesiologists’ difficult airway algorithm significantly.⁵ Despite new devices and techniques being added to the arsenal daily, the mainstay of difficult airway management remains flexible fiberoptic laryngobronchoscopic intubation. Fiberoptic intubation can be performed under a variety of conditions. However, one major decision must be made with every procedure: Will the patient be intubated while under general anesthesia, or does the patient need to be awake during intubation?⁶ Intubation under general anesthesia (even with inhalational induction and spontaneous respiration) carries the inherent risk of losing control of the difficult airway. For this reason, many anesthesiologists, on recognition of a difficult airway, elect to perform an awake intubation using either fiberoptic laryngobronchoscopy or awake direct laryngoscopy.

Direct laryngoscopy in an awake, unprepared patient can be extremely challenging. Excessive salivation and gag and cough reflexes can make intubation difficult, if not impossible, under awake conditions. In addition, the stress and discomfort may lead to undesirable elevations in the patient’s sympathetic and parasympathetic outflow. Several highly effective topical and regional anesthesia techniques have been developed to subdue these reflexes and facilitate intubation. Each of these techniques has the common goal of reducing sensation over the specific regions that will be encountered by the fiberoptic bronchoscope and endotracheal tube.

Relevant Anatomy

To decide on a proper approach to an awake fiberoptic intubation, one must determine what structures need to be anesthetized along the two basic routes of intubation (oral or nasal) to facilitate optimal surgical conditions in the context of patient-specific anatomic considerations. Each of these routes has a well-defined pattern of innervation that can be specifically blocked to provide adequate anesthesia.

The nasal cavity is innervated by the greater and lesser palatine nerves and the anterior ethmoidal nerve. The palatine nerves arise from the trigeminal nerve via the pterygopalatine ganglion and innervate the nasal turbinates and most of the nasal septum. The pterygopalatine ganglion is located posterior to the middle turbinate in the pterygopalatine fossa. The anterior ethmoidal nerve arises from the olfactory nerve (CN I) and innervates the nares and the anterior third of the nasal septum.⁷

The oropharynx is innervated by branches of the vagus, facial and glossopharyngeal nerves (Figure 19-1). These glossopharyngeal nerves travel anteriorly along the lateral surface of the pharynx, and the three branches provide sensory innervation to the posterior third of the tongue,⁸ the vallecula, the anterior surface of the epiglottis (lingual branch), the walls of the pharynx (pharyngeal branch), and the tonsils (tonsillar branch). The sensory innervation of the anterior two thirds of the tongue is provided by the trigeminal nerve (lingual branch of the mandibular division).⁸ Given that it is not a part of the reflex arcs controlling gag or cough, its blockade is not essential for comfort during fiberoptic intubation.

Figure 19-1. Innervation of the airway passages.

The internal branch of the superior laryngeal nerve is a branch of CN X (vagus nerve) (Figure 19-2). The superior laryngeal nerve provides sensory innervation to the base of the tongue, posterior epiglottis, aryepiglottic folds, and arytenoids.⁷ This branch originates from the superior laryngeal nerve lateral to the greater cornu of the hyoid bone. The recurrent laryngeal nerve provides sensory innervation of the vocal folds and trachea and motor function of all intrinsic laryngeal muscles except the cricothyroid supplied by the external branch of the superior laryngeal nerve.⁷

Clinical Pearls

Three major neural pathways supply sensation to airway structures (see Figure 19-1).

Terminal branches of the ophthalmic and maxillary divisions of the trigeminal nerve supply the nasal cavity and turbinates.

The oropharynx and posterior third of the tongue are supplied by the glossopharyngeal nerve.

Branches of the vagus nerve innervate the posterior epiglottis and more distal airway structures.

TECHNIQUES FOR ANESTHETIZING THE AIRWAY

Preparation for Awake Intubation

The process of intubating an awake patient requires careful preparation. The anesthesiologist must evaluate each patient’s needs on an individual basis. Nearly every patient experiences some degree of anxiety associated with the surgery, anesthesia, and perhaps outcome. For this reason, most patients require some degree of sedation and analgesia. For this purpose, it is best to use short-acting or reversible agents for sedation or agents that do not cause a considerable degree of respiratory depression. Some examples of commonly used medication for awake intubation include midazolam, alfentanil, and fentanyl. These sedatives/analgesics are particularly useful in this setting because of their easy titratability to effect easy reversal with flumazenil or naloxone. Similarly, dexmedetomidine does not cause respiratory depression and is suitable in this setting.⁹

Figure 19-2. Innervation of the larynx.

Antisialogogues should be considered before any airway instrumentation. Oral secretions may make visualization via the fiberoptic equipment difficult and may serve as a barrier to effective penetration of local anesthetic into the mucosa. Glycopyrrolate 0.4 mg given intramuscularly or intravenously helps to diminish secretions.¹⁰ Alternatively, atropine 0.5-1 mg may be used intramuscularly or intravenously to similar effect. Intramuscular administration is favored over intravenous administration to avoid undesired side effects such as tachycardia and, less commonly, psychosis (with atropine) (Table 19-1).

Topical Anesthesia of the Nose, Mouth, Tongue, & Pharynx

One way to achieve anesthesia for oral or nasal fiberoptic intubation is to topicalize the structures involved with a local anesthetic. Topicalization of the airway is the spreading of local anesthetic over a region of mucosa to achieve local uptake and neural blockade of that region.

By far, the simplest of these techniques involves the spraying or swishing of local anesthetic directly onto the mucosa of the mouth, pharynx, tongue, and/or nose. This can be accomplished with any of the many commercially available local anesthetics, particularly viscous lidocaine preparations and mixtures of benzocaine and tetracaine. One popular preparation, Cetacaine, is a pressurized solution of benzocaine, tetracaine, and butamben in a small canister, which delivers a spray via a long spray nozzle that is pointed in the desired direction (Figure 19-3). The anesthetic is delivered in an oily foam, which is absorbed rapidly into the mucosa and provides excellent topical anesthesia of the mucosa.

Alternatively, a 10-mL syringe can be filled with lidocaine 2-4% and sprayed via a small-bore single or multiperforated catheter or the working channel of the fiberoptic bronchoscope.¹¹ This arrangement produces a fine stream of local anesthetic liquid, which with sufficient aliquots directed at the target mucosa achieves an adequate topical anesthetic effect. The safety and efficacy of both techniques are well established. Even with large amounts of swallowed anesthetic, plasma levels of local anesthetic should not reach toxic levels.^12,13

Table 19–1.

Commonly Used Medications and Dosages With Their Reversal Agents

Topicalization by Use of Local Anesthetic Reservoirs

Topicalization can also be accomplished by the use of local anesthetic-soaked cotton pledgets or swabs. These are soaked in either viscous or aqueous solutions of local anesthetic and then left for 5-15 minutes on the region of mucosa that requires anesthesia. The cotton acts as a reservoir for the anesthetic agent, producing a dense block. This technique is especially effective in the nasal passages. In the past, cocaine-soaked pledgets were used because they resulted both in a superb local anesthetic effect and in localized vasoconstriction. This practice has fallen out of favor, however, as concerns about cocaine toxicity grew. In addition, because of cocaine’s high profile as an illicit drug, there are significant regulatory hurdles associated with stocking it in a hospital formulary (eg, DEA paperwork, theft, accurate accounting of usage). As a method of achieving similar results, most clinicians have used the technique of adding small concentrations of epinephrine (1:200,000 or less) or phenylephrine (0.05%) to lidocaine. Alternatively, a vasoconstricting nasal spray can be applied before application of the local anesthetic. This approach results in dry mucosa, which then can be more easily anesthetized with local anesthetic because the local anesthetic does not get diluted with nasal secretions or saliva. The resulting vasoconstriction is nearly as effective as that of cocaine and offsets lidocaine’s powerful vasodilatation.

Figure 19-3. Topicalization of the mouth mucosa using a ben-zocaine spray.

The application of highly concentrated local anesthetic-soaked cotton pledget reservoirs can be exploited to achieve highly specific nerve blocks as well. These methods are detailed later with the description of individual nerve blocks. More about topicalization can be found in Chapter 18.

Inhalation of Aerosolized (Atomized) Local Anesthetic

Inhalation of aerosolized local anesthetic is another simple technique to achieve oropharyngeal anesthesia (Figure 19-4). To perform this technique, local anesthetic is added to a standard nebulizer with a mouthpiece or face mask attached. The patient is then asked to inhale the local anesthetic vapor deeply. After a period of approximately 15-30 minutes, the patient should have inhaled a sufficient quantity of local anesthetic to achieve a reasonably good level of topical anesthesia throughout the oropharynx and trachea. Focused aerosolized local anesthetic from an atomizer is ideal for nasal intubation. A number of disposable commercially available syringe-powered atomizers are available but are deficient in achieving small particle size unless outfitted with a side-stream air/oxygen flow to enhance dispersion by virtue of the Venturi principle.

Figure 19-4. Anesthetizing airway using inhalation of aerosolized lidocaine.

For these techniques, lidocaine in concentrations of 0.5%-4% has been suggested; quicker and denser blockade is achieved by using concentrations in the range of 2-4%. This technique has a proven clinical track record of safety; however, little data are available regarding the blood levels of local anesthetic that are achieved using these techniques or regarding metabolism of swallowed local anesthetics. Parkes et al.¹⁴ showed plasma concentrations of 0.29-0.45 mg/L in healthy volunteers after inhalation of 10% lidocaine solution. Because these levels were well below the generally accepted 5 mg/L safe level, it can be inferred that inhaling a 2-4% lidocaine for 15-30 minutes should be safe in most patients, particularly as a stand-alone technique.¹⁴

The major advantage of this technique lies in its simplicity and lack of discomfort. In addition, very little working knowledge of the anatomy of the region is required for its successful implementation.

Although this technique may seem ideal, it does have some drawbacks that limit its usefulness. The main disadvantage is that the density of the anesthesia achieved throughout the airway is highly variable. Many patients still experience an intact cough reflex, which can make intubation technically challenging. The rate of onset of this technique is highly dependent on patient compliance. Many patients who need an awake intubation are incapable or unwilling to take deep breaths. Also, inhalation of local anesthetic vapors can lead to central nervous system depression in patients whose mental status may already be depressed owing to other disease processes.

Clinical Pearls

Topicalization is the simplest method for anesthetizing the airway.

Local anesthetic can be sprayed directly onto the desired mucosa.

Nebulization of lidocaine 2-4% via face mask or oral nebulizer for 15-30 minutes can achieve highly effective anesthesia of the oral cavity and trachea for intubation.