2.1 Airway Assessment and Management
All forms of surgery in the head and neck region require consideration of airway management, maintenance of ventilation with an adequate form of anesthesia, and prevention of concentration of oxygen in the operative field in the presence of cautery or laser. Various forms of airway management are discussed in the following paragraphs, considering anatomy, innervation, indications, instrumentation and equipment, and clinical context. The specific situation of the difficult airway is then discussed.
Airway Anatomy
There are two physiologic entry points to the airway: the nose and the mouth. The epiglottis, located at the base of the tongue, separates the oropharynx from the hypopharynx. The larynx is made up of a cartilaginous structure supported by muscles and ligaments.
Innervation
The sensory innervation above the epiglottis is provided by the trigeminal nerve (cranial nerve V, abbreviated CN V) and the glossopharyngeal nerve (CN IX); below the epiglottis, by the superior laryngeal and recurrent laryngeal branches of the vagus nerve (CN X). For more information on the cranial nerves, see Appendix B.
Nasal mucosa: by the pterygopalatine ganglion branch of the middle (maxillary) division of the trigeminal nerve (CN V2)
Posterior pharynx (including uvula and tonsils): by the continued branches from the pterygopalatine ganglion
Oropharynx and supraglottic area: by the glossopharyngeal nerve; branches of this nerve include the lingual, pharyngeal, and tonsillar branches
Trachea: by the recurrent laryngeal nerve
Larynx: sensory and motor innervation from the vagus nerve
Sensory: above the vocal folds, innervation is supplied by the internal branch of the superior laryngeal nerve; below the vocal folds, by the recurrent laryngeal nerve.
Motor: all muscles are supplied by the recurrent laryngeal nerve except for the cricothyroid muscle, which is supplied by the external branch of the superior laryngeal nerve.
Airway Equipment
Oral and Nasal Airways
In anesthetized patients, loss of airway tone allows the tongue and epiglottis to contact the posterior pharyngeal tissue, leading to obstruction. Artificial airway devices can be placed in the nose or mouth to provide an air passage. Nasal airways carry a risk of epistaxis and should be avoided in anticoagulated patients. These devices should also be avoided in patients with basilar skull fractures to avoid intracranial penetration of the airway device. If an airway device is indicated in a lightly anesthetized patient, the nasal route is generally tolerated better.
Facemasks
The facemask is designed to contour and conform to a variety of facial features, with the intention of creating an airtight seal capable of delivering gases from the anesthesia equipment.
Laryngoscopes
The intubating laryngoscopes most commonly used by the anesthesiology team have curved (Macintosh) or straight (Miller) blades and an open-blade design. Newer fiberoptic video rigid laryngoscopes are useful, such as the GlideScope (Diagnostic Ultrasound Corporation, Bothell, WA). There are a variety of operative laryngoscopes that may be useful for intubation, such as the Holinger or Dedo laryngoscope.
Airway Assessment
A complete airway examination looks at several aspects of the airway. It would be important to take note of difficulty with patient cooperation or consent or of the appropriateness of, or difficulty with, supraglottic airway placement, as well as difficult surgical airway access (see revised ASA Difficult Airway Algorithm [DAA], Fig. 2.1 ). There is a population of patients where the clinical examination belies the extent of their disease. However, the first priority is to identify patients who may be difficult to mask-ventilate or intubate. This is critically important: induction of general anesthesia without subsequently being able to ventilate the patient adequately is an acute, life-threatening emergency. Thus, identification of a patient with a potentially difficult airway before induction will enable the operative and anesthetic team to institute an appropriate plan and backup plans and to assemble and test airway equipment ahead of time.
Mallampati classification: Used to predict the ease of intubation by looking at the anatomy of the oral cavity while the patient is sitting upright in the neutral position ( Fig. 2.2 ).
Thyromental distance: Measured from the upper edge of the thyroid cartilage to the chin with head in full extension. A short thyromental distance (< 6 cm) equates with an anterior (superior) laryngeal position that is at a more acute angle, which makes the larynx difficult to visualize via direct laryngoscopy. It is a relatively unreliable test unless combined with other tests.
Mouth opening: Less than two finger-widths of mouth opening (trismus) suggests difficulty with intubation.
Cervical spine movements: Mobility of the atlantooccipital and atlan to axial joints may be assessed by asking the patient to extend the head while the neck is in flexion. Extension of the head with atlantoaxial joint immobility results in greater cervical spine convexity, which pushes the larynx anteriorly and impairs laryngoscopic view.
Temporomandibular joint (TMJ) mobility: Assessed by asking the patient to protrude the jaw while sitting up in the neutral position. Decreased mobility suggests greater difficulty with intubation.
Grade A: Lower incisors in front of upper incisors (good mobility)
Grade B: Lower incisors up to upper incisors
Grade C: Lower incisors cannot be protruded to touch upper incisors (poor mobility).
Cormack and Lehane grade: Used to classify the view on direct laryngoscopy. Previous documentation of laryngoscopy should include the grade of glottic view ( Fig. 2.3 ).
Preoperative Endoscopic Airway Evaluation: An airway evaluation technique that is readily available, is minimally invasive, and may provide enough information to reduce the use of awake intubation. The information sought by the anesthesiologist from this examination differs from that sought by the surgeon. The surgeon is interested in the extent and location of disease, the degree of preservation of function, and whether immediate intervention is required. The anesthesiologist wants to know whether there is sagittal plane access to the larynx, whether there are lesions that might interfere with supraglottic airway placement (such as an LMA), and whether there are lesions in the anterior part of the glottic opening that may be damaged by traditional laryngoscopy.
Preoperative Medication
Medications are often administered preoperatively to alleviate anxiety, to provide analgesia, or as aspiration prophylaxis ( Table 2.4 ).
Airway Management
Endotracheal Intubation
Indications for Endotracheal Intubation
Need for protection from aspiration
Altered level of consciousness (Glasgow Coma Score [GCS] score <8)
Respiratory distress
Severe pulmonary or multiorgan failure
Facilitation of positive pressure ventilation
Operative position other than supine
Operative site involving the upper airway
Disease involving the upper airway
One-lung ventilation
Confirmation of Endotracheal Intubation
Direct visualization of endotracheal tube passing between the vocal folds
Carbon dioxide present in exhaled gases in at least four consecutive breaths (positive end-tidal CO2)
Bilateral breath sounds
Absence of air entry during epigastric auscultation
Condensation of water vapor in endotracheal tube (ETT) during exhalation
Maintenance of arterial oxygenation
Chest X-ray with tip of ETT between the carina and thoracic inlet
Endotracheal Tube Size Recommendations
ETT size (mm): Age/4 + 4 for patients > 2 years old.
Length of insertion: 12 + Age/2. Add 2 to 3 cm for nasal intubation.
Pediatrics: Uncuffed ETTs are generally used in patients < 8 years old.
Types of Endotracheal Tube
• Ring-Adair-Elwyn endotracheal tubes. Ring-Adair-Elwyn (RAE) endotracheal tubes are preformed to fit in the nose or mouth and are commonly used in oral or pharyngeal surgery, particularly adenoidectomy or tonsillectomy. The shape prevents obstruction of the surgical field and fits surgical retractors such as the Crow-Davis retractor.
• Armored endotracheal tubes. These tubes are commonly used in head and neck surgery. They prevent kinking of the tube when the head is manipulated. This tube works well through a tracheotomy, as it can be curved inferiorly out of the surgical field without kinking and sutured in place temporarily.
• Laser-resistant endotracheal tubes. Laser-resistant ETTs are used in laser surgery, particularly treatment of laryngeal lesions. By preventing an interaction of inhaled oxygen with the laser, these tubes help prevent airway fires. The portion of the tube beyond the cuff(s) without laser-resistant metal wrap is flammable, and conflagration can still ensue if the laser breaches the barrier constituted by the cuff(s) and saline-soaked blocking pledgets and enters an oxygen-enriched tracheal environment.
• Nerve-monitoring endotracheal tubes. For thyroid and parathyroid surgery, a tube with contact electromyographic electrodes positioned at the level of the vocal folds permits intraoperative monitoring of recurrent laryngeal nerve integrity.
Endotracheal Intubation Procedure
Preoperative evaluation will help determine the route (oral vs. nasal) and method (awake vs. anesthetized) for tracheal intubation.
Equipment: laryngoscope with working light, appropriate-sized ETTs, oxygen supply, functioning suction catheter, functioning intravenous (IV) line, and anesthetic medications.
Cricoid pressure: an assistant′s thumb and forefinger depress the cricoid cartilage downward, compressing the esophagus against the underlying vertebral body. This prevents spillage of gastric contents into the pharynx during the period of time from induction of unconsciousness to placement of the ETT in the trachea.
Induction of anesthesia can be achieved by using IV or inhaled agents.
Endotracheal Intubation Complications
During intubation, possible complications include aspiration, dental damage, laceration of lips/gingiva/palate, laryngeal injury, esophageal intubation, endobronchial intubation, bronchospasm, and activation of sympathetic nervous system (elevated heart rate [HR] and blood pressure [BP]).
After intubation, possible complications include aspiration, laryngospasm, transient vocal fold incompetence, pharyngitis, and tracheitis.
Orotracheal Intubation
If there is no history of cervical spine instability, the patient′s head is extended into a “sniffing” position. This position aligns the oral, pharyngeal, and laryngeal axes such that the passage from the lips to the glottic opening is a straight line. The height of the operating room (OR) table should be manipulated so that the patient′s head is at the level of the clinician′s xiphoid cartilage ( Fig. 2.4 ).
The laryngoscope is introduced into the right side of the mouth. Advancing the blade posteriorly and toward the midline, the tongue is displaced to the left. Check that the lower lip is not pinched between the blade and the incisors. Placement of blade depends on which style has been selected:
Macintosh (curved) blade: The tip of the blade is advanced until the tip enters the vallecula (the space between the epiglottis and the base of the tongue).
Miller (straight) blade: The tip of the blade is passed below the laryngeal surface of the epiglottis, which is then lifted to expose the vocal folds.
Regardless of the blade selected, the laryngoscope is lifted upward and forward in the direction of the long axis of the handle. The upper incisors should not be used as a fulcrum for leverage, as this may damage teeth.
The vocal folds should be visualized prior to advancement of the ETT. Passing the ETT from the right, little resistance should be encountered. The balloon cuff of the ETT should pass 1 to 2 cm past the vocal folds. Once the proper position of the ETT is confirmed, it should be secured in place.
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