The complications of laryngoscopy and intubation include hypoxia, hypercarbia, dental and airway trauma, tube malpositioning, physiological responses to airway instrumentation, and tube malfunction. These complications can occur during laryngoscopy and intubation, while the tube is in place, or following extubation (Table 19–6).
Instrumentation with a metal laryngoscope blade and insertion of a stiff ETT often traumatizes delicate airway tissues. Tooth damage is a common cause of (relatively small) malpractice claims against anesthesiologists. Laryngoscopy and intubation can lead to a range of complications from sore throat to tracheal stenosis. Most of these are due to prolonged external pressure on sensitive airway structures. When these pressures exceed the capillary–arteriolar blood pressure (approximately 30 mm Hg), tissue ischemia can lead to a sequence of inflammation, ulceration, granulation, and stenosis. Inflation of an ETT cuff to the minimum pressure that creates a seal during routine positive-pressure ventilation (usually at least 20 mm Hg) reduces tracheal blood flow by 75% at the cuff site. Further cuff inflation or induced hypotension can totally eliminate mucosal blood flow.
Postintubation croup caused by glottic, laryngeal, or tracheal edema is particularly serious in children. The efficacy of corticosteroids (eg, dexamethasone—0.2 mg/kg, up to a maximum of 12 mg) in preventing postextubation airway edema remains controversial, but this approach is often used. Vocal cord paralysis from cuff compression or other trauma to the recurrent laryngeal nerve results in hoarseness and increases the risk of aspiration. The incidence of postoperative hoarseness seems to increase with obesity, difficult intubations, and anesthetics of long duration. Curiously, applying a water-soluble lubricant or a local anesthetic-containing gel to the tip or cuff of the ETT does not decrease the incidence of postoperative sore throat or hoarseness, and, in some studies, actually increased the incidence of these complications. Smaller tubes (size 6.5 in women and size 7.0 in men) are associated with fewer complaints of postoperative sore throat. Repeated attempts at laryngoscopy during a difficult intubation may lead to periglottic edema and the inability to ventilate with a face mask, thus turning a bad situation into a life-threatening one.
Unrecognized esophageal intubation can produce catastrophic results. Prevention of this complication depends on direct visualization of the tip of the ETT passing through the vocal cords, careful auscultation for the presence of bilateral breath sounds and the absence of gastric gurgling while ventilating through the ETT, detection of CO2 in exhaled gas (the most reliable automated method), chest radiography, airway ultrasonography, or the use of an FOB.
Even though it is confirmed that the tube is in the trachea, it may not be correctly positioned. Overly “deep” insertion usually results in intubation of the right mainstem bronchus because the right bronchus forms a less acute angle with the trachea than the left bronchus. Clues to the diagnosis of bronchial intubation include unilateral breath sounds, unexpected hypoxia with pulse oximetry (unreliable with high inspired oxygen concentrations), inability to palpate the ETT cuff in the sternal notch during cuff inflation, and decreased breathing-bag compliance (high peak inspiratory pressures).
In contrast, inadequate insertion depth will position the cuff in the larynx, predisposing the patient to laryngeal trauma. Inadequate depth of insertion can be detected by palpating the cuff over the thyroid cartilage. Because no one technique protects against all possibilities for misplacing an ETT, minimal testing should include chest auscultation, routine capnography, and occasionally cuff palpation.
If the patient is repositioned, tube placement must be reconfirmed. Neck extension or lateral rotation most often moves an ETT away from the carina, whereas neck flexion most often moves the tube toward the carina.
At no time should excessive force be employed during intubation. Esophageal intubations can result in esophageal rupture and mediastinitis. Mediastinitis presents as severe sore throat, fever, sepsis, and subcutaneous air often manifesting as crepitus. Early intervention is necessary to avoid mortality. If esophageal perforation is suspected, consultation with an otolaryngologist or thoracic surgeon is recommended. Vocal cord injury can likewise result from repeated, forceful attempts at endotracheal intubation.
Laryngoscopy and tracheal intubation violate the patient’s protective airway reflexes and predictably lead to hypertension and tachycardia when performed under “light” planes of general anesthesia. The insertion of an LMA is typically associated with less hemodynamic change. Hemodynamic changes can be attenuated by intravenous administration of lidocaine, opioids, or β-blockers, or deeper planes of inhalation anesthesia in the minutes before laryngoscopy. Hypotensive agents, including sodium nitroprusside, nitroglycerin, esmolol, nicardipine, and clevidipine, can attenuate the transient hypertensive response associated with laryngoscopy and intubation. Cardiac arrhythmias—particularly ventricular premature beats—sometimes occur during intubation and may indicate light anesthesia.
Laryngospasm is a forceful involuntary spasm of the laryngeal musculature caused by sensory stimulation of the superior laryngeal nerve. Triggering stimuli include pharyngeal secretions or passing an ETT through the larynx during extubation. Laryngospasm is usually prevented by extubating patients either deeply asleep or fully awake, but it can occur—albeit rarely—in an awake patient. Treatment of laryngospasm includes providing gentle positive-pressure ventilation with an anesthesia bag and mask using 100% oxygen or administering intravenous lidocaine (1–1.5 mg/kg). If laryngospasm persists and hypoxia develops, small doses of succinylcholine (0.25–0.5 mg/kg) may be required (perhaps in combination with small doses of propofol or another anesthetic) to relax the laryngeal muscles and to allow controlled ventilation. The large negative intrathoracic pressures generated by a struggling patient during laryngospasm can result in the development of negative-pressure pulmonary edema, particularly in healthy patients.
Whereas laryngospasm may result from an abnormally sensitive reflex, aspiration can result from depression of laryngeal reflexes following prolonged intubation and general anesthesia.
Bronchospasm is another reflex response to intubation and is most common in asthmatic patients. Bronchospasm can sometimes be a clue to bronchial intubation. Other pathophysiological effects of intubation include increased intracranial and intraocular pressures.
ETTs do not always function as intended. Polyvinyl chloride tubes may be ignited by cautery or laser in an oxygen/nitrous oxide–enriched environment. Valve or cuff damage is not unusual and should be excluded by careful inspection of the ETT prior to insertion. Obstruction of the ETT can result from kinking, from foreign body aspiration, or from thick or inspissated secretions in the lumen.
CASE DISCUSSION Evaluation & Management of a Difficult Airway
A 47-year-old man with a long history of tobacco and alcohol abuse presents for emergency drainage of a right-sided submandibular abscess.
What are some important anesthetic considerations during the preoperative evaluation of a patient with an abnormal airway? Induction of general anesthesia followed by direct laryngoscopy and oral intubation is dangerous, if not impossible, in several situations. (Table 19–7). To determine the optimal intubation technique, the anesthesiologist must elicit an airway history and carefully examine the patient’s head and neck. Any available prior anesthesia records should be reviewed for previous problems in airway management. If a facial deformity is severe enough to preclude a good mask seal, positive-pressure ventilation may be impossible. Furthermore, patients with hypopharyngeal disease are more dependent on awake muscle tone to maintain airway patency. These two groups of patients should generally not be allowed to become apneic—including induction of anesthesia, sedation, or muscle paralysis—until their airway is secured.
If there is an abnormal limitation of the temporomandibular joint that may not improve with muscle paralysis, a nasal approach with an FOB should be considered. Infection confined to the floor of the mouth usually does not preclude nasal intubation. If the hypopharynx is involved to the level of the hyoid bone, however, any translaryngeal attempt will be difficult. Other clues to a potentially difficult laryngoscopy include limited neck extension (<35°), a distance between the tip of the patient’s mandible and hyoid bone of less than 7 cm, a sternomental distance of less than 12.5 cm with the head fully extended and the mouth closed, and a poorly visualized uvula during voluntary tongue protrusion. It must be stressed that because no examination technique is foolproof and the signs of a difficult airway may be subtle, the anesthesiologist must always be prepared for unanticipated difficulties.
The anesthesiologist should also evaluate the patient for signs of airway obstruction (eg, chest retraction, stridor) and hypoxia (agitation, restlessness, anxiety, lethargy). Aspiration pneumonia is more likely if the patient has recently eaten or if pus is draining from an abscess into the mouth. In either case, techniques that ablate laryngeal reflexes (eg, topical anesthesia) should be avoided.
Cervical trauma or disease is a factor that should be evaluated prior to direct laryngoscopy. Cervical arthritis or previous cervical fusion may make it difficult for the head to be put in the sniffing position; these patients are candidates for bronchoscopy to secure the airway, as discussed previously. Trauma patients with unstable necks or whose neck has not yet been “cleared” are also candidates for bronchoscopy for tracheal intubation. Alternatively, laryngoscopy with in-line stabilization can be performed (Figure 19–37).
In the case under discussion, physical examination reveals swelling below the mandible and trismus that limits the patient’s ability to open his mouth. Mask fit does not seem to be impaired. CT of the head and neck suggests that the infection has spread along tissue planes and is displacing the airway to the left.
Which intubation technique is indicated? Oral and nasal intubations can be performed in awake patients. Whether the patient is awake or asleep or whether intubation is to be oral or nasal, it can be performed with direct laryngoscopy, fiberoptic visualization, or video laryngoscopy techniques.
Intubation may be difficult in this patient, however, because of limited mouth opening and distortion/displacement of the glottis. Induction of anesthesia should, therefore, be delayed until after the airway has been secured. Useful alternatives include awake fiberoptic intubation, awake video laryngoscopy, or awake use of optical stylets. The final decision depends on the availability of equipment and the experiences and preferences of the anesthesia caregivers.
Regardless of which alternative is chosen, an emergency surgical airway may be necessary. An experienced team, including a surgeon, should be in the operating room and all necessary equipment should be available and unwrapped. The neck can be prepped and draped.
What premedication would be appropriate for this patient? Any loss of consciousness or interference with airway reflexes could result in airway obstruction or aspiration. Glycopyrrolate would be a good choice of premedication because it minimizes upper airway secretions without crossing the blood–brain barrier. Parenteral sedatives should be very carefully titrated. Dexmedetomidine and ketamine preserve respiratory effort and could be used as sedatives. Psychological preparation of the patient, including explaining each step planned in securing the airway, may improve patient cooperation.
What nerve blocks could be helpful during an awake intubation? The lingual and some pharyngeal branches of the glossopharyngeal nerve that provide sensation to the posterior third of the tongue and oropharynx are easily blocked by bilateral injection of 2 mL of local anesthetic into the base of the palatoglossal arch (also known as the anterior tonsillar pillar) with a 25-gauge spinal needle (Figure 19–38).
Bilateral superior laryngeal nerve blocks and a transtracheal block would anesthetize the airway below the epiglottis (Figure 19–39). The hyoid bone is located, and 3 mL of 2% lidocaine is infiltrated 1 cm below each greater cornu, where the internal branch of the superior laryngeal nerves penetrates the thyrohyoid membrane.
A transtracheal block is performed by identifying and penetrating the CTM while the neck is extended. After confirmation of an intratracheal position by aspiration of air, 4 mL of 4% lidocaine is injected into the trachea at end expiration. A deep inhalation and cough immediately following injection distribute the anesthetic throughout the trachea. Although these blocks may allow the awake patient to tolerate intubation better, they also obtund protective cough reflexes, depress the swallowing reflex, and may lead to aspiration. Topical anesthesia of the pharynx may induce a transient obstruction from the loss of reflex regulation of airway caliber at the level of the glottis.
A relatively simple alternative to all of these is to allow the patient to breath atomized lidocaine for several minutes prior to instrumentation as is typically done for outpatient bronchoscopy.
Because of this patient’s increased risk for aspiration, local anesthesia might best be limited to the nasal passages. Four percent cocaine has no advantages compared with a mixture of 4% lidocaine and 0.25% phenylephrine and can cause cardiovascular side effects. The maximum safe dose of local anesthetic should be calculated—and not exceeded. Local anesthetic is applied to the nasal mucosa with cotton-tipped applicators until a nasal airway that has been lubricated with lidocaine jelly can be placed into the naris with minimal discomfort. Benzocaine spray is frequently used to topicalize the airway, but can produce methemoglobinemia, and for this reason we prefer lidocaine.
Why is it necessary to be prepared for a surgical airway? Laryngospasm is always a possible complication of intubation in the nonparalyzed patient, even if the patient remains awake. Laryngospasm may make positive-pressure ventilation with a mask impossible. If succinylcholine is administered to break the spasm, the consequent relaxation of pharyngeal muscles may lead to upper airway obstruction and continued inability to ventilate. In this situation, an emergency cricothyrotomy may be lifesaving.
What are some alternative techniques that might be successful? Other possible strategies include the retrograde passage of a long guidewire or epidural catheter through a needle inserted across the CTM. The catheter is guided cephalad into the pharynx and out through the nose or mouth. An ETT is passed over the catheter, which is withdrawn after the tube has entered the larynx. Variations of this technique include passing the retrograde wire through the suction port of a flexible FOB or the lumen of a reintubation stylet that has been preloaded with an ETT. These thicker shafts help the ETT negotiate the bend into the larynx more easily. Obviously, a vast array of specialized airway equipment exists and must be readily available for management of difficult airways (Table 19–8). Either of these techniques would have been difficult in the patient described in this case because of the swelling and anatomic distortion of the neck that can accompany a submandibular abscess.
What are some approaches when the airway is unexpectedly difficult? The unexpected difficult airway can present both in elective surgical patients and also in emergency intubations in intensive care units, the emergency department, or general hospital wards. Should video laryngoscopy fail even after attempts with an intubating bougie, an intubating LMA should be attempted (Figure 19–40). If ventilation is adequate, an FOB can be loaded with an ETT and passed through the LMA into the trachea. Correct tube position is confirmed by visualization of the carina.