Endoscopy includes laryngoscopy (diagnostic and operative) and microlaryngoscopy (laryngoscopy aided by an operating microscope) for conditions including vocal cord cysts and polyps and upper airway papillomatosis and malignancy, esophagoscopy, and bronchoscopy (discussed in Chapter 25). Endoscopic procedures may be accompanied by laser surgery.
Patients presenting for endoscopic surgery are often being evaluated for voice disorders (often presenting as hoarseness), stridor, or hemoptysis. Possible diagnoses include foreign body aspiration, trauma to the aerodigestive tract, papillomas, tracheal stenosis, tumors, or vocal cord dysfunction. Thus, a preoperative medical history and physical examination, with particular attention to potential airway problems, must precede any decisions regarding the anesthetic plan. In some patients, flow–volume loop (see Chapter 6), radiographic, computed tomography, ultrasound, or magnetic resonance imaging studies may be available for review. Many patients will have undergone preoperative indirect laryngoscopy or fiberoptic nasopharyngoscopy, and the information gained from these procedures may be of critical importance.
Important initial questions that must be answered are whether adequate positive-pressure ventilation via face or laryngeal mask is feasible, and whether the patient can be intubated using conventional direct or video laryngoscopy. If the answer to either question is “no,” the patient’s airway should be secured prior to induction using an alternative technique such as fiberoptic bronchoscope or tracheostomy under local anesthesia (see Case Discussion, Chapter 19). However, even the initial securing of an airway with tracheostomy does not prevent intraoperative airway obstruction due to surgical manipulation, a foreign body, or hemorrhage.
Sedative premedication should be avoided in a patient with threatening upper airway obstruction. Glycopyrrolate (0.2–0.3 mg) works more effectively and persistently when given intramuscularly rather than intravenously 1 h before surgery and may prove helpful by minimizing secretions, thereby facilitating airway visualization.
The anesthetic goals for laryngeal endoscopy include an immobile surgical field and adequate masseter muscle relaxation for introduction of the suspension laryngoscope (typically profound muscle paralysis will be sought), adequate oxygenation and ventilation, and cardiovascular stability despite rapidly varying levels of surgical stimulation.
Intraoperative muscle relaxation can be achieved by intermittent boluses or infusion of intermediate-duration nondepolarizing neuromuscular blocking agents (NMBs) (eg, rocuronium, vecuronium, cisatracurium), or with a succinylcholine infusion. Rapid recovery is important, as endoscopy is often an outpatient procedure. Given that profound muscle relaxation is often needed until the very end of the operative procedure, endoscopy remains one of the few remaining indications for succinylcholine infusions. Use of sugammadex (Bridion) to reverse profound degrees of rocuronium or vecuronium neuromuscular blockade is an alternative approach.
B. Oxygenation & Ventilation
Several methods have successfully been used to provide oxygenation and ventilation during endoscopy, while simultaneously minimizing interference with the operative procedure. Most commonly, the patient is intubated with a small-diameter endotracheal tube through which conventional positive-pressure ventilation is administered. Standard endotracheal tubes of smaller diameters, however, are designed for pediatric patients, and therefore are too short for the adult trachea and have a low-volume cuff that will exert increased pressure against the tracheal mucosa. A 4.0-, 5.0-, or 6.0-mm specialized microlaryngeal endotracheal tube (Mallinckrodt MLT) is the same length as an adult tube, has a disproportionately large high-volume low-pressure cuff, and is stiffer and less prone to compression than is a conventional endotracheal tube of the same diameter. The advantages of intubation in endoscopy include protection against aspiration and the ability to administer inhalational anesthetics and to continuously monitor end-tidal CO2.
In some cases (eg, those involving the posterior commissure or vocal cords), intubation with an endotracheal tube may interfere with the surgeon’s visualization or performance of the procedure. A simple alternative is insufflation of high flows of oxygen through a small catheter placed in the trachea. Although oxygenation may be maintained in patients with good lung function, ventilation will be inadequate for longer procedures unless the patient is allowed to breathe spontaneously.
Another option is the intermittent apnea technique, in which positive-pressure ventilation with oxygen by face mask or endotracheal tube is alternated with periods of apnea, during which the surgical procedure is performed. The duration of apnea, usually 2 to 3 min, is determined by how well the patient maintains oxygen saturation, as measured by pulse oximetry. Risks of this technique include hypoventilation with hypercarbia, failure to reestablish the airway, and pulmonary aspiration.
Another attractive alternative approach involves manual jet ventilation via laryngoscope side port. During inspiration (1–2 s), a high-pressure (30–50 psi) jet of oxygen is directed through the glottic opening and entrains a mixture of oxygen and room air into the lungs (Venturi effect). Expiration (4–6 s duration) is passive. Chest wall motion must be monitored and sufficient exhalation time allowed in order to avoid air trapping and barotrauma. This technique requires total intravenous anesthesia. A variation of this technique is high-frequency jet ventilation, which utilizes a small cannula or tube in the trachea, through which gas is injected 80 to 300 times per minute (see Chapter 58). Capnography will not provide an accurate estimate of end-tidal CO2 during jet ventilation due to constant and sizable dilution of alveolar gases.
C. Cardiovascular Stability
Blood pressure and heart rate often fluctuate markedly during endoscopic procedures for two reasons. First, some of the patients undergoing these procedures are elderly and have a long history of heavy tobacco and alcohol use that predisposes them to cardiovascular disease. In addition, the endoscopic procedure is, in essence, a series of physiologically stressful laryngoscopies and interventions, separated by varying periods of minimal surgical stimulation. Attempting to maintain a constant level of anesthesia invariably results in alternating intervals of hypertension and hypotension. Providing a modest baseline level of anesthesia allows supplementation with short-acting anesthetics (eg, propofol, remifentanil) or sympathetic antagonists (eg, esmolol), or both, as needed during periods of increased stimulation. Alternatively, some anesthesia providers utilize regional nerve block of the glossopharyngeal nerve and superior laryngeal nerve to help minimize intraoperative swings in blood pressure (see Case Discussion, Chapter 19).
Laser light differs from ordinary light in three ways: It is monochromatic (possesses one wavelength), coherent (oscillates in the same phase), and collimated (exists as a narrow parallel beam). These characteristics offer the surgeon excellent precision and hemostasis with minimal postoperative edema or pain. Unfortunately, lasers introduce several major hazards into the operating room environment.
The uses and side effects of a laser vary with its wavelength, which is determined by the medium in which the laser beam is generated. For example, a CO2 laser produces a long wavelength (10,600 nm), whereas yttrium–aluminum–garnet (YAG) lasers produce a shorter wavelength (1064 or 1320 nm). As the wavelength increases, absorption by water increases, and tissue penetration decreases. Thus, the effects of the CO2 laser are much more localized and superficial than are those of the YAG laser.
General laser precautions include suction evacuation of toxic fumes (laser plume) from tissue vaporization because they have the potential to transmit microbial diseases. When a significant laser plume is generated, individually fitted respiratory filter masks compliant with U.S. Occupational Safety and Health Administration (OSHA) standards should be worn by all operating room personnel. In addition, during laser procedures, all operating room personnel should wear laser eye protection and the patient’s eyes should be taped shut. Operating room windows should be covered and appropriately placed signage should be used to alert those entering the room that a laser device is in use.
The greatest risk during laser airway surgery is an airway fire. This risk can be moderated by minimizing the fraction of inspired oxygen (FiO2 <30% if tolerated by the patient) and can be eliminated when there is no combustible material (eg, flammable endotracheal tube, catheter, or dry cotton pledget) in the airway. If an endotracheal tube is used, it must be relatively resistant to laser ignition (Table 37–1). These tubes not only resist laser beam strikes, but they also possess double cuffs that should be inflated with saline instead of air in order to better absorb thermal energy and reduce the risk of ignition. If the proximal cuff is struck by the laser and the saline escapes, the distal cuff will continue to seal the airway. Alternatively, endotracheal tubes can be wrapped with a variety of metallic tapes; however, this is a suboptimal practice and should be avoided whenever use of a specialized, commercially available, flexible, stainless steel, laser-resistant endotracheal tube is possible (Table 37–2).
TABLE 37–1Advantages and disadvantages of various tracheal tubes for laser airway surgery. ||Download (.pdf) TABLE 37–1 Advantages and disadvantages of various tracheal tubes for laser airway surgery.
|Type of Tube ||Advantages ||Disadvantages |
|Polyvinyl chloride ||Inexpensive, nonreflective ||Low melting point, highly combustible1 |
|Red rubber ||Puncture-resistant, maintains structure, nonreflective ||Highly combustible1 |
|Silicone rubber ||Nonreflective ||Combustible,1 turns to toxic ash |
|Metal ||Combustion-resistant,1 kink-resistant ||Thick-walled flammable cuff, transfers heat, reflects laser, cumbersome |
TABLE 37–2Disadvantages of wrapping a tracheal tube with metallic tape. ||Download (.pdf) TABLE 37–2 Disadvantages of wrapping a tracheal tube with metallic tape.
No cuff protection
Adds thickness to tube
Not a device approved by the US Food and Drug Administration
Protection varies with type of metal foil
Adhesive backing may ignite
May reflect laser onto nontargeted tissue
Rough edges may damage mucosal surfaces
Although specialized, laser-resistant endotracheal tubes may be used, it must be emphasized that no endotracheal tube or currently available endotracheal tube protection device is reliably laser-proof. Therefore, whenever laser airway surgery is being performed with an endotracheal tube in place, the following precautions should be observed:
Inspired oxygen concentration should be as low as possible by utilizing air in the inspired gas mixture (many patients tolerate an FiO2 of 21%).
Nitrous oxide supports combustion and must be avoided.
The endotracheal tube cuffs should be filled with saline. Some practitioners add methylene blue to the saline to make cuff rupture more obvious. A well-sealed, cuffed endotracheal tube will minimize oxygen concentration in the pharynx.
Laser intensity and duration should be limited as much as possible.
Saline-saturated pledgets, although potentially flammable, should be placed in the airway to limit risk of endotracheal tube ignition and damage to adjacent tissue.
A source of water (eg, water-filled 60-mL syringe and basin) should be immediately available in case of fire.
These precautions limit, but do not eliminate, the risk of an airway fire; anesthesia providers must proactively address the hazard of fire whenever laser or electrocautery is utilized near the airway (Table 37–3).
TABLE 37–3Airway fire protocol. ||Download (.pdf) TABLE 37–3 Airway fire protocol.
1. Stop ventilation and remove tracheal tube.
2. Turn off oxygen and disconnect circuit from machine.
3. Submerge tube in water.
4. Ventilate with face mask and reintubate.
5. Assess airway damage with bronchoscopy, serial chest x-rays, and arterial blood gases.
6. Consider bronchial lavage and steroids.
If an airway fire should occur, all air/oxygen should immediately be turned off at the anesthesia gas machine, and burning combustible material (eg, an endotracheal tube) should be removed from the airway. The fire can be extinguished with saline, and the patient’s airway should be examined to be certain that all foreign body fragments have been removed.