Anesthesia for Endoscopy

ENDOSCOPY

Endoscopy includes laryngoscopy (diagnostic and operative), microlaryngoscopy (laryngoscopy aided by an operating microscope), esophagos-copy, and bronchoscopy. Endoscopic procedures may be accompanied by laser surgery.

Preoperative Considerations

Patients presenting for endoscopic surgery are often being evaluated for voice disorders (often present-ing 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 preopera-tive medical history and physical examination, with particular attention to potential airway problems, must precede any decisions regarding the anes-thetic plan. In some patients, flow–volume loops or radiographic, computed tomogra-phy, or magnetic resonance imaging studies may be available for review. Many patients will have under-gone preoperative indirect laryngoscopy or fiberop-tic nasopharyngoscopy, and the information gained from these procedures may be of critical importance.

Important initial questions that must be answered are whether the patient can be provided with positive-pressure ventilation with a face mask and rebreathing bag, and whether the patient can be intubated using conventional direct or video laryngoscopy. If the answer to either question is not “yes,” the patient’s airway should be secured prior to induction using an alternative technique (eg, use of a fiberoptic bronchoscope or a tracheostomy under local anesthesia; see Case Discussion). However, even the initial securing of an airway with tracheostomy does not prevent intraoperative air-way obstruction due to surgical manipulation and techniques.

Sedative premedication should be avoided in a patient with medically important upper airway obstruction. Glycopyrrolate (0.2–0.3 mg intramus-cularly) 1 hr before surgery may prove helpful by minimizing secretions, thereby facilitating airway visualization.

Intraoperative Management

The anesthetic goals for laryngeal endoscopy include an immobile surgical field and adequate masseter muscle relaxation for introduction of the suspension laryngoscope (typically the result of profound muscle paralysis), adequate oxygenation and ventilation, and cardiovascular stability despite rapidly varying levels of surgical stimulation.

A. Muscle Relaxation

Intraoperative muscle relaxation can be achieved by intermittent boluses or infusion of intermediate-duration nondepolarizing neuromuscular block-ing agents (NMBs) (eg, rocuronium, vecuronium, cisatracurium), or with a succinylcholine infusion. However, profound degrees of nondepolarizing block may prove difficult to reverse and may delay return of protective airway reflexes and extubation. Given that profound relaxation is often needed until the very end of the surgery, endoscopy remains one of the few remaining indications for succinylcholine infusions. Rapid recovery is important, as endos-copy is often an outpatient procedure.

B. Oxygenation & Ventilation

Several methods have successfully been used to provide oxygenation and ventilation during endos-copy, while simultaneously minimizing interference with the operative procedure. Most commonly, the patient is intubated with a small-diameter endo-tracheal tube through which conventional posi-tive-pressure ventilation is administered. Standard tracheal 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 high pressure against the tra-cheal mucosa. A 4.0-, 5.0-, or 6.0-mm specialized microlaryngeal tracheal tube (Mallinckrodt MLT®, Mallinckrodt Critical Care) is the same length as an adult tube, has a disproportionately large high-vol-ume low-pressure cuff, and is stiffer and less prone to compression than is a conventional tracheal 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 CO ₂.

In some cases (eg, those involving the poste-rior commissure or vocal cords), intubation with a tracheal 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 tra-chea. 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 tech-nique, in which ventilation with oxygen by face mask or endotracheal tube is alternated with peri-ods of apnea, during which the surgical procedure is performed. The duration of apnea, usually 2–3 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 connecting a manual jet ventilator to a side port of the laryngoscope. 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. It is crucial to monitor chest wall motion and to allow sufficient time for exhalation 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 80–300 times per min-ute . Capnography will not provide an accurate estimate of end-tidal CO ₂ during jet ven-tilation due to constant and sizable dilution of alveo-lar gases.

C. Cardiovascular Stability

Blood pressure and heart rate often fluctuate strik-ingly during endoscopic procedures for two rea-sons. First, some of these patients are elderly and have a long history of heavy tobacco and alcohol use that predisposes them to cardiovascular dis-eases. In addition, the procedure is, in essence, a series of physiologically stressful laryngoscopies and interventions, separated by varying periods of minimal surgical stimulation. Attempting to main-tain a constant level of anesthesia invariably results in alternating intervals of hypertension and hypo-tension. Providing a modest baseline level of anes-thesia allows supplementation with short-acting anesthetics (eg, propofol, remifentanil) or sympa-thetic antagonists (eg, esmolol), as needed, dur-ing periods of increased stimulation. Alternatively, some anesthesia providers use regional nerve blockof the glossopharyngeal nerve and superior laryn-geal nerve to help minimize intraoperative swings in blood pressure (see Case Discussion).

Laser Precautions

Laser light differs from ordinary light in three ways: it is monochromatic (possesses one wavelength), coherent (oscillates in the same phase), and colli-mated (exists as a narrow parallel beam). These char-acteristics 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 a yttrium–aluminum–garnet (YAG) laser produces 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 the evacuation of toxic fumes (laser plume) from tissue vaporiza-tion; these have the potential to transmit microbio-logical diseases. When significant laser plume is generated, fitted respiratory filter masks compliant with Occupation Safety and Health Administration 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. The greatest risk of laser airway surgery (if an endotracheal tube is used) is an airway fire.This risk can be moderated by using a technique of ventilation that minimizes the fraction of inspired oxygen (Fio2) and can be eliminated if there is no combustible material (eg, no flammable tube or catheter) in the airway. If an endotracheal tube is used, it must be relatively resistant to laser igni-tion (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 endo-tracheal tube is possible (Table 37–2).

Although specialized, laser-resistant endotra-cheal tubes may be used, it must be emphasized that no endotracheal tube or currently available endotra-cheal 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 should be avoided.

·        The endotracheal tube cuffs should be filled with saline. Some practitioners add methylene blue to signal cuff rupture. A well-sealed cuffed tube will minimize oxygen concentration in the pharynx.

·        Laser intensity and duration should be limited as much as possible.

·        Saline-soaked pledgets (completely saturated) should be placed in the airway to limit risk of endotracheal tube ignition and damage to adjacent tissue.

·        A source of water (eg, 60-mL syringe) 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).

If an airway fire should occur, all air/oxygen should immediately be turned off at the anesthe-sia 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 combustible fragments have been removed.