Hazards Associated with Laser Surgery in the Airway of the Horse: Implications for the Anesthetic Management

Summary

Over the past two decades, the use of lasers has become an integral part of upper airway surgery in the horse as it allows for more precise tissue dissection, produces less bleeding and tissue trauma, and reduces the incidence of postoperative complications. Nevertheless, laser surgery in the anesthetized horse also holds novel hazards for patient and operating room personnel alike. Certainly laser surgery constitutes a challenge as operation of a high-energy temperature source in the airway, often in vicinity of the endotracheal tube, bears the risk of accidental tube ignition and subsequent airway fire. This danger is of particular concern during inhalation anesthesia, when patients breathe oxygen-rich gas mixtures that readily support tube combustion. Therefore laser surgery in the respiratory tract requires a detailed pre- and intraoperative communication and cooperation between surgeon and anesthesiologist, and a specific anesthetic management tailored to the individual surgical procedure and laser instrument being used. By following prescribed safety procedures and precautions, and employing appropriate measures in an emergency situation, both surgeon and anesthesiologist can markedly reduce the risk of potentially life threatening complications.

Introduction

The term laser is an acronym for light amplification by stimulated emission of radiation. Light emitted by a laser differs from natural light in that it is coherent (waves are all in phase with each other in space and time), collimated (highly directional), monochromatic light (light of one wavelength), and of high energy [1-3]. Since the introduction of lasers into medicine in the early 1960s, laser technology has progressed rapidly and is now widely used in all surgical disciplines. A variety of surgical lasers are available today with different physical properties (Table 1). The destructive effect of a given laser output is dependent upon the wavelength emitted, the power density projected, the time the tissue is exposed to the high-energy source, and the blood supply to the tissue. Laser light might be accurately focused on small areas evaporating tissue and cauterizing small blood vessels. Long wavelength lasers, such as the carbon dioxide (CO2) laser, transfer energy to tissue and water, therefore tissue penetration is shallow (approximately 0.01 mm). The neodymium:yttrium aluminum garnate (Nd:YAG) laser, on the other hand, operating with a shorter wavelength than the CO2 laser readily penetrates tissue up to a depth of 5 mm.

CO2, carbon dioxide laser; He-Ne, helium-neon laser; Nd:YAG, neodymium:yttrium aluminum garnate laser; GaAlAs, gallium aluminum arsenide diode laser.

Use of lasers in the airway of the horse requires technology that allows transmission of the laser beam through fiberoptic delivery systems, small and flexible enough to fit the biopsy channel of an endoscope in order to reach the surgical field in the respiratory tract [4]. The Nd:YAG laser became the first commercially available device that fulfilled this condition. More recently the gallium aluminum arsenide (GaAlAs) diode laser has entered the veterinary market as well and is now widely available for transendoscopic laser surgery in the airway [5]. As a result of this technological progress, the laser is today an established instrument for upper airway surgery in the horse and offers the advantages of less traumatic tissue dissection with reduced risk for bleeding and postoperative complications (e.g., laryngeal edema) as well as faster return to preoperative training activity. The close proximity of laser beam and endotracheal tube (ETT) in the surgical field, however, is of great concern to the anesthesiologist as it inflicts significant risk upon the equine patient during general anesthesia. This chapter will describe common problems associated with the use of lasers in airway surgery, lists precautions to be considered to minimize the risk of laser-related complications as well as rapid corrective measures to be taken in the event of complications.

Common Laser Applications in Equine Airway Surgery

Over the past years, the use of lasers in equine upper airway surgery has become increasingly popular [6,7]. Among frequently performed procedures that involve transendoscopic laser surgery are correction of epiglottal entrapment, vocal cordectomy and laryngeal sacculectomy, partial soft palate resection (staphylectomy), excision of arytenoid cartilage granulomas (resulting from arytenoid chondritis), ablation of pharyngeal lymphoid hyperplasias and pharyngeal masses, and removal of sub- or dorsal epiglottic cysts or granulomas. Lasers have also been effectively applied in tissue biopsies, creation of a salpingopharyngeal fistula, treatment of conditions such as progressive ethmoid hematoma, guttural pouch tympanitis, choanal atresia, axial deviation of the aryepiglottic fold, tracheal ulcers and pyogranulomas, and debridement of dorsal epiglottic abscesses. A detailed description of the surgical techniques and instruments used for those procedures is beyond the scope of this chapter and the interested reader is referred to recent review papers and equine surgery textbooks [6-11].

Hazards Associated with Laser Surgery in the Airway

The American National Standards Institute (ANSI) has classified most medical lasers, including the CO2, Nd:YAG, and GaAlAs diode lasers, as Class IV, or most hazardous lasers on the basis of their optical emissions [12]. Direct intrabeam viewing or contact with the laser beam are considered the most dangerous, but also specular or diffuse reflection of laser light may damage skin or eyes of surgeons, other operating room personnel and the patient alike, unless appropriate safety measures are taken (see Table 2). While light emitted from lasers in the far infrared portion of the spectrum, such as the CO2 laser, is highly absorbed by all surfaces and may damage only the outer layers of the eye, causing corneal ulceration and opacification, infrared light from the Nd:YAG or GaAlAs diode lasers is transmitted through the cornea and lens and, thus, may damage the retina [12]. These risks are minimal during transendoscopic laser surgery in the airway, where the threat of direct exposure is reduced.

In general, airway surgery has a disproportionately larger potential for complications than surgery in other parts of the body because surgical manipulation in the airway may lead to serious impairment of respiratory function and complications in the recovery period. This applies regardless of whether or not a laser instrument is used or the patient is awake or anesthetized. Hence, general complications associated with surgery in the respiratory tract, such as airway obstruction, aspiration (blood, tissue particles, etc.), hypoventilation, hemorrhage, cardiac arrhythmias (e.g., vagal or sympathetic nerve stimulation), or postoperative laryngeal edema, may also occur during or after laser surgery in the airway.

The main problem associated with use of lasers in airway surgery is the introduction and operation of a high-energy temperature source in the patient’s airway [13]. Though reported to be rare in the horse, inappropriate handling of the laser fiber tip and/or endoscope may cause inadvertently deep tissue trauma, particularly when exposure time is long and power density is high or the instrument is operated in the continuous (opposed to pulsed) mode [6,7]. Dependent on location, those lesions may cause extensive damage to mucosal and submucosal tissue, blood vessels, nerves, and laryngeal or tracheal cartilage. In addition, in human patients transesophageal or bronchopleural fistulas and pneumothorax are reported complications of laser-induced tissue trauma [14,15].

In intubated anesthetized patients breathing O2-enriched gas mixtures the use of lasers in the airway bears a significantly higher risk of an airway fire. Any inadvertent misdirecting of the laser beam may cause heat damage to the ETT, considering that the tube is typically in close proximity to the laser target in the surgical field. In the best of circumstances, the laser may hit the tube for only a very brief moment causing no more damage than a small, partial, or perhaps complete, puncture of the tube. Significant leakage of inhalant gases into the surgical field can occur if the perforation of the tube wall is complete. Any puncture of the cuff, the most vulnerable part of the ETT, will immediately cause it to collapse, resulting in a major leak that mandates immediate discontinuation of positive pressure ventilation (PPV) and significantly increases the risk of aspiration. Dependent on contact time, the intense heat generated by lasers can ignite all ETT materials commonly used in anesthesia [16]. This risk is particularly high if the laser beam hits any dark colored labels or marks on the tube’s surface, as laser light absorption is increased in those spots. Once ignited, the anesthetic gases that pass through the ETT may readily support combustion, leading to a "blowtorch effect" (Fig. 1), best described as flames streaming from the distal (tracheal) end of the burning ETT and causing severe burn injuries that may reach far into the bronchial tree [17]. Due to a Venturi effect, flames of the laser combustion may sweep a jet-like stream of hot gases and soot particles into the lower airways worsening tissue damage in trachea and lungs (Fig. 2). Extensive thermal injuries associated with fire and gas explosion in the airway may affect pharynx, larynx, trachea and lower airways. Most dangerous are thermal lesions of epiglottis and tracheobronchial tree, which may cause severe posttraumatic edema (leading to partial or complete airway obstruction) and impairment of pulmonary gas exchange [18].

Figure 1. A blowtorch effect may result from a laser impingement and subsequent ignition of the proximal part of the endotracheal tube. Once ignited, the oxygen-rich anesthetic gases passing through the endotracheal tube may support combustion, leading to a blowtorch fire and immediate heat damage to the cuff that then rapidly collapses.

Figure 2. This diagram depicts the Venturi effect that may occur during positive pressure ventilation when the cuff of the endotracheal tube suddenly collapses due to a blowtorch fire in the airway. In the moment of cuff deflation, gas under high pressure (jet stream) exiting the distal end of the endotracheal tube entrains ambient air because the pressure around and behind the distal end of the tube becomes lower (relative negative pressure area N) with respect to the pressure in the mainstream gas flow (positive pressure area P). This further enhances the propulsive force of the blowtorch flame.

Besides the laser source itself, "laser smog", a fume formed during tissue coagulation by the laser, is another source of danger [17,19]. It contains organic materials (e.g., xylene or toluene) that are noxious and mutagenic. Particularly in the non-intubated horse smoke particles are easily inhaled by the patient and may cause an inflammatory response in the lower respiratory tract with or without clinical symptoms such as coughing and forceful breathing [20]. While of less concern to the patient when intubated and connected to an anesthetic circuit, laser smog can be hazardous to surgeons and other operating room personnel as well. They may develop bronchial inflammation with bronchospasm, alveolar edema and potentially diffuse pulmonary atelectasis when inhaling the smoke [19]. Mixing of laser smog with O2 may occur where there is an insufficient seal of the ETT cuff, or a cuff deflation, and may rapidly produce a highly explosive gas mixture that further supports or enhances combustion [19].

The ease with which tube combustion occurs may depend on the tube material, inhaled gas composition, duration of laser exposure and the power density [18]. Silicon tubes burn more easily than red rubber and polyvinyl chloride (PVC) ETTs [21]. Studies comparing the combustibility of red rubber versus PVC have produced conflicting results. While Patel and Hicks [22] reported a higher heat resistance of red rubber tubes as compared to PVC tubes upon direct exposure to a laser, Wolfe and Simpson [16] found opposite results under similar experimental conditions. The risk of ETT combustion is significantly increased with inspired O2 concentrations exceeding 30% (FiO2 > 0.3) [23-25]. As reported by Wolfe and Simpson [16], red rubber tubes combust at a significantly lower O2 concentration (18%) than PVC tubes (26%). Silicone, much like red rubber, can ignite in room air [23]. In addition, the incidence of "flaring" of carbonized tissue increases as the O2 concentration at the surgery site increases [19].

Besides direct heat injury to the airways, patients may demonstrate severe respiratory symptoms resulting from the toxic effects of waste products of tube combustion. Burning of PVC produces hydrochloric acid and vinyl chloride, both of which can cause severe airway irritation and bronchoconstriction [26]. Red rubber tubes are stiffened with compounds that reduce their flammability, but when ignited produce thick black smoke which then may be inhaled by the patient; fortunately, the smoke does not seem to contain irritating substances [27]. Silicone, if ignited, rapidly becomes a brittle ash that crumbles easily and, hence, tends to quickly accumulate in lower airways and lung [22].

Safety Precautions for use of Lasers

General Precautions

Although the use of lasers for minimally invasive airway surgery of the horse has proven to be relatively safe, important safety procedures must be followed in order to protect both patient and operative personnel from the hazards of lasers [6]. Laser safety training is available for veterinarians and veterinary technicians with certification courses administered by the American College of Veterinary Surgeons (4401 East West Highway, Suite 205, Bethesda, MD 20814-4523, USA) and the American Society for Laser Medicine and Surgery (2404 Stewart Square, Wausau, WI 54401, USA). It is recommended that all operating room personnel become familiar with a Laser Safety Protocol, which lists the main safety precautions for use of lasers in surgery (Table 2). Also all new equipment and procedures should be carefully reviewed before being allowed to operate in the hospital on patients.

The most efficient way to protect the eye from the laser is to avoid any contact with its emitted light and to use instruments with matte surfaces that diffuse reflected beams. Whenever a surgical laser is used, doors to the operating room should be closed and access restricted. A conspicuous warning sign (e.g., "Danger - Laser radiation: Do not enter without appropriate eye protection") should be displayed on the outside of all doors of the surgery suite, indicating the type of laser used and the form of eye protection required [1,6,12]. This will prevent any accidental exposure of animal health care personnel to the laser beam when entering the room. Surgeon, anesthesiologist and all other personnel in the operating room must wear goggles of specific colors in order for the laser light to be absorbed. There is only one exception to this rule. Eyes can be protected from CO2 laser radiation by any glasses or goggles because far infrared light emitted by this laser is absorbed by almost all surfaces [12]. It is important to note that tinted goggles may impair the anesthesiologist’s ability to evaluate changes in the horse’s mucous membrane color and to read screens of certain ECG or other physiological data monitors, particularly those using color-coding for display of various traces or data. The eyelids of the horse may be closed and covered with moist gauze patches in order to avoid any remotely possible exposure to the laser beam.

Safe transendoscopic laser surgery in horses requires a laser-compatible endoscope [6,7]. This includes an eyepiece that is equipped with an interchangeable filter, which filters out the wavelength emitted by the specific laser being used. Otherwise the surgeon is at risk of eye injury when viewing the operative field during laser operation without the obligatory protective eyewear that is specific to the wavelength of the laser being used. Use of a video recording/monitor system attached to the endoscope does not only offer the surgeon the advantage of a magnified display of the surgical field, but also allows the anesthesiologist to carefully monitor the progress of the surgical procedure and the position of the laser tip in relation to the airway and ETT at any given time. In this case, the filter mentioned before will prevent an optical flare, which may occur upon activation of the laser, and may obscure visibility for the surgeon or cause complete "whiteout" of the video screen. Activating the laser only when the tip of the laser fiber is located within the body cavity and the surgical image is viewed on a television monitor further minimizes the risk of ocular injury to surgeon, patient and operating room personnel.

The tip of laser-compatible endoscopes usually contains a ceramic element to protect the tip of the endoscope from heat generated by the laser beam [6]. Keeping the tip of the laser fiber at least 1 cm beyond the tip of the endoscope when performing surgery will reduce the amount of heat applied to the surface of the endoscope and minimize the accumulation of debris on the surface of the lens and thus decreases the risk of ignition of this material and/or the distal end of the endoscope.

Of less concern than eye injury is skin damage associated with lasers. This may range from a minor erythema to a full-thickness burn, and may affect primarily the patient’s skin close to the surgical site and, much less commonly, the hands of the surgeon or his assistant [12]. More severe damage can occur when drapes or other flammable material in close vicinity to the operating field ignite. In order to prevent burns and accidental exposure during laser use, the patient’s skin around the operative field (e.g., a tracheostomy site) should be covered with wet towels.

As mentioned before, evaporation of tissue by laser energy produces smoke known as "laser smog" that may cause bronchial inflammation and bronchospasm, nausea, vomiting and lacrimation in susceptible individuals [1,12,19]. Adequate suction applied close to the site of smoke production or continuous gas evacuation from the suction channel of the endoscope will reduce environmental pollution and thus protect both surgeon and operating room personnel from the effects of smoke inhalation, and at the same time facilitate the operator’s view of the surgical field. Wearing of special laser masks that prevent inhalation of smoke particles can further help reduce the risk of adverse effects of laser plume in susceptible individuals.

Specific Anesthetic Considerations during Laser Surgery in the Airway of Horses

Because of the described hazards, especially that of tracheal tube fire, any laser procedure in the airway of the horse under general anesthesia requires thorough pre- and intraoperative communication between anesthesiologist and surgeon and mutual preparedness to solve unexpected complications as a team.

Before induction of anesthesia, a plan should be formulated for the various steps in both administering anesthesia via the airway and performing surgery as well. A detailed knowledge of the location and type of surgical procedure to be performed is essential for the anesthesiologist, who has to develop a strategy that fulfills the goal of satisfactory surgical access to the airway while maintaining a safe ventilatory pathway. Some laser procedures in the upper airway of horses (e.g., arytenoidectomy, excision of large subepiglottic granulomas) are best performed through a ventral laryngotomy incision and hence require a tracheostomy for placement of an ETT, thereby providing an optimally secure airway with minimal risk of an airway fire during laser operation. Similarly, nasal intubation for procedures in more rostral areas of the larynx often achieves both a safe airway and relatively unhindered access to the surgical site with little chance for the laser beam getting into contact with the ETT and eliciting an airway fire. Whenever circumstances allow for a significant spatial separation of surgical field and airway (or ETT), a routine inhalation anesthetic protocol might be used safely. In all other situations precautions need to be taken to minimize the fire hazard associated with laser surgery in the airway.

Since all ETTs used in large animal anesthesia are made of inflammable material (silicone rubber, red rubber or PVC), protection of these tubes from laser damage can be achieved by carefully wrapping them with self-adhesive, non-reflective aluminum tape in a spiral fashion with overlapping edges, beginning just above the ETT’s cuff and ending at the Y-piece adapter [19]. However, laser beams may still penetrate these metallic foils and/or may be reflected off the metallic surface into surrounding tissues. In addition, the tape may not always adhere adequately to the tube and may loosen or break off during intubation or extubation, resulting in aspiration of or airway obstruction with tape particles. As mentioned before the cuff is the most vulnerable part of any ETT and, when ruptured, allows a massive leak of anesthetic gases, leading to hypoventilation of a ventilated patient as well as providing an O2-rich environment for ignition of the tube [19]. Filling the ETT cuff with water or saline (possibly mixed with methylene blue as an indicator of rupture) may reduce fire by dispersing the heat energy [29], but might also result in excess pressure on the tracheal mucosa. When the cuff is punctured by the laser beam the fluid can act as an immediate fire extinguisher [30]. Laser resistant ETTs that use opaque or foam coverings, even metal inserts, are commercially not available in sizes appropriate for use in horses, and have met with only limited success in retarding airway fires [1,12,23].

Both total intravenous (TIVA) and inhalant anesthetic techniques are appropriate for horses undergoing general anesthesia for laser surgery in the upper respiratory tract. This applies particularly when lasers are used for only a limited period of time during the initial or final phase of the surgical procedure. In horses inhalation anesthesia offers generally a greater advantage for intermediate and long-term procedures (> 60 - 90 minutes), though progress has been made recently to improve techniques using TIVA in horses for prolonged general anesthesia [31]. In general, horses undergoing airway laser surgery, except maybe for very short procedures, should be intubated to avoid inhalation of "laser smog" and aspiration of tissue debris and blood. In most anesthetized horses breathing room air will result in hypoxemia, thus necessitating O2 supplementation, which however, will negate the possible advantages of TIVA in terms of reducing the fire hazard. Anesthetic vapors currently in common use in equine anesthesia (halothane, isoflurane, and sevoflurane) are not inflammable in clinically used concentrations [1].

Regardless of the technique used, it is recommended to use the lowest concentration of inspired O2 (FiO2) that is compatible with adequate oxygenation of the patient [32], because, as mentioned before, the higher the O2 concentration the greater the risk that combustible material hit by the laser beam will ignite [23-25]. Based on the authors’ experiences, concentrations of 25 - 40% (FiO2 < 0.4 with PPV) are usually adequate, except perhaps in some larger animals such as unusually heavy Warmblood or draft horses. Though not commonly used in horses, it is important to stress that also nitrous oxide (N2O) supports combustion, when it disintegrates into nitrogen (N2) and O2 at temperatures above 450°C (N2O --> N2 + 1/2 O2 + heat) [25]. Thus, N2O should be strictly avoided during laser surgery. Room air, nitrogen (N2), or helium (He) should be used to reduce FiO2, which effectively decreases the risk of igniting the ETT and minimizes the chance of producing a highly explosive gas mixture in the event of an ETT cuff failure (leakage or deflation) that would allow laser smog to mix with the oxygen of the inspired gas.

Regardless of the anesthetic protocol and ventilatory technique (spontaneous or mechanical ventilation) used, administration of an inspired gas mixture low in FiO2 requires careful monitoring of the patient’s oxygenation status. Continuous recording of the inspired O2 concentration combined with pulse oximetry (SPO2) and/or repeated arterial blood analysis (SaO2, PaO2) is mandatory to detect signs of oxygen desaturation accurately, allowing for rapid corrective measures. Though PPV might often not be absolutely necessary, it is the authors’ experience that anesthetized horses breathing spontaneously gas mixtures low in FiO2 (< 0.4) tend to desaturate significantly more frequently than mechanically ventilated horses. However, it is important to recognize that if the ETT is ignited by a laser burst, PPV will further support the development of the above described "blowtorch" effect and must be immediately discontinued [17,30].

Postoperative edema of and bleeding from operated tissues are still a common complication following laser surgery in the upper airway of the horse [6,7]. Pre- or intraoperative administration of non-steroidal anti-inflammatory drugs (phenylbutazone) and eventually glucocorticoids (prednisolone, dexamethasone) helps minimize inflammatory reactions that otherwise may lead to potentially life threatening airway obstruction during the recovery period. If it is anticipated that edema formation will be more severe or may persist despite anti-inflammatory medication, a tracheostomy with subsequent tracheal tube placement distal to the surgical site may be indicated to maintain an open airway. Hemorrhage from the surgical site can be reduced by preoperative irrigation of the mucosal surface area with solutions containing an adrenergic vasoconstrictor (e.g., epinephrine, norepinephrine) [6].

Prior to moving the patient to the recovery stall at the end of the anesthetic, the head should be lowered to allow drainage of blood clots, cell debris and remaining flush solution. Subsequently, the (most commonly nasally placed) ETT should be pulled temporarily up to the level of the surgical site with its cuff inflated, as this further facilitates clearing of remaining blood clots from the airway. The cuff of the ETT should remain inflated throughout the recovery period till the horse is standing to prevent aspiration of blood from persisting hemorrhage. At that time, the ETT can be withdrawn with the cuff still partially inflated. If the ETT is rather small in diameter for the size of the horse, thus significantly increasing the patient’s work of breathing, the cuff might be deflated already earlier, however not before an adequate cough reflex has returned.

Management of Airway Fire and other Complications

Serious complications associated with laser surgery in the airway of the horse are very rare and then often due to unfamiliarity with the specific laser being employed or due to disregard of appropriate safety precautions described above [3,6,7]. Though the incidence of airway fires during laser surgery in the horse is unknown, earlier studies in human medicine, conducted at times when laser-resistant ETTs were not yet available, report an incidence of 0.4 - 1.5% [17,33]. Since it is impossible to totally eliminate the risk of fire when a laser is used in the airway, the entire operating room team must be familiar with all steps to be taken in the event of an airway fire, prepare for them prior to surgery and be constantly on the alert during the procedure [12,28]. If immediate steps are followed to prevent fire from extending down the tracheobronchial tree (Table 3), and if appropriate secondary and postoperative steps are taken in evaluating and treating the injuries that occurred, the morbidity and mortality from a laser fire in the airway can be minimized [12,30,34,35].

In the event of an airway fire, immediate disconnecting of the anesthetic breathing circuit from the tracheal tube is essential to stop any gas flow which otherwise would enhance and maintain the airway fire. Subsequently, removal of the burning ETT must be accomplished as quickly as possible to avoid further thermal and chemical damage to the airway. Remaining flames on the removed ETT or on tube fragments that have fallen off during extubation must be extinguished immediately to prevent ignition of surgical drapes or similar inflammable material. Equally important, the surgeon should instantaneously withdraw the laser’s fiberoptic delivery system and the endoscope (if used for transendoscopic laser application) and irrigate the surgical site with sterile saline or another isotonic aqueous solution to prevent any further smoldering of tissue or tube fragments left in the airway. For these reasons, aqueous solutions must be immediately available to anesthesiologist and surgeon alike during laser surgery in the airway. If there are no obvious tube fragments left in the airway or other material obstructing the airway, and following appropriate suctioning of the airway, the patient may be reintubated and ventilated with a gas mixture containing an O2 concentration not higher than necessary to ensure adequate oxygenation of the patient. Following these immediate steps, larynx and tracheobronchial tree should be thoroughly inspected using an endoscope to evaluate the damage that occurred during the airway fire. The findings obtained during this examination will determine how to proceed with secondary measures and subsequent postoperative management of the patient (Table 3).