Lasers have become a common instrument for surgical and nonsurgical therapy in equine medicine. The many different tissue interactions that can be produced, the precision of its use, and the ability to apply laser energy to less accessible areas are the great advantages of the laser compared with other forms of therapy.
Laser is an acronym for Zight amplification of stimulated emission of radiation. The light emitted by lasers works according to the basic properties of light and electromagnetic radiation, but it is very different from the light produced by more common light sources such as incandescent bulbs, fluorescent lamps, or sunlight. The similarity between laser light and common white light is that all light consists of particles (photons) that travel through space in unique waveforms. White light consists of a mixture of many different wavelengths. Each color of visible light has its own characteristic wavelength. Visible light has an electromagnetic spectrum of wavelengths that range from approximately 400 nm to 700 nm.
Laser light can be within the visible spectrum of light, but it differs significantly from white light because of its monochromacity, collimation, and coherence. Laser light consists of a single wavelength or an extremely narrow range of wavelengths, and is therefore considered “monochromatic.” Also light emitted from bulbs or headlights diverge rapidly, but laser light has a very narrow cone of divergence. Finally, light waves can travel through space without any fixed relationship to each other, meaning they are incoherent. If all waves are lined up together so their peaks and valleys match, they are in phase, or coherent. Laser light is coherent, and white light is not.
The components to create laser light are an active medium, a power source, an optical resonator, and an output coupler (partially transmitting mirror). The active medium is the material that determines the wavelength of the laser. The medium can be a gas, a liquid, a solid material, or a junction between two plates of semiconductor materials. The power source is the pump that stimulates the emission of radiation and the type of energy used as a power source is determined by the lasing medium. The optical resonator can be thought of as mirrors on either side of the medium that reflects the light back into the medium for “amplification.” The output coupler allows a portion of the laser light contained between the two mirrors to leave the laser resonator in the form of a beam.
Lasers are characterized in two main ways. They can be delineated by the medium (diode, CO2, neodymium: yttrium-aluminum-garnet [Nd:YAG]) or the power output (pulsed vs continuous). A general classification system also exists for laser power and safety (classes I-IV). Classes I and II are low-risk lasers with a power of less than 1.0 mW. “Cold” or therapeutic lasers are class III lasers. All surgical lasers are class IV (>0.5 watts). Although the power is measured in watts, the power density is termed “irradiance” and is the amount of power per unit of surface area. Irradiance is equal to the laser power output/laser beam size (W/cm2). Therefore a larger beam size of a given power will have a smaller irradiance. The number of joules depicts the total energy, which is equal to the laser output (watts) multiplied by the exposure time (seconds). The “energy fluence” is equal to joules/laser beam size, and measures the total amount of energy directed to the tissue during a treatment. An understanding of this fact is important because the effectiveness of a particular laser is determined not only by its wavelength but also by how it is used.
Laser light interacts with tissue in several ways. It can be absorbed, transmitted, reflected, or scattered. The percentage of each interaction is dependent on the characteristics of the tissue and the laser light. The amount of absorption is dependent on the wavelength of the light relative to the chromophore content of the tissue (hemoglobin, keratin, protein, water, melanin). Each chromophore has its own absorption spectrum for different wavelengths of light. If the light is absorbed it is transformed into heat energy. Heating tissue to 60° C will lead to coagulation of proteins, and heating tissue to higher than 100° C will result in vaporization. Thus lasers will yield different biologic effects dependent on the energy absorption coefficient. Although vaporization and coagulation can be seen at the time of surgery, a zone of thermal injury exists beyond what can be seen at surgery. If a large amount of energy is expended that is not strictly focused on the area of interest, excessive swelling and trauma to the tissues may occur postoperatively.
Lasers have become a common tool to speed healing in many different types of injuries. The lasers used for this purpose differ greatly from surgical lasers. Therapeutic lasers are considered “cold” or low-power lasers and fall into classes II and III. They may induce some heat but no greater than that which would be felt from a 60-W bulb held close to the skin. The benefits of these lasers are the analgesic effects caused by alterations in nerve conduction and wound healing caused by stimulation of changes in intracellular calcium that ultimately results in increased protein synthesis and collagen production. The most common lasers employed are the gallium arsenide (GaAs) and helium neon (HeNe) lasers at a distance of 1 to 2 mm from the surface of the target tissue for a total energy density of 5 J/cm2.
Although surgical lasers have existed since 1960, it was not until lasers could be applied through small flexible fibers that these tools had an enormous impact on equine surgery. These fibers can be passed down the biopsy channel of a videoendoscope and employed under videoendoscopic control. This development revolutionized the treatment of upper respiratory conditions by providing the surgeon an opportunity to approach lesions within the nasal cavity, larynx, and pharynx without making a surgical skin incision. This new procedure also provided a technique for cutting a fairly reactive and very wellvascularized tissue that is precise and provides significant hemostasis.
The two most common lasers used for upper respiratory surgery are the diode and Nd:YAG lasers. They have wavelengths of 980 nm and 1064 nm, respectively, and can pass down a small flexible optical quartz fiber without significant disruption of wavelength. The diode laser has two main advantages compared with the Nd:YAG. The diode laser is a much smaller unit (less than 15 lb) and is significantly less expensive than the Nd:YAG. The major disadvantage of the diode laser is its power limitation of 25 W, whereas the Nd:YAG can exceed 50 W. Other lasers such as the CO2 cannot pass down a small fiber effectively because of their much larger wavelength and therefore cannot be used with a standard videoendoscope. Although the CO2 laser wavelength is strongly absorbed by water and therefore is an excellent precise cutter, it has only poor-to-fair coagulating capability. The diode or Nd:YAG wavelengths are diffusely absorbed by all protein molecules and therefore have greater coagulation capabilities, although they do not cut as well as the CO2 laser.
The laser can be used in contact or noncontact mode. Most surgeries can be performed with a bare fiber (no special tip) in contact mode. This method provides very accurate, controlled cutting and hemostasis of the small vessels in the respiratory mucosa, and provides the surgeon some tactile sense of the procedure. A lower power setting of 14 to 18 W is sufficient in most cases. This also means that a small very portable diode laser can be employed. If the laser is used correctly, little lateral thermal damage should occur. The surgeon resects the tissue by dragging the fiber across the tissue as he or she would lightly drag a scalpel blade. The types of surgeries commonly done in this fashion include axial division of epiglottic entrapment, resection of subepiglottic or pharyngeal cysts, vocal cord resections, resection of granulation tissue, and treatment of guttural pouch tympanites.
With noncontact laser surgery, the fiber is held 3 to 5 mm away from the target tissue. A higher power setting of 40 to 60 W is commonly required to work effectively, which requires an Nd:YAG laser. Noncontact surgery is used mostly for ablation of cystlike structures such as ethmoid hematomas or pharyngeal cysts and to vaporize membranous structures.
A great advantage of the use of lasers in respiratory surgery is that many of the surgeries can be done on the standing, sedated animal on an outpatient basis. This fact also equates to a shorter, easier postoperative management because no skin incision is present. Procedures can be performed with the animal standing in the stocks with just intravenous sedation such as xylazine (0.44 mg/kg). Repeated half doses or a longer-acting agent may be required depending on the procedure and the experience of the surgeon. With the horse sedated, a twitch is normally not required to pass the endoscope. The horse’s head can be suspended from cross ties for support, but an individual must always be positioned at the horse’s head for safety and to alter the head position as needed. Topical anesthetic is applied to the area of interest through polyethylene tubing that is advanced down the biopsy channel of the endoscope. The horse often swallows while the anesthetic is applied, and application should be intermittently suspended to make certain the anesthetic is applied appropriately in between swallows. The anesthetic is usually effective for approximately 2 hours, so the animal is not allowed to eat for 1 to 2 hours after surgery.
Laser safety should always be considered. Although the laser is used within the respiratory cavity, surgical personnel should still wear laser safety glasses as a precaution against any misfiring of the laser. The laser should always be kept in the standby mode when not being used. If a procedure is performed with the horse under general anesthesia near an endotracheal tube, the oxygen concentration should be decreased with helium to dramatically reduce the risk of spontaneous ignition. Smoke evacuation is usually not necessary in contact laser surgery in the standing horse but may be required in noncontact work or when the horse is under general anesthesia.
Antiinflammatory medication is the cornerstone of postoperative management in the upper respiratory tract. Phenylbutazone (4.4 mg/kg) and dexamethasone (0.044 mg/kg) are given in the immediate postoperative period. Both medications are recommended for several days at a decreased dose depending on the type of surgery and anticipated degree of inflammation. Local antiinflammatory medication can also be administered through a 10-Fr catheter that is advanced through the nasal passage into the nasopharynx. Ten milliliters of a mixture of dimethyl sulfoxide, glycerine, and dexamethasone solution are administered slowly through the catheter while watching the horse swallow. This mixture is administered twice daily for as long as 7 days.
Antimicrobials are not commonly given unless the surgeon is working on areas of thickened scar tissue where the vascularity may be compromised, or extensive use of the laser is required. Although vaporization of all tissue with the laser results in a sterile incision, the adjacent tissues of the throat and mouth can easily contaminate the open wound bed at the conclusion of surgery. Surgical inexperience can lead to greater thermal injury than visually appreciated and increased susceptibility to infection even in healthy tissues, particularly when the laser is used on subepiglottic tissues.
Although the laser has become an invaluable tool for many upper respiratory surgeries, its improper use can create significant trauma and irreparable damage. Great care should be taken to use only as much energy as necessary to complete the task and minimize extraneous firing. When used appropriately, the laser greatly diminishes the need for more extensive surgery and speeds the recovery of the patient.