Kenneth E. Sullins.
Noncontact application of laser energy requires relatively high‐power settings and high‐power densities for an adequate tissue effect. Smaller fibers transmitting 20–25 watts can vaporize small areas but burn out very rapidly. With higher outputs such as 50 watts, more tissue effect is accomplished, but bare fibers still tend to overheat at these levels. A fiber burning out inside an endoscope can badly damage the scope. Nd:YAG lasers that can be fitted with gas‐cooled coaxial fibers contain a 600 μm (highest power density possible) quartz fiber passed through a plastic tube that conveys cooling gas or liquid (Figure 12.11c). A metal tip joins the two at the end of the fiber, enabling the fiber to be used to deliver noncontact laser energy, or it can be fitted with a sapphire tip for contact lasing. Compared to the bare quartz fiber, higher powers can be transmitted without burning out the fiber. Care must be taken not to touch tissue with the cooling port, because clogging will cause the fiber to burn out. If the fiber tip burns out, it must be refitted with a new tip or replaced [27]. If the metal tip flares during burnout, it should be cut off the fiber before withdrawing the fiber from the endoscope or the metal edges could lacerate the biopsy channel in the endoscope [2].
Laser Safety
American National Standard (ANSI) for Safe Use of Lasers in Health Care Facilities Z136.3 is the authority for medical laser safety in the United States. All surgical lasers are secured with a key lock and separate interlock required to operate the machine. A designated Laser Safety Officer responsible for lock security, warning signs during surgery, and other required safety measures are advisable.
Appropriate eye protection is required for all surgical laser wavelengths. Clear glass with protection from all angles is adequate for the CO2 laser, but optical density recommendations are specific for the near‐infrared and other wavelengths and should be followed for the specific laser. The patient’s eyes must be considered as well. Since surgical lasers discussed here are not in the visible spectrum, a low energy helium‐neon laser aiming beam is used. However, prolonged direct exposure, particularly to the eye, can still cause damage.
All smoke generated from tissue should be evacuated using a filtered laser smoke evacuator. In spite of reports that insignificant concentrations of bacteria become aerosolized [28] and that horses are not adversely affected by routine upper airway laser surgery [29], there is sufficient evidence that infectious, carcinogenic and irritant material is present in laser smoke [30, 31]. The vaporized debris and potentially viable cells or pathogens should not be inhaled by humans or the patient. Surgical suction is inadequate for this task because it is less efficient, and the suction lines will eventually foul.
The surgical field should be protected by barriers. Towels or lap sponges soaked with sterile water or saline limit CO2 laser energy from burning tissue off the field or drapes. Wet sponges should be held behind tissue that the laser could penetrate completely. Laser beams reflected from metallic surgical instruments retain sufficient energy to affect tissue or personnel. Anodized or matte finished instruments to limit reflection can be purchased.
Accelerants should be avoided. Saline should be used instead of alcohol for surgical prep. Heliox (oxygen diluted with helium) can be substituted for pure oxygen when operating close to the airway with the horse under general anesthesia. If these few simple rules are followed, laser surgery is as safe as any other surgery.
Specific Complications of Laser Surgery in Horses
Patient Complications
Definition
Complications associated with the patient secondary to laser use
Risk factors
Laser type
Tissue type
Length of deployment
Depth of laser penetration
Pathogenesis
General surgery
General surgery using lasers consists of primary incisions, excision of masses followed by primary closure or leaving the wound open for second intention healing, and ablation/vaporization of tissue or masses also left open for second intention healing. Incisions intended for primary closure are sensitive in that skin margin viability must be preserved. Appropriate power density, rate of laser movement and separating tissue tension prevents collateral heating of the skin margins that could lead to marginal skin slough and incisional dehiscence. Carbon dioxide lasers are the best choice for these types of general surgery because tissue can be precisely incised with almost no collateral heating with appropriate instrument settings and surgical technique. The approximate appropriate power density expecting primary healing for a skin incision is 5,000 W/cm2 [10].
If either of these is in question, sutures set back from the skin margins 2–3 extra millimeters could help prevent dehiscence.
The Nd:YAG /diode laser is not as precise as the CO2 laser for skin incisions. The quartz fibers or sapphire tips produce more collateral heating of tissue. The power should be set high and the handpiece advanced in a single pass with skin tension separating the margins as it progresses. Tension relieving sutures provide some insurance against marginal necrosis of tissue. Noncontact delivery of the Nd:YAG/diode laser produces too much collateral heating for reliable primary tissue healing and can risk subsurface tissue.
Either of the above lasers can be used for excision of masses where skin margins are not a concern. However, the CO2 laser is more efficient for large masses and there are no fibers to wear out. Where important structures lie deep close to the surgical site, the near infrared lasers should be restricted to contact delivery.
While all laser surgery removes some tissue along the lines of incision/dissection, ablation/vaporization removes all the target tissue by non‐contact delivery of laser energy. Masses ablated/vaporized are usually comparatively small or they would have been excised. Examples include smaller dermal melanomas, squamous cell carcinomas or similar masses. Limited to noncontact delivery, the CO2 laser ablates tissue efficiently and safely, because it is highly absorbed by water and tissue penetration is limited. Still, surrounding tissue collateral heating should be minimized. The computerized scanner described earlier greatly increases efficiency and reduces collateral heating. Nd:YAG/diode lasers efficiently ablate dark‐colored soft tissue masses but can still over penetrate to deeper tissue. Contradicting the opening sentence, a spherical sapphire tip on a gas‐cooled fiber of an Nd:YAG laser can be used to perform “contact ablation” by “painting” the lesion away similar to a burr on bone. Importantly, the sapphire tip largely limits the effect to the surface. Only heat is delivered to the tissue so collateral heating must be watched but deeper penetration of laser energy will be limited.
Prevention
Lesions on the inner pinna of the ear are good illustrations. A 4‐mm sarcoid can easily be ablated with either CO2 or near infrared lasers. The CO2 laser will efficiently ablate the mass (no matter what color it is) and care must be taken not to overheat the underlying cartilage. Noncontact Nd:YAG/diode lasers will also ablate the mass but less efficiently and dark skin on the other side of the ear can swell, slough or change color. Holding an ice pack on the opposite side of the ear is a helpful safety measure.
Pooled
