Anesthesia and Laser Surgery

Chapter 42


Anesthesia and Laser Surgery



The term laser is an acronym for light amplification by the stimulated emission of radiation. Stimulated emission of radiation was first proposed by Albert Einstein in 1917, and the first laser was developed by Arthur Schawlow and Charles Townes in the late 1950s; Townes went on to win the Nobel Prize in physics for his work on masers and lasers. The first laser, a 694-nanometer (nm) ruby laser, was built by Theodore Maiman in 1960. Lasers now have widespread use in a variety of unrelated fields, including construction, communications, the military, energy production, and the entertainment industry. Lasers are also used in spas and beauty salons for laser hair removal (LHR) by cosmetologists and estheticians.


Medical lasers were introduced in the mid-1960s when the ruby and the argon-ion lasers were first used in retinal surgery. With the development of smaller and more powerful lasers, the use of lasers expanded to all surgical specialties and are frequently used in laparoscopic procedures, surgery of the upper and lower airways, endoscopic arthroscopic procedures, and transmyocardial and transluminal revascularization. Capable of making incisions as small as 0.5 microns, the laser scalpel offers definite advantages over traditional surgery, including improved access to operative sites, greater precision in tissue destruction and removal, increased anatomic preservation, and controlled hemostasis. Surgical lasers have revolutionized the field of dermatology and esthetic surgery.1 In dentistry, lasers are used for gum reshaping, drilling, and whitening of teeth. Lasers continue to play a major role in ophthalmology, in large part because of the introduction of the excimer laser in the 1980s. In 1995 and 1999, the Food and Drug Administration (FDA) approved excimer lasers for photorefractive keratectomy (PRK) and laser-in-situ keratomileusis (LASIK), respectively. LASIK is now one of the most common surgical procedures in the United States. Medical lasers have other therapeutic applications, including photodynamic therapy (PDT) in which laser-activated drugs are used to destroy cancer cells. Medical lasers are commonly used even in private medical offices.



Basic Principles of Lasers



Light


Electromagnetic radiation is a broad spectrum of heat energy composed of radio waves, microwaves, infrared waves, visible light waves, ultraviolet waves, x-rays, and gamma rays (Figure 42-1). A wavelength is the distance between two successive points on a periodic wave that has the same phase. The wavelength decreases and the frequency increases as the electromagnetic spectrum moves from radio waves to gamma waves. Ultraviolet, visible, and infrared waves with a wavelength range of 200 to 1000 nm make up the optical portion of the electromagnetic spectrum. Visible light includes a rainbow of colors—red, orange, yellow, green, blue, indigo, and violet—with a very narrow range of wavelengths of 400 nm (violet) to 700 nm (red). The ultraviolet and infrared portions of the optical spectrum are invisible to humans. Infrared radiation is perceived as heat, and ultraviolet radiation causes a chemical reaction in human skin with little heat production.



Light can be described as both a wavelength and a particle of energy called a photon. The energy of an electromagnetic wave is proportional to its frequency and inversely proportional to its wavelength. Because of its short wavelength and high frequency, ultraviolet radiation contains a lot of energy that can damage the skin. The energy of a photon is defined as the energy emitted when an electron falls from an excited orbit to one of lower energy; the energy between the two orbits defines the wavelength of the emitted photon. The following equation describes the relationship between the energy and the wavelength of light.


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(E = energy in joules, h = Planck’s constant (6.63 × 10−34 J-s), c = the speed of light (2.998 × 10−8 m/s), and λ = wavelength in meters).



Spontaneous and Stimulated Absorption and Emission of Energy


In an atom, electrons exist in specific locations called orbits or orbitals. A specific level of energy is associated with each orbital. The ground state or orbital closest to the nucleus has the lowest energy state; the higher orbitals have the greatest energy. If an electron is to move to a higher orbital, it must gain the difference in energy between the two orbitals; conversely, it will lose energy if it falls to a lower orbit. Electrons have a tendency to return to the ground state spontaneously, releasing a photon of light energy in the process. This is called spontaneous emission of radiation. The energy of the photon and therefore its wavelength is dependent on the energy difference between the two orbits. Light produced by fluorescent lights or incandescent bulbs is a result of electrons changing orbits and returning to ground state. Both the energy of the released photon and the wavelength of the light are proportional to the energy difference between the excited state and the ground state of the atom. Light produced by spontaneous emission is composed of different wavelengths and frequencies; consequently, the photons oscillate randomly (noncoherence), and the light disperses as it travels.


An atom can be “pumped” up to a higher energy level by a stimulating photon if its energy is the same as the energy difference between the two orbitals. A stimulating photon can also cause an atom in an excited state to undergo decay and release energy. When an atom is struck by a stimulating photon, it decays back to its ground state and emits a second photon. If the energy of the stimulating photon is equal to the energy difference between the excited and ground states of the atom, the emitted photon will have the same wavelength, energy, frequency, and direction as the stimulating photon. This process is known as stimulated emission of radiation (Figure 42-2). The two photons can strike other excited atoms and stimulate additional emission of photons, resulting in a sudden burst of coherent radiation as all the atoms return to ground state in a rapid chain reaction.



Properties of laser light that differentiate it from fluorescent or incandescent light include coherence, directionality, and monochromaticity. Laser beam photons have the same wavelength and oscillate synchronously in identical phase with one another (coherence). Laser light moves in a parallel, narrow beam (spatial coherence) over long distances and displays minimal dispersion. This spatial coherence, known as collimation, allows the laser light to be focused on a very small area (Figure 42-3).



Reflectors placed on the moon allow calculation of the distance to the moon by the length of time it takes for a laser light to return to earth. Reflection of a laser beam can reduce the collimation and increase the dispersion, especially if the reflecting surface has a matte or dull finish; however, the reduction in collimation and increased dispersion of a reflected laser beam is insignificant if the reflecting surface is smooth and shiny.


Laser light is composed of specific and discrete wavelengths; consequently, the light emitted is monochromatic and specific for each laser. Just as white light is composed of multiple colors, some lasers are tunable and can emit light at several different wavelengths. However, tunable lasers can only emit one color or wavelength at a time. A typical light bulb is more powerful than a laser, but its light is not collimated, and the dispersion of the light reduces its intensity. In contrast, the intensity of a 1-milliwatt (mW) laser can be six times that of a 100-watt (W) incandescent bulb. Although a typical laser emits only a few milliwatts of power, from a distance of 100 feet lasers can produce a highly intense beam of 1 to 2 mm that can be 1 million times more concentrated than light from an incandescent source.



Components of a Laser


A laser is a device that creates and amplifies a narrow, intense beam of coherent light. It consists of an energy source, an optical resonating cavity, and a laser medium to create the laser light (Figure 42-4). Lasers require an external energy source to transfer or pump up the energy of the laser medium. The electrons in the lasing medium absorb the energy and move to a higher energy state. Flash lamps, continuous light, high-voltage discharge, diodes, or another laser can be used as the energy source. Electric current is used to excite gas lasers, such as carbon dioxide (CO2) and argon (Ar) lasers. Liquid and solid state lasers, such as the potassium-titanyl-phosphate (KTP) laser, require activation by a flash lamp or another laser.



The optical resonating cavity, a tubelike structure, provides optimal amplification of the laser beam. It contains the lasing medium and a mirror at each end of the tube. When the lasing medium is excited by the outside energy source (e.g., flash lamp, electric current), the atoms are “pumped” to a higher energy level, increasing the number of atoms in the excited state. Population inversion is necessary for stimulated emission of radiation, and it occurs when more atoms are in an unstable excited state than the resting state. When one of the atoms spontaneously decays back to its ground state, it releases a photon that “stimulates” another excited atom to decay back to its ground state, releasing another photon. The wavelength, frequency, phase, and direction of the second photon are identical to those of the first photon. The mirrors reflect the excited photons back into the resonating cavity at approximately 186,000 miles per second, where they travel back and forth in a parallel fashion, stimulating the release of more photons from other excited atoms and amplifying the resultant laser light. One of the mirrors is partially transparent and allows a very thin beam of the coherent, collimated, and monochromatic laser light to exit and focus on the target tissue.


The laser medium can be a solid, gas, liquid, or semiconductor that is stimulated to a metastable state when pumped with an external energy source. Lasers are commonly named after the laser medium that determines the wavelength output of the laser. Solid-state lasers such as the neodymium: yttrium-aluminum-garnet (Nd:YAG) laser use a solid matrix that is doped with a small amount of impurity (dopant). It is the impurity, that is, Nd, that provides the energy source for the laser. Solid-state lasers are more powerful than gas lasers and require optical pumping. Gas lasers use a variety of gases as lasing media, including argon, CO2, helium, helium-neon, and krypton and require an electrical source of energy for pumping. Complex dyes, dissolved in a liquid such as alcohol, constitute the lasing media in liquid lasers. Optically pumped, liquid lasers are tunable over a broad range of wavelengths, mostly in the visible spectrum. Excimer lasers use electrical stimulation to produce a dimer of a halogen such as chlorine and fluorine and an inert gas such as argon, krypton, or xenon. The dimer is unstable and quickly breaks down into its constituent atoms, releasing energy in the form of light. Semiconductor lasers, also called diode lasers, are composed of semiconductor crystals that are pumped by a high-intensity current. They are commonly used in compact disc players, laser printers, and laser pointers. The gallium-arsenide laser is an example of a semiconductor laser.



Modes of Operation


The majority of medical lasers deliver only one wavelength, and laser selection is dependent on the desired effect on the targeted tissue. The wavelength or color of the laser light is dependent on the laser medium, and the effect on tissue is dependent on the wavelength. In addition to selecting the appropriate wavelength, the surgeon must use the appropriate exposure time and energy density (power setting) to achieve the intended photomechanical, photothermolytic, or photochemical effect.


A laser beam can be delivered in a continuous wave (CW), pulsed-wave, or Q-switched mode. In the CW mode, the laser continues to emit a steady beam as long as the laser medium is excited. Output is measured in watts and can vary significantly among lasers. For example, the power of the helium-neon laser is measured in milliwatts, whereas the output of the more powerful CO2 laser is measured in kilowatts. Power density of the beam (irradiance or flux) varies from a few watts per square centimeter to hundreds of watts per square centimeter.


Collateral tissue damage can be expected if the laser beam is held on tissue longer than the thermal relaxation time (i.e., time it takes for 50% of the laser energy to be thermally conducted to surrounding tissue). Pulsing the laser beam or scanning a continuous beam allows time for concentration of the energy, limits the exposure time, and minimizes thermal damage. In the pulsed mode, the laser emits peak energy levels in individual pulses from femtoseconds (quadrillionths of a second) to seconds. The power of a pulsed laser is measured in joules, and energy intensity is expressed as joules per square centimeter.


The duration of the laser beam is limited by computerized scanning of the laser beam in a preset pattern before delivery of the beam to the tissue. In the Q-switched mode, the laser emits high-energy, ultrashort pulses (approximately 10 to 250 nanoseconds [nsec]). A shutter is placed in the optical path to allow the buildup of a large population inversion. After release of the shutter, the electrons fall rapidly to ground state, releasing a large amount of energy that is measured in megawatts.


Laser effects on tissue can be controlled by the mode of delivery. The Nd:YAG laser is used in the CW mode for coagulation of tumors, in the pulsed mode for hair removal, and in the Q-switched mode for tattoo removal. In the CW mode, the CO2 laser can be focused very tightly and used for incision, much like a scalpel, whereas the defocused CO2 laser can be used to vaporize a larger area of tissue. When delivered through a scanning device, the laser beam can remove a predetermined thickness of skin.


Fiberoptic cables are used for delivery of laser beams with visible and near-infrared wavelengths. Articulated arms with reflecting mirrors mounted in tubes are used to direct the beam of a far-infrared laser (CO2). Additional devices may be attached to the fiberoptic cables or articulated arms, including slit lamps for use on the eye, operating microscopes, and insulated fibers for use with endoscopes. Contact laser probes (sapphire) attached to the distal end of a fiberoptic bundle transform the light energy into heat for precise cutting and reduced penetration.



Effects of Lasers on Biologic Tissues


Lasers are associated with rapid and precise vaporization or coagulation of tissues and are commonly used in a variety of unrelated diagnostic and therapeutic procedures. Laser light is monochromatic and has very selective effects on biologic tissues. The degree of laser light transmission, scattering, reflection, or absorption is dependent on the tissue and the wavelength of the light. Absorption of the light is necessary for the laser to be effective; if the tissue transmits, reflects, or scatters the light, the laser will have little or no effect on the tissue. A specific wavelength may be absorbed by one type of tissue and transmitted by another. Biologic tissues can be thought of as an aqueous solution of light-absorbing molecules. Chromophores, such as hemoglobin and melanin, and water are the main absorbing components, and they determine the reaction of the tissue to the laser light. To be effective, the laser light must match the absorptive property of the tissue. If light absorption occurs, the laser light is converted to heat; vaporization or ablation of the tissue occurs when the temperature reaches 100° C. As the tissue is vaporized, the thermal energy of the laser beam cauterizes capillaries and provides immediate hemostasis. A lower temperature would produce tissue coagulation or denaturation rather than ablation.


A tissue’s reaction to light absorption depends on the wavelength, intensity, and exposure time of the light. Powerful, short pulses of laser light cause an explosive tissue expansion (photomechanical reaction), whereas low-power, long pulses cause a rapid increase in temperature (photothermal reaction) that results in tissue vaporization and coagulation. When applied for longer durations, low-power lasers can cause a chemical reaction or change in specific molecules (photochemical reaction). Laser light is also used to activate photosensitizing medications that are selectively absorbed by a specific tissue (photodynamic reaction). The effectiveness of nonthermal laser-assisted techniques is dependent on the ability of special drugs (photosynthesizers) to produce cytotoxicity in the presence of oxygen (O2) after stimulation with light of an appropriate wavelength.


Tissue absorption is greatest with longer wavelengths such as the far-infrared wavelength of the CO2 laser (10,600 nm). The CO2 laser beam is completely absorbed by water in the first few cellular layers, resulting in explosive vaporization of the top layer but little or no damage to the underlying tissues. Excimer lasers (ultraviolet) are associated with an even more superficial effect because of their strong absorption by water. The light from lasers with visible wavelengths, such as the ruby, argon, and krypton lasers, is transmitted by water and absorbed by cells that contain dark pigment. It can penetrate the skin and the cornea to coagulate pigmented or vascular lesions. The light from near-infrared lasers, such as the Nd:YAG, is transmitted rather than absorbed by water. Because they have a greater tissue penetration, near-infrared lasers are better suited for deeper procedures such as tumor debulking. Advantages of lasers include precision, access to remote sites in the body, reduced blood loss, reduced damage to adjacent tissue, and improved patient satisfaction. A disadvantage of laser therapy may be delayed wound healing.



Medical Lasers


The major types of lasers used in medicine are far-infrared (CO2), mid-infrared (erbium [Er]:YAG, holmium [Ho]:YAG, Nd:YAG), near-infrared (diode), visible (ruby, krypton, argon, copper, and gold vapor), and ultraviolet (excimer). Commonly used surgical lasers are listed in Table 42-1.




Carbon Dioxide Laser


The CO2 laser has wide application and is the most commonly used surgical laser. The infrared light produced by the CO2 laser (10,600 nm) is invisible to the human eye, and a low-power helium-neon (He-Ne) laser (633 nm) is incorporated to provide a visible red beam for surgical aim. Because it emits light in the infrared region of the electromagnetic spectrum, the CO2 laser is a powerful but dangerous laser. Infrared radiation is heat, and this laser basically melts through whatever its beam is focused on, including steel. It is a very precise laser; with the lateral zone of damage less than 0.5 nanometer (nm) from the area of incision. The CO2 laser beam is not transmitted by quartz, glass, or other transparent material and must be delivered as a free beam or through a rigid endoscope with a mirrored, articulated arm. The CO2 laser light is strongly absorbed by water, and vaporization of cells occurs within the first 100 to 200 µm of the irradiated surface. It can be used in both the CW and pulsed-wave mode. Focused into a tight beam, the CO2 laser can be used for cutting. Defocusing the beam decreases the power density, and the tissue will be vaporized, or ablated. The CO2 laser is used extensively in general surgery, orthopedics, gynecology, urology, and otolaryngology and is associated with minimal blood loss. When used with a scanning device, thin layers of the skin are ablated for skin resurfacing during cosmetic surgery. Because of its high power, the CO2 laser is widely used in industry for cutting, drilling, and welding.



Yttrium-Aluminum-Garnet Lasers


The lasing medium of the YAG laser is a YAG crystal rod doped with atoms of rare earth minerals, which accounts for the different properties of the YAG lasers. YAG lasers can be used in the CW, pulsed-wave, or Q-switched mode.







Visible Lasers









Dye Lasers


Organic fluorescent materials are dissolved in a solvent such as methanol and are typically pumped with a flashlamp or another laser. The energy levels of the dyes are very close to one another and allow the lasers to release a wide range of wavelengths. In the CW and pulsed mode, dye lasers have wavelengths of 400 to 1000 nm. They can produce extremely short pulses (measured in trillionths of a second [picoseconds]). The major advantage of the dye laser is the ability to tune the wavelength to maximize the laser-tissue interaction. Dye lasers are used in dermatology for excision of vascular and pigmented lesions, in urology for treatment of urinary calculi, and in oncology for photodynamic therapy. The pulsed dye laser (PDL) uses a rhodamine dye to emit a yellow laser beam at 577 to 585 nm (peak absorption of hemoglobin). It is the laser of choice for treatment of port-wine stains and thick, red scars.



Excimer Lasers


Derived from the terms excited and dimer, excimer lasers use a medium composed of a reactive noble gas (chlorine or fluorine) and an inert halogen gas (argon, krypton, or xenon). When the medium is electrically stimulated, an unstable pseudomolecule (dimer) is produced. As the dimer breaks down to its constituent atoms, it releases light in the ultraviolet range that is strongly absorbed by water. Excimer lasers have a photochemical effect on targeted tissues (pulsed mode), with minimal thermal effect on the underlying tissue. The very short wavelength (ultraviolet) is capable of high resolution and has applications in microscopic surgery. Examples of excimer lasers include the argon-fluoride (193 nm), krypton-fluoride (249 nm), xenon-chloride (308 nm), and xenon-fluoride (351 nm) lasers. They are currently used in ophthalmology for photorefractive keratectomy (PRK) and laser-in-situ keratomileusis (LASIK). Other uses include removal of arterial plaques and treatment of psoriasis.


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May 31, 2016 | Posted by in ANESTHESIA | Comments Off on Anesthesia and Laser Surgery

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