Laser Risks and Preventive Measures for the Staff

Chapter 20 Laser Risks and Preventive Measures for the Staff



Vangie Dennis, RN, CNOR, CMLSO


This is a remarkable age in which humanity rides the surging crest of a great wave of scientific and technologic achievement. Technologic advancements have fashioned an ingenious tool to develop a device that produces a type of light energy that has never been seen before—the laser. Light may come from a chemical reaction, light may come from energy conversions, but light from lasers is unique.



HISTORY OF LASERS


In the seventeenth century Sir Isaac Newton discovered that sunlight was composed of a rainbow of many colors. He believed that light was propagated by tiny particles, but could only speculate about the origins of this phenomenon. By the end of the nineteenth century it was known that light was a visible manifestation of a primary force of nature, electromagnetism. This also includes radio waves, microwaves, and x-rays. Light energy, like atoms, emits other waveforms of energy as it spontaneously changes from one energy state to another. By itself, a spontaneous process cannot produce light. It was in the twentieth century that Albert Einstein predicted that, under special circumstances, photons (particles of light) could bombard a substance and could stimulate the release of identical photons. This process is called stimulated emission. It had never been described before.


The word LASER is an acronym for Light Amplification by Stimulated Emission of Radiation (Bertolotti, 2004). A laser beam is merely light; however, it is light that is amplified by stimulated emission, unlike normal light, which is produced by spontaneous emission. The differences between laser light and normal light are part of the reason that the laser has so rapidly assumed, and is continuing to assume, such a prominent position in science and medicine. Understanding the physics of the laser is necessary to understanding and optimizing safe practice within the perioperative setting.


The concept of a laser actually began in 1913 when Niels Bohr developed his theory of the atom (Carruth and McKenzie, 1986). This theory uses the model so familiar today of the atom consisting of a nucleus around which electrons circulate. Based on this model, Bohr was able to describe the atomic basis for the production of light energy by spontaneous emission. Using Bohr’s model, in 1917 Albert Einstein speculated on the possibility of both spontaneous and stimulated emission. However, the existence of stimulated emission remained primarily a theory until 1954, when Charles H. Townes, Nikolay Basov, and Alexander Prokhorov were able to physically produce measurable stimulated emission with the MASER (Microwave Amplification by Stimulated Emission of Radiation) (Carruth and McKenzie, 1986).


Based on work with the maser, Charles H. Townes and Arthur L. Schawlow published a paper in 1958 discussing the possibility of extending the maser principles into the optical portion of the electromagnetic spectrum (Bertolotti, 2004). A year earlier, Gordon Gould, a graduate student at Columbia University, developed the principle of laser. He recognized that he had something great, so he had the notebook containing this theory notarized. He received bad legal advice, however, and did not pursue a patent until 9 months after Townes and Schawlow had submitted their patent. After many years of legal battles, Gould finally received credit and recognition for coining the word laser (Ball, 2004). However, it was not until 1960 that Theodore H. Maiman of Hughes Research Laboratories developed the first visible laser, a ruby laser (Maiman, 1960). Following this breakthrough, the entire area of visible and near-visible lasers grew rapidly. The development of other lasers quickly followed: the helium-neon (He-Ne) and neodymium:yttrium-aluminum-garnet (Nd:YAG) lasers were developed in 1961. The first semiconductor laser was developed in 1962, and 1963 brought the development of the carbon dioxide (CO2) laser.


Laser technology continues to grow as physicists strive to develop laser in the ultraviolet spectrum and into the x-ray spectrum. Similar growth is seen in the clinical and commercial use of lasers. Nowhere is this growth more evident than in the medical applications of laser, not only in surgery, but also in diagnosis and treatment. The growing prominence of lasers in medicine is rapidly approaching the point where no physician’s education will be considered complete until his or her training has included some knowledge of basic laser physics and experience with laser applications. Lasers today have a myriad of uses. They are used as target designators for DNA research, in magnetic resonance imaging (MRI) medical systems, and to create holograms.



LASER PHYSICS



Light Properties


In understanding the fundamentals of laser physics, it is essential to first have a basic understanding of the areas of science that deal with light properties and electromagnetic energy. Understanding the fundamentals of laser physics is directly proportional to understanding the safety practices of operating room personnel.


Light can be described as a beam or a ray. This beam or ray of light travels in a straight line until it encounters another object. Once surface contact is made, three things may occur: (1) the light can be absorbed by the surface, (2) it can be reflected back from this surface, or (3) it can penetrate and be transmitted beyond this surface.


When light is absorbed by a surface or material, the energy of the light beam is normally transformed into heat energy. This thermal effect of light energy absorption is the principle behind laser surgery. The laser light is absorbed by the tissue and transformed into thermal energy, causing the desired ablation or coagulation of tissue. Thus laser surgery is similar to many other means of delivering heat energy to the tissue. However, it is by far the most efficient and most precise means of delivering thermal energy to date. It is this efficiency and preciseness that makes the laser so valuable to all fields of surgery (Laserscope, 2002).


The reflection of a laser beam from a surface is primarily a safety hazard in laser surgery. All laser light can be reflected off any shiny surface, and the flatter and smoother the surface, the worse the safety hazard. Surgical tools with roughened or blackened surfaces have been developed to minimize the risk for inadvertent reflections. There are times, in procedures such as those in the upper airway, when reflection of the laser energy serves as a means of diverting the thermal energy away from the endotracheal tube.


The transmission of laser light through a surface is a factor that is very much dependent on the frequency or wavelength of the laser light and the properties of the tissue at that frequency. For example, glass transmits light in the visible spectrum almost completely, while completely blocking the transmission of the far-infrared energy produced by the holmium:yttrium-aluminum-garnet (Ho:YAG) or CO2 laser. When light transmits into any solid surface, the light will interact. Each particle will then reflect, absorb, or transmit the light off or through the surface layer. This random reflection, absorption, and transmission scatters the light in all directions within the material. The deeper the light is able to transmit within the material, the more the scattering and spreading of the light energy. Laser energy at wavelengths that transmit deeply into biological tissue will cause thermal destruction at greater depths, but will also cause a subsurface area of thermal damage, the radius of which is larger than the radius of the laser beam spot size (Laserscope, 2002).



Laser Energy Generation


The physical basics of light energy are the foundation of the generation of laser light. Stimulated emission does not occur in nature. The environmental conditions must be very precisely controlled to make stimulated emission predominant over spontaneous emission. The key to laser energy generation lies within the laser tube, or optical resonator cavity. The laser tube is almost always cylindrical, designed so that laser propagation can occur in all directions. Within the laser tube is the active medium, which contains the atoms whose electrons undergo an energy-level transition to produce the laser light.


The name of a particular laser is derived from the active medium within the laser tube, which is responsible for the actual lasing. The medium can be a solid, a liquid, or a gas. A solid medium is usually in crystal form, although semiconductor lasers, so important in the communication industry, are also considered solid-state lasers. The potassium-titanyl-phosphate (KTP) laser and the Nd:YAG lasers are among the most common solid-state lasers. Liquid lasers are almost exclusively dye lasers. Dye lasers are very versatile, because, by changing the concentration or type of dye within the resonant chamber, the laser can actually be tuned to almost any desired frequency. However, a major drawback to dye lasers is a severe limitation in the amount of output power possible. Dye lasers are often used in ophthalmology and dermatology procedures. Gas lasers are by far the most common and popular types of lasers. Examples of gas lasers include the CO2, the helium-neon, and the argon laser. Gas lasers are generally considered to be the most economical, efficient, and reliable type of laser (Carruth and McKenzie, 1986).



Characteristics of Laser Light


Laser light produced by stimulated emission possesses three properties that distinguish it from ordinary light produced by spontaneous emission. These three properties are monochromaticity, coherence, and collimation (Laserscope, 2002).


Monochromaticity means that all of the photons of light emitted by the laser are at one frequency and are moving in the same space and time. However, laser light is as close to being at one frequency as is physically possible. White light produced by spontaneous emission consists of light of all visible frequencies or all colors mixed together. Even visible light that might be considered a single color actually consists of a considerably wider spectrum of frequencies than light produced by stimulated emission. Therefore laser light is one color of light—monochromatic (Laserscope, 2002).


Coherence is a term referring to the wave nature of light and describes the fact that all of the peaks of the sine waves representing each photon are exactly in phase with each other, or both in the same space and time. In other words, all of the peaks of the waves exactly coincide, allowing all of the numerous small oscillations to accumulate. White light produced by spontaneous emission consists of all different frequencies. Therefore it is impossible for the waves of the white light to sum up consistently as does monochromatic light, because each frequency has its own wavelength, and the difference in wavelength will ensure that the photons of different frequencies will be out of phase the majority of the time. White light is composed of many colors of the light spectrum and will cancel each frequency out. With laser light, because of the coherency, the frequency will become in sync. This accounts for the brilliance so often seen with a laser beam (Laserscope, 2002).


Collimation describes the fact that all of the photons, or waves, produced by the stimulated emission process are going in exactly the same direction, parallel to each other. This property allows the stimulated emission photons to remain in phase while traveling for a long distance away from the laser source. Light produced by spontaneous emission tends to spread in all directions, as does the light produced by a light bulb. Although spontaneous emission light can be “collimated” using special optical lenses or mirrors to form a beam, it can never be as well collimated as light produced by stimulated emission (Laserscope, 2002).


Several characteristics of laser light result from these three properties. Although white light can be used to form images, laser light cannot form images, primarily because the laser light is collimated and coherent. Image formation requires light to travel in a wide range of directions from a given point. Because laser light photons all travel in the same direction, the light cannot be used to form an image. It can, however, be used to form holograms, but these are not “true” images (Bertolotti, 2004).


Laser light can be focused to an extremely sharp point using a lens. White light, however, can never be focused to a sharp point, because it consists of a mixture of light frequencies. Each frequency of light will have its own individual focal point after passing through a physical lens. Therefore, when the red portion of the spectrum is focused, the blue portion is not, and vice versa. Laser light can be focused to such a fine point that the molecules of air will literally explode if their breakdown voltage threshold is exceeded.



Operating Parameters


Three operating parameters determine precisely what the laser beam will do when exposed to tissue during surgery. These parameters are power, spot size, and exposure time. These parameters are used in combination to achieve the desired tissue effects.



Power


Power is an operating parameter expressed in watts. When the laser is turned on, the laser surgeon must decide what power setting will be used for the procedure. This parameter should be dictated by that surgeon’s own experience and by consensus recommendations. The power setting usually is the highest recommended wattage with the shortest duration of exposure (time impacting tissue) so that thermal conductivity does not become a concern.



Spot Size


Spot size is the second operating parameter and describes the diameter of the minimum spot size achievable with a given lens. This parameter is typically given in millimeters. It is important to remember that spot size specifications are usually given for the spot size diameter, not the spot size radius, which is typically used in power density calculations. The spot size determines how concentrated the beam’s power will be at the focal point of the laser beam. The minimum spot size is usually fixed by the focal length of the focusing lens. The spot size can be changed by using another lens with a different focal point (Laserscope, 2002).



Exposure Time


Time is the feedback parameter during laser use. Generally it is given in seconds, but it can be expressed in milliseconds (1 msec = 1 × 10−3 seconds). The longer the beam impacts tissue at a given power and spot size, the deeper that beam will penetrate. A surgeon uses time as the feedback mechanism in the same manner that a surgeon uses the feeling of pressure as feedback when cutting with a scalpel. There is also a similar learning curve that must be mastered when first using the laser. Learning to use the scalpel takes time and experience to master how much pressure is necessary to make a cut that is neither too shallow nor too deep. Learning to use the laser also necessitates the accumulation of experience to master how much time on tissue is required to cut or ablate tissue to the depth desired.



Power Density


Power density is a parameter that combines two of the three operating parameters to describe how concentrated the laser beam is on the surface of the tissue. Power density determines the effect of the laser beam when applied to the tissue. High power densities of the CO2 laser correlate with vaporization of tissue, whereas lower power densities are associated with tissue coagulation. Power density is expressed in watts per square centimeter (W/cm2), although occasionally this parameter is given in watts per square millimeter (Ball, 2004).


Increasing the power density will increase the laser beam’s ability to vaporize as opposed to coagulate. The power density is directly proportional to the power setting. Therefore doubling the power will double the power density. However, the power density is inversely proportional to the square of the spot size diameter. Thus decreasing the spot size by half will quadruple the power density (Bertolotti, 2004).


The power density is influenced to a much greater extent by the beam spot size than it is by the power setting alone. Because of this relationship, a laser can be easily taken from full vaporization to full coagulation merely by focusing and then defocusing the laser beam. When the delivered laser light is at the focal point (which is determined by the focusing lens), the laser beam will have its maximum power density and will perform vaporization most efficiently. Backing away from the focal point, or defocusing, allows the laser beam to diverge and enlarge the spot size, thus decreasing the power density.


Radiant exposure is the true determinant of the amount of thermal damage caused by the laser. Radiation exposure takes into account all three of the operating parameters of the laser using the following equation:



image



Radiant exposure is directly proportional to the amount of thermal damage done to the tissue and is a better description of the tissue effect than is power density (Bertolotti, 2004). The actual time the laser impacts tissue is extremely difficult to estimate, particularly when the laser beam is in almost constant motion over the tissue surface as it is during most surgical procedures. Most descriptions of laser surgical procedures deal with the parameter of power density, leaving variations in the application of the time parameter to the discrimination of the individual laser surgeon.



EFFECTS ON BIOLOGICAL TISSUE


A laser beam’s effect on tissue will vary with the frequency of the laser light and the type of tissue with which the laser interacts. Two properties of the tissue are often used to describe this interaction: the coefficient of absorption and the selective absorption (American National Standards Institute [ANSI], 2005)



Coefficient of Absorption


The coefficient of absorption describes how a particular substance absorbs a specific frequency of light. Because soft tissue is normally 70% to 90% water, water is used as the standard for biological tissues; it is also useful as a model for nonpigmented soft tissue. Water is a particularly good model for describing the interactions of different lasers with the clear structures of the eye.


The three major lasers used in medicine are the KTP, the Nd:YAG, and the CO2. Because each laser operates at a fairly specific wavelength, each laser is associated with its own, individual coefficient of absorption.


The coefficient of absorption is a factor describing how rapidly the particular frequency of energy will be absorbed with depth. Low coefficients of absorption often correspond to very high percentages of scatter. If the energy is not well absorbed and is not highly reflected, it will attempt to transmit beyond the surface. As some laser energy penetrates, it allows for a large degree of scattering. Nd:YAG laser energy, in particular, scatters to a great degree, especially in tissue. In fact, scattering is the main factor that limits the penetration of the Nd:YAG laser in tissue. The absorption coefficient commonly given for 1064-nm wavelengths (Nd:YAG) in tissue are taking into account not only absorption but also scattering. Because “tissue” is not uniform in composition, these values will vary a great deal for different types of soft tissue and even for the same type of soft tissue from different locations (ANSI, 2005).



Selective Absorption


Selective absorption takes into account the absorption of laser energy by constituents of the medium other than water. Laser energy in and near the visible spectrum is readily absorbed by pigments and dyes.


Normal white light is composed of all of the frequencies or colors of electromagnetic energy in the visible spectrum (e.g., red, yellow, blue). Pigments absorb certain frequencies of the spectrum very well and other frequencies not as well. The color of an object determines how well those wavelengths are absorbed. In effect, the pigments take certain portions of the visible spectrum out of white light by absorption, allowing the portions that are perceived as the color to be transmitted, reflected, and scattered. For example, hemoglobin is a very good absorber of the blue and green portions of the visible spectrum, allowing the red portion to be perceived as the color of hemoglobin. Thus hemoglobin is a very good absorber of the blue-green argon and the green KTP laser energies, but not as good an absorber of the red ruby or helium-neon laser energies. Hemoglobin also absorbs the near-infrared energy from the Nd:YAG lasers fairly well, although not as well as blue and green light. Melanin tends to absorb all frequencies of visible light equally well.


The selective absorption by pigmented tissues is the principal reason why the argon laser was initially so popular in ophthalmic surgery. The blue-green light energy from the argon laser transmits easily through the clear structures of the eye with minimal absorption by the lens and aqueous humor while being readily absorbed by the pigmented retina. Thus the laser energy is preferentially absorbed in the retinal area of the eye as is desired for performing many retinal procedures. The argon laser is now being replaced by the KTP 532-nm wavelength and diode laser technology. Similarly, both the KTP and Nd:YAG laser are extremely useful for the treatment of surface pigmented lesions, allowing laser energy to be absorbed primarily in the pigmented areas while sparing (relatively) the nonpigmented areas (Dennis, 2008).



Wavelength Versus Transmission


The properties of soft tissue absorption and transmission are dependent upon the laser wavelength. The properties of soft tissue determine the amount of laser energy that will be reflected from, absorbed by, or transmitted between air and tissue. Transmission or penetration into soft tissues is very good for low-frequency electromagnetic energy.


Beginning in the very-low-frequency infrared (invisible) portion of the spectrum, transmission begins to increase with increasing frequency or decreasing wavelength. A high degree of transmission is seen in the x-ray spectrum, which makes these frequencies particularly good for medical imaging.


The far-infrared energy produced by the CO2 laser (wavelength = 10,600 nm) has extremely low transmission in soft tissue. Thus the CO2 laser energy is very well confined to the upper 0.1 mm of the tissue surface. The near-infrared energy produced by the Nd:YAG laser (wavelength = 1064 nm) penetrates deeper in nonpigmented soft tissue than the CO2 laser energy and scatters significantly. In fact, it is primarily the scattering by the tissue rather than absorption that limits the depth of penetration of Nd:YAG energy. The visible blue-green energy of the argon laser (wavelength = 488 nm) or the KTP (wavelength = 532 nm) would penetrate the deepest of the three lasers if the tissue were completely devoid of pigment and hemoglobin. However, the presence of pigment and blood, even in relatively avascular tissues, changes the depth of penetration drastically as a result of selective absorption. Therefore the depth of penetration by the argon or KTP laser energy is less than that of the Nd:YAG laser energy, and in extremely pigmented tissues the penetration depth can closely approximate that of the CO2 laser (ANSI, 2005).



Effects of Temperature on Tissue


Laser surgery is an accurate and efficient method of performing thermal surgery, in contrast to other modalities used in operating rooms. Of course, other considerations are the operator’s experience and understanding of the tool. The actual surgery results from the conversion of the laser energy into heat. In tissue, protein denaturation occurs at and above 60 ° C (140 ° F), which results in irreversible tissue injury. At 100 ° C (212 ° F), water converts into steam. At some point between 300 ° and 400 ° C (572 ° and 752 ° F), tissue will begin to carbonize, resulting in surgical smoke or plume. At approximately 530 ° C (986 ° F) and in the presence of oxygen, tissue will burn and evaporate. Although these temperature-related effects are similar for all medical lasers once the energy is converted to heat, the differences in absorption of the various wavelengths by the various components of tissue make a great difference in the actual heat conversion during laser surgery (ANSI, 2005).


Heat coagulation of tissue is generally a slow process until carbonization begins. When carbonization occurs, energy absorption increases as does the rise in surface temperature. This is true for all types of medical laser energy because carbon is such a good absorber in the infrared and visible portion of the electromagnetic spectrum. A laser beam should be activated continuously on carbonized tissue. This is particularly true for the CO2 laser, but also applies to the Nd:YAG, the diode 830 nm, the KTP, and the argon lasers. Carbon will absorb laser energy extremely well and will rapidly heat the surface of the tissue to 300 ° C to 500 ° C (572 ° to 932 ° F). Surface temperatures this high will significantly extend the zone of thermal necrosis underlying the tissue surface as thermal conductivity spreads to heat the adjacent structures (ANSI, 2005).



LASER SAFETY


When lasers are introduced into a health care environment, whether in a hospital, surgery center, or a physician’s office, health care professionals must be prepared to address issues of safety for both the staff and the patient. All lasers present safety hazards and should be used in accordance with established regulations, standards and recommended practices, manufacturer’s recommendations, and institutional policies. Laser safety is based on knowledge of the specific laser wavelength being used, its instrumentation and delivery systems, mode of operation, laser parameters, power densities, absorption in tissues, and associated risks specific to the wavelength.



Federal Regulatory Agencies and Nongovernmental Controls


Lasers are classified as medical devices and are subject to regulation. The Performance Standards for Light-Emitting Products (21 CFR 1040) provides specifications for manufacturers of medical laser systems. Becoming acquainted with the organizations, laws, and standards regulating or affecting the use of lasers in a medical setting familiarizes the health care professional with the information necessary to develop and implement an appropriate laser safety program.


The federal Performance Standards for Light-Emitting Products (21 CFR 1040.10) (U.S. Food and Drug Administration, U.S. Department of Health and Human Services, 2009) requires that the operating manuals from the manufacturers provide the following information: calibration, maintenance, warning signs, nominal hazard zone, specific laser eyewear, and safety instructions that the laser safety officer can incorporate into standard operating procedures.



FDA and the National Center for Devices and Radiological Health


All medical lasers are regulated by the U.S. Food and Drug Administration (FDA) under the Medical Device Amendments to the Food and Drug Act, which apply primarily to laser manufacturers. These regulations are enforced by the National Center for Devices and Radiological Health (NCDRH). The FDA regulates more than 250 types of lasers, including those intended for medical and surgical use. The federal guidelines require manufacturers to classify the medical laser systems based on the laser’s ability to cause damage to the eye and skin. They are categorized under FDA classification of medical devices as class III, subdivision class 4, because of the potential hazards. Manufacturers must conform to all safety requirements of the federal standard. They must also be approved by the NCDRH before any marketing or testing of a laser for a particular clinical application or use and must comply with the labeling requirements (ANSI, 2005).

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Aug 5, 2016 | Posted by in ANESTHESIA | Comments Off on Laser Risks and Preventive Measures for the Staff

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