Lasers in Otolaryngology
Introduction:
The laser (Light Amplification by Stimulated Emission of Radiation) is being used in otolaryngology for nearly half a century. It is now accepted as part of the surgical armamentarium.
Principle of laser:
Normally, an atom is in a stable form which means that the electrons are equal to the number of protons, and are revolving around the nucleus in a fixed orbit. When given energy, the electrons change their orbits away from the nucleus and in this stage the atoms are said to be in an "excited state". This excited state does not last long. Such atoms in excited state release their absorbed energy automatically (spontaneous emission) and returns to the original state. If photons are made to strike these excited atoms, the decay of the atoms is accelerated and both the incident and the absorbed photons are released. This process is known as stimulated emission. This stimulated radiation is amplified with the help of mirrors. Lasers are thus electromagnetic radiations. Their wavelenths are determined by the lacing medium which could be argon, carbondioxide, Nd:YAG, helium etc.
The optical cavity containing the lasing medium has a 100% reflective mirror at one end and a semi-reflective mirror at the other end. The photons travelling along the axis of the mirrors are reflected and thus continue travelling within the optical cavity and stimulates the release of more photons. This reflection produces a temporally and spatially coherent beam of light that escapes via the semireflective mirror as the laser beam. The first laser machine used ruby as the lasing medium.
Tissue interactions with lasers:
Laser light falling on tissues may be reflected, scattered, transmitted or absorbed. Out of all these interactions only absorbed light causes a tissue reaction.
Reflection:
Part or whole of laser light is reflected back
Scatter:
Laser energy scatters in the tissues and its penetration deep into the tissues becomes limited. Shorter the wavelenth of laser more of the energy is scattered.
Transmission:
The light is transmitted through the tissue without causing any effect on tissues through which it passed. Argon laser has been used to coagulate retinal vessels without any damage to cornea, lens or the vitreous.
Absorption:
Laser energy is absorbed by the tissue. It is exactly this absorbed energy that produces the effect on the tissue. The main substance that absorbs the laser is called the primary chromophore. Absorption produces mainly kinetic excitation of the absorbing molecules. This kinetic excitation produces thermal effects ranging from reversible hyperthermia through enzyme deactivation, protein denaturation, and coagulation to dehydration, vaporization and carbonization.
The effect of laser on tissue depends on the absorbed energy. At a temperature of 60 degrees centigrade, there is protein denaturation, but tissues can recover from here. At 80 degrees centigrade there is degradation of collagen tissue and at 100 degrees centigrade, cells and their pericellular water convert into heat that causes tissue ablation. Lasers can hence be used to cut, coagulate blood vessels or vaporize the tissue. When a burn is caused by a laser beam there is some degree of collateral damage.
Zones of tissue damage can be divided into:
Zone of vaporization:
A crater is created due to tissue ablation and vaporization leaving behind only a few flakes of carbon.
Zone of thermal necrosis:
This zone is just adjacent to the zone of vaporization. There is associated tissue necrosis. Small blood vessels, nerves and lymphatics are sealed.
Zone of thermal conductivity and repair:
This zone recovers with time.
Properties & effects of lasers:
Depending on the wavelength of laser energy, it produces the following effects:
Photothermal:
This produces heat energy that is used to cut, coagulate or vaporize tissues.
Photoacoustic:
It can be used to break stones and is used in lithotripsy.
Photochemical:
Ultraviolet lasers with wavelength of 248 and 312 nm can ionize DNA and RNA respectively. They could even be carcinogenic. This effect of specific lasers (argon) has been used in photodynamic therapy to selectively destroy cancerous tissue.
Photodissociation:
This effect of laser breaks C-C bonds, divides collagen without heating it. This feature is made use of in excimer laser in LASIK procedures to reshape the cornea for refractive errors.
Classification of laser as per electromagnetic spectrum:
Visible lasers:
Visible light has a wavelengthe of 400-700 nm. Lasers that fall within this range are known as visible lasers. They have different colors from violet to red (VIBGYOR). Since these laser beams are visible they dont require a separate aiming beam to focus them. Argon laser has blue color (488-514 nm). KTP laser (512nm) is also in the visible range and has blue green color.
Invisible lasers:
Lasers in the ultraviolet zone (1-380nm) and infrared zone (>760 nm) are not visible. Infra red lasers are further subdivided into near infrared lasers (760-2500 nm), mid infrared lasers (2500-50,000 nm). There are no lasers in the far infrared zone.
Lasers that can be transmitted through optical fibers include:
Argon KTP Nd:YAG Er:YAG Ho:YAG Diode laser
Lasers used in ear surgeries include:
Argon - 514 nm KTP -532 nm Carbondioxide - 10,600 nm Er:YAG - 2960 nm
In ear surgeries lasers are used to vaporize small glomus tumors, acoustic neuromas, small A-V malformation, granulation tissue or adhesions in the middle ear cavity. Lasers have also been used to perform myringotomy, perforation of foot plate during stapes surgery, and coagulation of membranous posterior canal in BPPV.
Operational parameters:
1. Wave length of laser beam : The exact properties of laser depends on its wavelength
2. Power: Is actually the output from the machine and is measured in watts. Higher the power, more is the energy delivered to the tissues
3. Exposure tine: It is measured in seconds
4. Spot size: This is actually the area exposed to the laser beam. Spot size is the minimum at focal length. Focussed beam is used for cutting and decofocussed beam is used for coagulation / ablation of tissues.
5. Power density: It is the power delivered per unit area of spot size and is measured in watts/square cm. This indicateste intensity of the beam.
6. Exposure to laser: This value is the power density multiplied by duration of exposure in seconds and is measured in joules/ cubic cm.
Laser delivery mode:
Continuous mode:
It provides constant stable energy as the active medium is continuously kept in a stimulated mode.
Pused mode:
This gives interrupted beam as the active medium is intermittently activated for a short time.
Q - switched mode:
This mode provides very short pulses in a controlled manner. Pulses range between 10 ns and 10 milliseconds.
Advantages:
Precision
Rapid ablation of tissues
Excellent hemostasis
Minimal post op pain
Minimal tissue oedema
Disadvantages:
Expensive
Expensive to maintain
Safety precautions need to be taken
Types of lasers used in otolaryngology:
Argon laser:
This lies in the visible spectrum. Does not eed pointing ray. It is absorbed by hemoglobin. Hence it is used to treate portwine stain, hemaingioma and telengiectasis. When focussed to a small point it can vaporize the target tissuee. This laser is used to create a hole in the foot plate of stapes. It needs a drop of blood to be placed over the foot plate for this effect to occur.
KTP laser:
This laser also lies in the visible spectrum. It has a wavelength of 532 nm. These waves are absorbed by hemoglobin and can be delivered via optical fibers. This laser is also used ins tapes surgery, endoscopic sinus surgery to remove polypi, inverted papilloma and vascular lesions.
Nd:YAG laser:
This laser has a wavelength of 1064 nm and lies in the infrared zone. It is in the invisible range and requries a separate aiming beam of visible light to focus. It can pass through clear fluids and is also absorbed by pigmented tissue as the case may in eye and urinary bladder. In otolaryngology it has been usedto debulk tracheobronchial and esophageal lesions for palliation.
Carbondioxide laser:
It has a wavelength of 10,400 nm in the invisible range. It requires an aiming beam of visible light to focus the laser beam. This laser does not pass through optical fibers. It needs special articulated arms and mirrors that reflect the laser beam to the spot of the lesion. This laser beam is absorbed by tissues high in water content and is not color dependent. Reflection and scatter through tissues is minimum. Its tissue effect is in depth and in adjacent tissues laterally. Clinically it is used in laryngeal surgeries to excise papillomas.
Diode laser:
This has a wave length of 600-1000 nm. It can be delvered via optical fibers and is moderately absorbed by melanin and hemoglobin. Diode lasers are used for turbinate reduction, laser assisted stapedectomy and tonsillar ablation.
Safety precautions:
Eye protection to surgeon and assistant by wearing goggles. Wavelenght specific glasses should be worn to prevent retinal damage. Patient's eye should be protected by double layer of saline soaked cotton eye pads. All exposed areas of face are covered by saline soaked pads.
Endotracheal tubes:
Wave specific tubes are available. Rubber tubes are better than PVC as they are more resistant to laser hits. PVC tubes when hit by laser can generate toxic fumes. These endotracheal tubes should be covered by reflective aluminium foils. The cuff of the endotracheal tube should be inflated with blue dye mixed saline and covered with wet cottonoids. In case of accidental hit by laser blue color effusion will warn the surgeon.
Anesthetic gases:
Non inflammable gases are used. Halothane / enflurare are preferred to nitrous oxide. Concentration of oxygen should not exceed 40%.
Smoke evacuation:
Constant suction should be used to suck out fumes released out of laserization of tissue.
