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Print version ISSN 0034-7094
Rev. Bras. Anestesiol. vol.54 no.1 Campinas Jan./Feb. 2004
Laser surgery and anesthesia*
Cirugía a láser y anestesia
Odilar Paiva Filho, M.D.I; José Reinaldo Cerqueira Braz, TSA, M.D.II
do Programa de Pós-Graduação em Anestesiologia da FMB-UNESP.
Anestesiologista do Hospital Beneficência Portuguesa de São José
do Rio Preto, SP
IIProfessor Titular do CET/SBA do Departamento de Anestesiologia da FMB-UNESP
BACKGROUND AND OBJECTIVES:
Laser surgeries pose risks both to patients and the medical staff. This article
aimed at presenting basic notions for laser usage and rules for improving laser
CONTENTS: Notions of physics applied to laser, safety rules and procedures in case of adverse events with laser application are presented.
CONCLUSIONS: When operated by trained professionals, and provided safety rules are followed, laser is useful and safe both for patients and the medical staff.
Key Words: ANESTHESIA; EQUIPMENTS; laser
JUSTIFICATIVA Y OBJETIVOS:
Las cirugías con la utilización del láser presentan riesgos para
el paciente y para el grupo médico. El objetivo de este artículo es
mostrar nociones básicas sobre el láser y reglas para aumentar la
seguridad de los procedimientos con su utilización.
CONTENIDO: El presente artículo contiene nociones de física aplicadas al láser, reglas de seguridad y la conducta en caso de ocurrencia de eventos adversos con la utilización del láser.
CONCLUSIONES: Concluimos que, cuando manipulado por profesionales entrenados, y respetadas las normas de seguridad, el láser es útil y seguro, tanto para el paciente cuanto para el equipo médico.
Laser is the acronym for "Light Amplification by Stimulated Emission of Radiation". In principle, laser has military application as a marker for training targets. It is also used in communications, aircraft navigation systems, scanners, metal work, photography, holography and Medicine. In Medicine, laser is used to more precisely dissect tissues, coagulate small veins, maintain sterility conditions and promote less inflammatory response. Provided safety rules are followed, laser decreases postoperative morbidity as compared to conventional operating techniques, especially in laryngeal surgeries 1. This study aimed at helping health professionals to safely use laser beams.
LASER BEAM DEVELOPMENT
Laser beam development was only made possible by the advent of Quantic Physics, which has allowed the work with atomic structure models and electromagnetic radiation.
Many light sources are heated gases or solids, such as electrically heated tungsten filaments of standard light bulbs, which emit wavelength spectra of relative and temperature-dependent intensities. Simplifying the study of different variables, classic physicists have created a "cavity radiator". This was a solid artifact with a lateral hole which, when heated to a certain temperature, would make light inside the hole brighter than light outside the hole, regardless of radiator's material, but temperature dependent. However, a lot had still to be explained.
In 1900, Max Planck made important discoveries. He has concluded that atoms could not continuously irradiate, but only in "quanta", and only when they jumped from a high energy to a low energy state. However, light process was still considered a wave.
If light is reflected by a metal surface, than electrons are released from the metal - photoelectric effect. If metal is positioned in a vacuum tube with an external circuit, it is possible to measure the voltage through which photoelectric current is interrupted and to calculate photoelectrons energy. It has been observed that:
1. Photoelectrons are emitted as soon as light appears;
2. Current flow is a function of light intensity;
3. Photoelectrons energy is a function of light frequency.
There has been a failed attempt to explain this phenomenon by the wave theory. However, Albert Einstein in 1905 was able to explain the phenomenon. Einstein has postulated that light was not "continuous waves" but rather "quanta", which he has called photons. Based on Panck's theory that atoms would behave like "electromagnetic oscillators" being able to emit or absorb light from the cavity, but only in characteristic frequencies, Einstein has used Planck's formula:
E = nhv
E = energy;
n = an integer;
h = Planck's constant;
v = frequency (represented by the Greek letter epsilon).
And has postulated that atoms had "E" energy according to Planck's formula. A single photon may interact with a single electron and the energy conveyed to this electron would depend only on photon energy and frequency. By increasing light intensity, the number of launched photon-electrons is increased and, as a consequence, current is increased. Emission will instantaneously start, provided all energy needed for one photon-electron release is contained in a single photon.
When a photon interacts with an electron, there may be insufficient energy to trigger the process. In this case, the photon is absorbed by the electron, but the electron is not released from the atom. However, it jumps to a higher energy level forcing the atom to go from its basic state with E1 energy to a higher E2 energy state. If the absorbed photon has E = hv energy, than electron's energy increase will be E2 - E1, which will be equal to hv.
Approximately 10-8 seconds later, the electron jumps back to its previous state re-emitting the photon with hv energy. The excited atom has no preferential space direction so the photon may be irradiated in any direction while the atom goes in the opposite direction. This process is called spontaneous emission. If a group of atoms is excited this way, it will irradiate photons in different randomized directions; so, as atoms, they will return to their basic state.
However, if a photon with hv energy interacts with an electron at a level of E2 energy, the electron is forced to lower to its basic level with the emission of a second photon. This phenomenon is called stimulated emission and is the basis for laser beams activity.
The most important point about stimulated emission is that both photons leave the atom with the same wave phase, direction and length of the entering photon. If this would be done differently, they would interfere with themselves and be cancelled, violating the energy conservation rule. Both are called coherent photons. If a group of atoms is excited this way, initial photon beam will be increased by additional photons and will be "amplified".
If a material is in thermal balance at a T temperature, low energy atoms distribution will be markedly higher as compared to high-energy atoms distribution. If N1 is atoms density in low state and N2 is more excited atoms density, than N2/N1 ratio is given by:
N2/N1 = e(kt/hv)
t = temperature in Kelvin;
k = Boltzmann's constant.
If the material is at 103 K, then N2/N1 = 10-5. So, only one atom in 105 is in excited state.
The condition in which the number of excited atoms exceeds those in basic state is a non-balance condition, called population reversal. This condition is necessary to maintain laser activity. If a large amount of atoms is in basic state, there will be only spontaneous absorption and emission. If population reversal is maintained, there will be stimulated emission. The process used to maintain population reversal is called pumping.
So, quantic physics produces laser beams starting for example from helium, neon and argon which, arranged in an adequate system and submitted to pressure varying 1 to 3 Torr, receive a high voltage current, going through all previously described phases and emitting laser beams.
In summary, laser is the electromagnetic irradiation emitted by a device using light amplification by stimulated emission radiation at wavelengths varying from 180 mn to 1 mm. Electromagnetic spectrum varies from gamma rays to electricity. Primary laser radiation wavelength, both for military and medical applications, includes ultraviolet, visible and infrared spectra.
There are several types of laser: solid state, semiconductors, liquid and gas lasers. Carbon dioxide, argon and Nd-YAG lasers, which may be transmitted by fiber optics, have medical applications 2.
LASER BEAMS MEDICAL APPLICATIONS
Fiber optics laser allows for precise microsurgeries even in difficult to access areas. Useful in superficial surgeries, it may also be used in laparoscopies, endoscopies, chest surgeries, ophthalmology, gynecology, plastic surgery, urology, neurosurgery and ENT. It concentrates in a small and precise area a large amount of energy which vaporizes tissues causing instantaneous blood vessels and lymphatics cauterization 2.
As live tissues are basically made up of water, carbon dioxide laser, which is well absorbed by water, will be rapidly absorbed by the first cell layers, vaporizing these cells without injuring adjacent tissues. Carbon dioxide laser beams are invisible and have a helium-neon laser beam to become visible to human eyes and to allow target location. Argon and Nd-YAG lasers, with less water absorption content, lead to less vaporization and more thermal coagulation, being used in ENT, retina and anterior chamber ophthalmic surgeries, in dermatology to remove spots and hemangiomas, and in plastic surgeries to remove tattoos, due to its absorption by several pigments, including hemoglobin. Nd-YAG laser has also been used in photocoagulation of respiratory and digestive tract hemorrhages. So, different lasers have different and specific clinical applications 3-5.
Tissue vaporization produces gases and smoke which, in addition to decreasing operating field visibility, have fine particles of up to 0.3 mm responsible for atmosphere contamination. Gases produced may trigger gas embolism, especially during thoracoscopies, laparoscopies and tracheal tumor resection as a consequence of blood vessels and hollow organs perforation 6. Inadvertent heat transfer is also a disadvantage of laser and the recommendation is to use it intermittently and in moderate power (15 W).
Nd-YAG laser penetrates the eye and may promote burns and blindness, while argon and carbon dioxide laser, absorbed by tissue water, cause corneal injuries. However, the most feared laser-related adverse event is inflammability 4. This requires ventilation with low oxygen inspired fractions and tracheal tubes especially developed to prevent combustion within respiratory ways. Tracheal tubes are manufactured as from an aluminum or copper coil to prevent fire and, sometimes, they are equipped with auto-inflated foam cuffs to prevent cuff rupture 5.
Some tracheal tubes are developed with dim material to decrease laser reflection which could damage adjacent tissues. Others are made of silicone-coated material which, according to studies, has lower combustion risk due to higher resistance, but if burned, they produce toxic silica-containing ashes 6.
Red rubber and PVC tubes are at risk of burning inside airways. When submitted to 15 W power, PVC tubes may burn in just 3.7 seconds and red rubber tubes in 16.5 seconds 6. Standard tubes coating with paper or aluminum tape to prevent combustion should be avoided - although being the lowest cost alternative due to the high cost of especially developed tubes - because they do not effectively prevent combustion and may leave the cuff unprotected against laser beams. However, aluminum tape-coated tracheal tube is accepted in Brazil. Tape should be wrapped around the tube as from the portion immediately above the cuff, without leaving folds which may damage the mucosa. Cuff should be protected with gauze moistened with saline and should be filled with gauze stained with methylene blue. The staining allows for fast and easy detection in case of cuff rupture 1,6,7.
Among special tubes for laser surgeries there are: Norton, made of coiled metal rings without cuff; Bivona Fome-Cuff, aluminum tube coated with silicone with polyurethane foam cuff; Xomed Laser-Shield, made of silicone and non-reflecting metal particles; Malinckrodt Laser-Flex (Figure 1) made of coiled and flexible metal rings with two distal plastic cuffs; and Sheridan Laser-Trach, made of red latex covered with atraumatic screen with latex distal cuff 6.
LASER IN CLINICAL PRACTICE
Laser has been widely used in upper respiratory tract surgeries, especially larynx, and also in dermatology, urology, gynecology, ophthalmology, chest and GI tract surgeries 8-11.
The integration between the surgical team and the anesthesiologist is recommended for the safe use of laser in laryngeal surgeries. The anesthesiologist shall ensure airways patency and adequate ventilation; surgeons should adequately and safely operate. Before using the laser, oxygen inspired concentration should be decreased to safe values, thus preventing combustion. Oxygen inspired fraction shall not exceed 0.40 and should be as low as possible to maintain adequate oxygen saturation 1. Upper respiratory tract sharing increases the risk for obstruction and accidental tracheal extubation, burning and other airway, face and eye injuries, in addition to contaminating the team with the smoke generated from tissue vaporization.
Before inducing anesthesia, and adequate vein should be catheterized for hydration and drugs infusion, and patient should be monitored with cardioscopy, pulse oximetry, capnography, blood pressure monitor and gases analyzer or monitor able to determine oxygen inspired fraction, when available.
Recommendations for laser surgeries are:
1. Use adequate cover or goggles for team's eyes protection;
2. Presence in the room of the minimum number of people needed for the surgery;
3. Laser equipment operation by qualified professional;
4. Keep room lights on. Pupil with decreased diameter due to higher exposure to light minimizes laser-induced retina injury;
5. Remove jewels. Objects like necklaces, earrings or rings may reflect laser beams and burn other tissues or sites;
6. Maintain laser trigger in the horizontal position below the face level to protect eyes and avoid doing any other task while handling beam-emitting laser pen. Be sure that the stool or chair where the laser operator is sitting is firm. This prevents randomized shots which could cause accidents;
7. Use adequate eye protection (goggles) and be alert not to cause accidental shots and burn other tissues or objects with the laser;
8. Prefer tracheal tubes especially designed for laser surgeries and protect adjacent tissues with gauze moistened with saline solution;
9. Do not use inflammable or potentially inflammable substances, even for cleaning and disinfecting any material used;
10. Decrease oxygen inspired fraction to 0.40 or less during laser use to better prevent combustion; and replace nitrous oxide by nitrogen or helium whenever possible. Nitrous oxide feeds combustion when decomposed, originating oxygen atoms. The decomposition of 2 moles nitrous oxide generates 1 mol oxygen, enough to favor combustion 12. Helium, due to low viscosity and better thermal conductivity allows for the use of smaller tubes, which improves operating field and delays ignition 5,6;
11. Protect patient's eyes with gauze moistened with saline solution;
12. Fill tracheal tube cuff with methylene blue-containing saline solution to help diagnose cuff rupture.
Minimum halothane, enflurane and isoflurane concentrations for combustion in 30% oxygen were determined by Leonard (1975) and are: 4.75%, 5.75% and 7%, respectively 13.
There are no restrictions to the use of opioids, hypnotics or neuromuscular blockers during laser surgeries. One alternative to special or aluminum-coated tracheal tubes is to perform laser surgeries without tracheal intubation ventilating lungs through Venturi jets and providing a better operating field for the surgeon. In these cases, the procedure shall be short, preoperative fast strictly respected and patients should be premedicated with antiemetics. Venturi-type jets ventilation is achieved with 100% oxygen since air carried from airways will decrease oxygen inspired fraction. Jet ventilation is not free from adverse events and may result in barotraumas, subcutaneous emphysema, pneumothorax or pneumo- mediastinum, gastric distention and residues carried to the trachea. In addition, jet ventilation may be difficult or even impossible in obese patients or patients with decreased chest or lungs compliance. Although the use of aluminum tape coated tubes is accepted, the recommendation is to always use tracheal tubes especially designed for laser surgeries, such as Xomed laser Shield, Bivona Fome-Cuffed, Norton or Malinckrodt Laser Flex 14-17. Laryngeal mask is a good alternative and has been shown to be effective in uvulopalatoplasties to treat snoring with laser, since it is easy to insert and remove and in some cases it is possible to use the laser without removing the mask from the larynx 18,19.
LASER AND ADVERSE EVENTS
Among laser-induced adverse events the most feared is airways fire. Tracheal tube burning inside airways is severe and potentially lethal, in addition to causing several difficult to treat sequelae.
In case of airways fire, the recommendations are:
1. Remove fire source, in this case, the laser;
2. Stop ventilation, disconnect ventilatory circuit and extubate patient;
3. Fight fire with abundant saline solution;
4. Ventilate patient under facial mask with 100% oxygen;
5. Perform rigid laryngoscopy and bronchoscopy to evaluate injury, remove burnt tracheal tube fragments and debride injured tissue if needed;
6. Reintubate patient and start mechanical ventilation;
7. Evaluate the degree of airways involvement and the need for tracheostomy;
8. If there are face and oropharynx burns, these should also be treated;
9. Perform control chest X-rays;
10. Steroids are controversial.
Adequate material and strict adherence to safety rules and recommendations are critical for the success of medical laser 10.
Adverse events with laser described in the literature are mostly related to the non-adherence to safety rules. The development of adequate materials to be used with laser and the adequate training of professionals have increased its safety. So, the simple adherence to safety rules and laser manipulation by qualified professionals are the best method to prevent adverse events. Among other possible laser-induced adverse events - such as hemorrhages, gas embolism, cuff rupture - soft tissue edema is the latest postoperative event. So, patients submitted to airway surgeries with laser should remain in observation for a longer period, even if they early meet Aldrete-Kroulik criteria for PACU discharge. Acute respiratory failure after successful use of laser to treat subglottic tracheal stenosis four hours after surgery has already been reported 20. On the other hand, a retrospective study with patients submitted to laryngological surgeries has shown 1% incidence of adverse events in laser surgeries and from these, 0.75% were related to the use of laser and 0.25% were unrelated to it. For the medical team, the highest risk is eye burning. There is an international classification for continuous operation lasers related to eye burns (Chart I) 21.
The conclusion is that although laser-induced adverse events may be severe and even lethal, these events may be prevented by the simple adherence to safety rules and recommendations, which makes laser a useful and safe tool for several surgical specialties.
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Submitted for publication March 18, 2002
Accepted for publication May 5, 2003
* Received from Departamento de Anestesiologia da Faculdade de Medicina de Botucatu (FMB - UNESP), Programa de Pós-Graduação em Anestesiologia