ANESTH ANALG 1983;62:218-29

218

Anesthesia for Laser Surgery

Jeanne M. Hermens, MD, Michael J. Bennett, MD, PhD, and Carol A. Hirshman, MDCM

The laser beam represents a source of energy that can be focused to an extremely high intensity and is ca- pable of vaporizing materials. Increasingly, surgeons are using lasers for procedures requiring precise focal tissue removal or photocoagulation. Such precision is the result of reducing the size of the surgeon’s scalpel to the microscopic level. Laser surgcal techniques have the additional advantages of a bloodless operative field and complete sterility. However, this technology has led to a new set of problems for anesthesiologists. We need to understand the fundamental principles and applications of lasers in the operating room because lasers can be dangerous even at a distance and certain beams are invisible. Without proper precautions, the laser can ignite some anesthetic gases or endotracheal tubes and damage normal tissue.

This paper reviews selected aspects of the man- agement of patients undergoing laser surgery. The principles of laser technology will be outlined briefly. The current range of surgical procedures employing lasers will be summarized. Lastly, we will present our recommendations on anesthetic management of laser microlaryngeal surgery with the C 0 2 laser, empha- sizing currently available preventive measures. To this end, the published literature is summarized and the experience of our department is reviewed.

Principles of Laser Technology The first laser was developed by Maiman in 1959 (1) using a ruby crystal as an active medium. Rapid de- velopment of different lasers soon followed using gases, liquids, crystalline solids, and glasses as active media (2,3). Lasers are characterized by the production of a beam of intense, nearly monochromatic, highly co- herent radiation in the infrared, visible, or near ul- traviolet regions of the electromagnetic spectrum (Fig. 1)-

The word laser is an acronym for Light Amplifi-

Received from the Department of Anesthesiology, The Oregon Health Sciences University, Portland, Oregon 97201. Accepted for publication September 18, 1982.

Reprint requests to Dr. Hermens.

0 1983 by the International Anesthesia Research Society

cation by Stimulated Emission of Radiation. To dis- cuss the operation of a laser, we will need a few terms and concepts from modern physics. For simplicity, our discussion will be limited to an idealized molec- ular system but similar considerations apply to real molecular and atomic systems.

We know that molecular systems contain only cer- tain discrete amounts of energy. A molecule with the lowest possible amount of energy is said to be in the ground state, while molecules with more than this amount of energy are said to be in an excited state. Any molecule in an excited state tends to lose energy much as water tends to lose energy by running down- hill. One of the principal ways in which an excited molecule can cool off (or lose energy) is by emission of a light wave or photon. The wavelength of a given photon is characteristic of the energy difference be- tween the particular excited state from which it was emitted and the lower energy state (not necessarily the ground state) in which it leaves the molecule. On an atomic scale, this is the mechanism of the familiar discharge lamp or neon light. Photons can also be absorbed by a process inverse to emission in which a molecule is given additional energy (or promoted to a higher energy state) by the absorption of a photon whose wavelength corresponds to the energy differ- ence between the initial and final molecular states. A situation in which there are more molecules in a par- ticular excited state than some other state of lower energy (again, not necessarily the ground state) is said to be a population inversion. Figure 2 shows a highly stylized schematic representation of some of these terms. For example, a photon of wavelength corre- sponding to photon 4 could be absorbed by a molecule in the second excited state and raise its energy to the third excited state. There are three radiative processes involved in a system such as we have described: ab- sorption of a photon with a raising of the energy state of the molecule; spontaneous emission of a photon from an excited molecular state; and stimulated emis- sion, in which a photon released from one molecule interacts with another molecule in the same excited state and causes a decay in that system, resulting in the release of two photons in step (or coherent) with

LASER SURGERY ANESTH ANALG 219 1983;62:218-29

LASER ABSORPTION

(eg Xenon flash or electrical current1

Temporal coherence

Spatial coherence

-5-

SPONTANEOUS EMISSION

Monochromaticity 100 Watts

100 Watts

Figure 1. Pictorial description of a laser, showing the external en- ergy source feeding the lasing medium and the coherence prop- erties of the beam contrasted with the properties of a more con- ventional light source. (Reprinted with permission of the publisher from Surg Neurol 1980;14:1-10.)

each other. Figure 3 shows these processes diagrammatically.

As a specific example of a laser, consider a tube filled with gas containing COz through which an elec- trical discharge is passed. By appropriate choice of gas mixture and pressure, much of the energy of the electrical discharge can be made to excite the mole- cules of COz. In this discharge, there is a population inversion between two of the excited states of the CO, molecule. If carefully aligned mirrors are applied to

Figure 2. Schematic description of the ground state, first few ex- cited states, and some possible photon wavelengths from a small collection of hypothetical molecules excited by an electrical dis- charge. Photon 2 would be a possible laser wavelength.

FROM ELECTRICAL 4th Excited D ISCH ARG E

State 9 Photon 5 3rd Excited

State

2nd Excited Inverted State

Population Inversion

& State 1 K w Z 1st Excited ,

State

Ground State

Photon 1

10' DS 106 to' 108 109 Number of Molecules per State

STIMULATED

-+1

L

Figure 3. Three radiative processes are involved in laser action: absorption of a photon raises an atomic or molecular system to a higher (excited) energy state; spontaneous emission of a photon occurs when a system that is in an excited state spontaneously decays to a lower energy state; and stimulated emission, in which a photon interacts with a system in an excited state to cause a decay of that system, resulting in the release of two photons (the original photon plus the photon emitted by decay of the system). The two photons are of equal wavelength and are coordinated in time and space. (Reprinted with permission of the publisher from Surg Neu- rol 1980;14:1-10.)

each end of the tube, a photon that happens to be emitted exactly along the axis of the tube by spon- taneous emission from one of the molecules in the inverted state can bounce back and forth between the mirrors, each time stimulating the emission of addi- tional photons by the other molecules in the inverted state. Thus, the intensity of the beam can grow to very large values. Because stimulated emission is in step with the stimulating photon, the beam is co- herent and, because two specific energy states are involved, the beam is of a single wavelength (mono- chromatic). Conversely, if a population inversion does not exist absorption dominates and the beam tends to die away instead of growing.

Coherent monochromatic light can be focused into an extremely small spot, much smaller than the in- coherent and polychromatic light from familiar sources such as an incandescent filament. This fact, along with the high intensity of the laser beam, makes it possible to achieve extremely high-power densities in the focal spot of the laser beam capable of vaporizing any material.

ANESTH ANALG 220 1983;62:218-29

HERMENS ET AL.

Our example used a molecular gas excited by an electrical discharge; other types of lasers use liquids, crystalline solids, or glasses with excitation by inco- herent light sources such as flash lamps, chemical reactions, or even other lasers.

With a few exceptions, each lasing medium emits a specific wavelength (4,5) (Table 1). These wave- lengths interact with biologic tissues in different ways; hence, their application to medicine may vary, as dis- cusssed below.

Certain lasers are operated only in short pulses while others are used in either a pulsed or a contin- uous fashion. Most of the lasers used for surgery are either of the continuous or long pulse (fractions of a second) variety. Exceptions are the ruby crystal laser used in ophthalmology and the neodymium yttrium aluminum garnett (NdYAG) laser. Short, high-pow- ered pulsed lasers can cause significant spattering of tissue and, theoretically infection from microorgan- isms or particles scattered about by microexplosions from the laser pulses.

Surgical Uses of Lasers Medical applications use lasers of several of the com- mon types. Gas lasers, such as the helium neon laser, are commonly used for alignment during radiologic or other laser procedures. The continuous COz laser is commonly used for excision of soft tissue. The con- tinuous argon laser is used primarily for excision of pigmented lesions or photocoagulation. The crystal- line solid ruby laser is a pulsed device commonly used in ophthalmology for retinal reattachment. In these applications, the total energy per pulse is small enough that spatter does not occur. Pulsed glass lasers, such as the NdYAG lasers are beginning to be used for excision of soft tissue, particularly pigmented soft tis- sue. With the precision, lack of bleeding, reduction of tissue reaction, and preservation of normal tissue

made possible by lasers, it is not surprising that nearly all surgical specialties are exploring their uses.

The medical or surgical use of lasers is dependent on the interaction of the energy produced with bio- logic tissues. The total absorption of laser energy is determined largely by absorption in tissue water and tissue pigmentation. The ruby laser produces a wave- length of 0.695 pm, which is very poorly absorbed by water but well absorbed by pigmented tissues such as melanin and hemoglobin. Thus, the ruby laser can safely penetrate the anterior structures of the eye and photocoagulate vascular and pigmented retinal le- sions. The COz laser, whose wavelength is 10.6 pm, is strongly absorbed within the first 200 pm of any tissue traversed, and therefore can damage all surface soft tissue cells. As illustrated in Table 2, the C02 laser has been employed for surgical removal of la- ryngeal lesions, skin lesions, and an ever-increasing number of similar procedures. As with the ruby laser, energy from argon and NdYAG lasers is preferentially absorbed by pigmented tissues.

Table 2 is a sampling of the surgical procedures that have employed lasers. The list is not all inclusive nor does the list imply that laser surgery is necessarily the procedure of choice.

Hazards of Laser Surgery Hazards to the Operating Team and Operating Room Personnel Because of the very high intensity of typical laser beams, there are significant safety considerations in their use. Specular reflection, as from a mirror surface, changes the direction of a laser beam without changing its focal properties and can thus direct the full power of the beam in an unintended direction. One way to reduce this hazard is to use matte rather than mirrorlike sur- faces because diffuse reflection from a matte surface reduces energy density.

Table 1.

Beam character Transmitted

Kinds of lasers Wavelength ( p ) Color by fiberoptics Laser mode

Gas Continuous Helium neon 0.633 Red Yes

Argon 0.488 and 0.515 Blue-green Yes Continuous co, 10.6 Invisible No Continuous

Crystalline

Glass

Solid

Ruby 0.695 Red Yes Pulsed

NdYAG 1.06 Invisible Yes Pulsed

LASER SURGERY ANESTH ANALG 22 1 1983;62:218-29

Gastroenterology

General surgery

Gynecology

Neurosurgery

Ophthalmology

Otolaryngology

Argon

Argon and NdYAG

COZ

Argon

Ruby Argon CO,

Table 2. Selected Samples of the Surgical Uses of Lasers Medical/surgical field Type of laser Procedures

Dermatology/plastic surgery CO, Excision of thermal eschars (6); hemangiomata, keratosis, telangectasia, spider nevi, basal cell carcinoma, malignant melanomas, squamous cell carcinoma, Kaposi's sarcoma (7,8); multiple pilonidal sinuses, vulvar cavernous hemangioma (9); excision of large skin tumors in patients with low platelets, subcutaneous mastectomies with silastic implants, augmentation mammoplasties with silastic implants (10).

Port wine stains (11,12), hemangiomas, removal of tatoos.

Photocoagulation of bleeding gastric erosions (13,14), esophageal varices, Osler-Weber-Rendu lesions, carcinomas, vascular malformations, excision of polyps (15). Both use fiberoptic bundles to deliver the laser energy to the lesions (16).

Splenectomy, breast lump excision, gastrectomy, mastectomy, colostomy, bilateral adrenalectomy (17).

Excision of cervical, vaginal, and vulvar neoplasias (18); benign vaginal adenosis, condyloma, tumor volume reduction in unresectable tumors (19), tuboplasties (20).

Excision of benign brain tumors, neuromas, and spinal cord arteriovenous malformations (2).

Glaucoma therapy (21); photocoagulation for proliferating diabetic retinopathy (22), ocular histoplasmosis (23); retinal detachment.

Photocoagulation of diabetic retinopathy (24). Tympanoplasty, rnyringotomy, stapedectomy (25). Excision of laryngeal and tracheal papillomas (26,27);

vocal cord polyps, nodules, keratoses, and localized cord carcinomas (28-30); choanal atresia (31); subglottic stenosis (32); soft tissue lesions in the neck (lymphangioma, neurofibroma, subglottic hemangioma) (33).

bronchoscope (34). NdYAG Excision of tracheobronchial lesions via a fiberoptic

The eye is the most susceptible tissue to injury by laser radiation (35). Visible laser beams, such as those of the ruby or argon laser, are strongly absorbed by pigmented structures but pass easily through the cor- nea and lens of the eye. Therefore, they are associated with significant hazard to the retina (36). Beams from the near infrared lasers such as the NdYAG laser can also be transmitted into the eye, although there is significant deposition in the aqueous tissues of the cornea and lens. Because CO, laser beams are ab- sorbed within the first 200 pm of tissue, they are not a hazard to the retina but to the cornea. To prevent eye damage, all personnel in the operating room must wear safety glasses appropriate for the laser in use (37). Each kind of laser requires eye goggles designed to absorb energy of the wavelength emitted by the

specific laser in use. They should fit well around the forehead and have side shields to protect the lateral margins of the orbit. Each should be clearly marked with the wavelength for which it provides protection.

Absorption of any laser pulse on the skin causes a burn whose severity is primarily determined by pen- etration depth and by the total energy deposited by the laser beam. Assuming that the laser is used cor- rectly and that a beam will not be directed anywhere but at the lesion being treated, skin protection for personnel is not usually necessary (38). Reflected beams from instruments or retractors are possible but dam- age to skin distant from the operative site would be unlikely because the energy density decreases rapidly beyond the focal point.

When a laser is being used, a conspicuous sign

ANESTH ANALC 222 i983;62:218-29

HERMENS ET AL.

noting that a laser is in use should be placed on the outside of the door along with extra safety glasses for anyone entering the room.

Hazards to the Patient The major hazards from surgery are of two types: fires and destruction of normal tissue. Fires can occur when the laser strikes a combustible object such as an en- dotracheal tube. Destruction of normal tissue can oc- crr when a directed or reflected laser beam strikes unprotected tissues.

Fire Hazards. The principal problem is that the fo- cused laser beam is an intense heat source capable of igniting almost all rubber and plastic materials. The ease with which this occurs depends on the material itself, the gas environment surrounding the material, and the focus of the laser beam.

In the earliest clinical trials with COz lasers, it was recognized that the laser beam can directly penetrate and ignite an endotracheal tube (39,40). The highest risk of fire exists in those surgical situations where the endotracheal tube is in the laser operative field, particularly in airway surgery. The reported incidence of tube ignition varies from 0.4% to 1.5% in patients undergoing laryngeal surgery with the COz laser (41- 44). Thus, fire danger and its preventioii are of par- ticular importance to anesthesiologists.

All commercially available rubber and plastic tubes can be ignited by a well-focused laser beam in 100% oxygen. Nitrous oxide supports combustion almost as well as oxygen. Therefore, reducing the inspired oxygen concentration and replacing it with nitrous oxide does not eliminate the hazard (45). The use of oxygen in varying concentrations with air rather than with nitrous oxide has been advocated (42). However, many tubes will burn in as little as 25% oxygen (43). Halothane, isoflurane, and enflurane are not flam- mable, not combustible, and do not support com- bustion although they can be decomposed by very high temperatures.

There has been controversy in the literature over which kind of endotracheal tube, red rubber or plastic (26,46-49), is more resistant to penetration and ig- nition. The important fact is that both kinds of tubes can be ignited when used with frequently employed concentrations of anesthetic gases (49). Both types have been described in case reports of intraoperative fires (41-43,50); therefore, both types need protection to minimize the risk.

Besides direct laser ignition of the tube, indirect ignition can also occur under special circumstances (49). The interior surface of a protected tube can be ignited by burning pieces of tissue inhaled into the

tube (Figs. 4 and 5). This is commonly called arcing, even though no electrical discharge is involved.

Several methods are currently available to mini- mize the risk of ignition of the endotracheal tube, e.g., no tube in the airway, protection of a rubber or plastic tube by wrapping the outside surface with a variety of materials, or use of a noncombustible tube. The choice of a specific technique depends on the site of the surgery, the availability of the equipment, and the size of the patient. Details of each of these techniques are described in the section on anesthetic management.

Damage to Normal Tissue. A poorly aimed or re- flected laser beam can damage normal tissue. A tra- cheal tear was recently attributed to laser damage in a patient who developed respiratory distress and pneumomediastinum after extubation (51). Good technique by the surgeon and an immobile target will minimize tissue injury. In the operative field, tissue adjacent to the lesion can be protected by water-moist- ened gauze pads, sponges, or swabs (38). The pa- tient’s eyes should be taped closed and covered with moistened gauze or eye pads. Because misfiring or misdirecting the laser beam is unlikely, skin protec- tion beyond the routine surgical drapes is usually not necessary.

Anesthetic Management Not all of the procedures listed in Table 2 require the services of an anesthesiologist, and the majority of procedures requiring anesthesia pose few problems beyond routine protection of the eyes of the patient and the operating team. However, laser microsurgery of the airway presents particular problems to the anes- thesiologist because of conflicting needs of the sur-

Figure 4. Vinyl plastic endotracheal tube removed from a patient following indirect ignition. (Reprinted with permission of the pub- lisher from Arch Otolaryngol 1980;106:639-41.)

LASER SURGERY ANESTH ANALG 223 1983;62:218-29

A plan with options in case of total obstruction must be agreed upon before induction of anesthesia.

Premed ica t ion Many patients, particularly those with papillomato- sis, require multiple surgical procedures. They are often upset and frightened about yet another proce- dure. There is great temptation to heavily premedi- cate difficult children. Premedication with narcotics, diazepam, and even barbiturates promote tongue and jaw relaxation and respiratory depression and should be avoided in patients with compromised airways. Phenothiazine derivatives are also best avoided as they are reported to increase excitatory phenomena (52).

Figure 5. TWO vinyl plastic tubes ignited indirectly by the laser beam. Upper tube is wrapped with aluminum tape. (Reprinted with permission of the publisher from Arch Otolaryngol1980;106:639- 41 .)

Induction of Anesthesia

geon and anesthesiologist for airway access and the fire hazards associated with the laser beam. We shall therefore concentrate on this particularly difficult group of patients.

Preoperative Eva1 uat ion A general medical assessment is always necessary. In addition, in patients with airway lesions the degree of obstruction and adequacy of ventilation must be carefully assessed. Larger lesions, such as papillomas, may cause severe obstruction. Bleeding into a lesion can rapidly convert partial obstruction to total ob- struction, particularly in children. Occasionally, a large polyp may cause complete airway obstruction during induction of anesthesia. The general appearance of the patient, the quality of the voice, and the respi- ratory pattern give important information on the de- gree of respiratory obstruction. Examination of the mouth and neck are essential in assessing anatomic abnormalities that may give clues to potentially dif- ficult airway management and intubation problems. In addition, the medical record is invaluable in de- scribing airway pathology and equipment used suc- cessfully in the management of previous anesthetics. Laboratory tests are also useful. Spirometry and flow volume curves can determine the degree of obstruc- tion. Arterial blood gas tensions should be measured if spirometry is abnormal. X-rays, CT scans, and tom- ograms of the larynx provide additional information on anatomical changes.

Communication between the anesthesiologist and surgical team is essential in the management of any patient with known or suspected airway obstruction.

If there is no airway obstruction, standard inhalation or intravenous techniques of induction can be used. In children with minimal airway obstruction, rectal barbiturates may be useful.

In the patient with a compromised airway, all equipment that might be required must be immedi- ately at hand. This includes a variety of laryngoscope blades, airways, endotracheal tubes, and broncho- scopes as well as a tracheostomy set. A surgeon ca- pable of performing a tracheostomy rapidly must be in attendance. An awake intubation with careful vis- ualization of the cords is a safe way to proceed in a cooperative patient. Blind intubation runs the risk of trauma to the lesions, which may convert a partial obstruction to complete obstruction, and is best avoided.

If visualization is difficult, intubation over a fiber- optic bronchoscope can be performed. Considerable skill is required to perform this procedure quickly and smoothly. Bronchoscopes that fit through a 4.5-mm tube are available. A 3-mm pediatric ventilating bron- choscope can be used in the awake patient with a severely compromised airway. Once an airway is es- tablished, the patient can then be anesthetized with intravenous agents or anesthetic gases via the bron- choscope. A tracheostomy performed under local anesthesia may be required in the presence of a se- verely compromised airway or other complicating factors,

In children and some patients unable to cooporate with an awake intubation, inhalation induction using halothane and oxygen with spontaneous ventilation is frequently the technique of choice. Ketamine should probably be avoided in the patient with a compro- mised airway as it tends to increase airway reflexes

ANESTH ANALG 1983;62:218-29

224 HERMENS ET AL.

and may predispose to obstruction and laryngo- spasm. Prolonged laryngeal irritability has been ob- served with ketamine use (40), particularly after extubation.

Choice of Anesthetic Technique Three main techniques are currently available to avoid the fire hazards associated with the use of conven- tional endotracheal tubes during laser surgery of the airway: no tube in the airway, protection of the ex- ternal surface of a conventional tube, or use of a non- combustible tube.

No Tube in the Airway. SPONTANEOUS VENTILA- TION. Following induction and topical anesthesia of the larynx, maintenance of anesthesia is accomplished by insufflation of nitrous oxide-oxygen mixtures with a potent nonflammable inhalation anesthetic via nasal catheters (53,54). This has the advantage of an un- interrupted view of the larynx at all times. However, it has disadvantages. Complete control of the airway is lacking. Apnea or hypoventilation may occur with secondary cardiac arrhythmias from too deep a plane of anesthesia. Conversely, laryngospasm from too light a plane of anesthesia may occur even with constant vigilance as to the adequacy of depth of anesthesia. The vocal cords do not remain immobile, making the surgery more difficult and increasing the risk of dam- aging normal tissue. Potent anesthetic gases are ex- hausted through the open mouth making adequate scavenging of these gases difficult.

VENTURI VENTILATION. Venturi ventilation, which has long been used to provide ventilation through an open bronchoscope for endoscopy procedures (55- 57), has been extended to laryngoscopic procedures by securing an injection needle to a straight-blade laryngoscope (45,58-61). Oxygen or an oxygen-ni- trous oxide mixture, forced through the needle by high pressure, entrains room air. Anesthesia is in- duced and maintained with intravenous agents and often N20. Muscle relaxation is required and is fre- quently accomplished using a continuous infusion of succinylcholine. The laryngoscope, blade, larynx, and trachea must be aligned in a straight line and the anterior part of the larynx must be cleared of ob- structing lesions before its use. The advantages of this system are an unobstructed view, a motionless field, and adequate ventilation. The disadvantages include all the risks of Venturi ventilation: pneumothorax, pneumomediastinum, inflation of the stomach, as- piration of resected material, complete respiratory ob- struction, and dehydration of mucosal surfaces (59,62,63). Norton recommends using 1% aqueous li- docaine spray that serves to humidify the gases and

diminish postanesthetic laryngospasm when using this technique (64).

Protection of the External Surface of a Conventional En- dotracheal Tube. A red rubber or vinyl plastic tube, 1- 2 mm smaller than usually employed, is chosen. The smaller tube allows better visualization of the oper- ative field. A cuffed tube is usually used for adults and an uncuffed tube for pediatric patients. The en- dotracheal tube can be wrapped with metal tape (Fig. 6). Aluminum or copper adhesive-backed metal tapes are suitable; lead tape is not, as it is easily broken or melted and the vapors are toxic. The tube is wrapped in a spiral manner beginning near the tip or cuff and ending at the level of the uvula. The foil at the tip of the tube is carefully trimmed with scissors or blade. Tape edges are smoothed to decrease soft tissue trauma. Tubes are not wrapped longitudinally because of the high incidence of kinking of the tube and poor adhe- sion of the tape when applied in this way.

Copper tape has the highest coefficient of reflec- tivity for far infrared wavelengths, although alumi- num tape is quite satisfactory. The tapes are available in a variety of widths. We feel that widths of 0.4-1.0 cm allow the smoothest edges when wrapping and therefore decrease the risk of soft tissue trauma. Cop- per tape is available at art stores, craft shops, variety stores, and radio supply stores. Aluminum tape is available through the 3 M Company and radio supply stores.

There are several potential complications from tubes wrapped with metal tape. Rough edges can injure the pharyngeal and laryngeal tissues. Pieces of tape can loosen (Fig. 7), break off, and be aspirated (65) (Fig. 8). The wrapped tube can become kinked by foil

Figure 6 . Two vinyl plastic endotracheal tubes wrapped with metal tape (upper tube is wrapped with copper and lower tube with aluminum tape) and a metal endotracheal tube (Norton tube) in the center.

LASER SURGERY 225 ANESTH ANALG 1983;62:218-29

Figure 7. Aluminum-tape wrapped endotracheal tube removed from a patient. The tape adhered poorly to the tube.

compression. Theoretically, the laser beam can be re- flected from the metal surface and cause tracheal bums, although no such cases have been reported with the possible exception of the tracheal tear mentioned pre- viously (51).

To avoid some of these complications, Patil et al. (66) recommend wrapping tubes with muslin strips instead of metal tape. The muslin is soaked in water or saline before intubation. The water dissipates the laser energy and prevents ignition. The tube wrap- ping must be kept moistened with water or saline throughout the procedure. This can be accomplished by intermittently injecting water onto the muslin with a syringe or by incorporating a small tube such as an epidural catheter (67) into the muslin wrapping and injecting water or saline through the catheter. The major disadvantages of muslin-wrapped tubes are their bulkiness, the need for repeated moistening, and the increased fire hazard should drying occur.

Figure 8. Piece of aluminum tape removed from the trachea of a patient with acute airway obstruction.

Figure 9. Laser ignition of an acrylic-coated vinyl plastic endotra- cheal tube through which oxygen was flowing.

Recently, coating vinyl plastic tubes with dental acrylic has been advocated (68). The acrylic-coated tubes are rigid and bulky and application of the acrylic is time consuming. Moreover, in an oxygen-enriched atmosphere, the laser beam can easily penetrate the acrylic and ignite the endotracheal tube (Fig. 9).

Tube ignition may occur despite good preventive techniques. Tape may slip or break off leaving a por- tion of the tube unprotected. Muslin may become dry and thereby be ignited. Even when tube wrappings are intact, the tip or the inside surface of the tube may ignite from sparks generated by laser vaporiza- tion of adjacent tissue. If ignition occurs, disconnect the endotracheal tube from the gas source immedi- ately as most of these tubes do not burn in air. Then remove the tube. Rapid direct access to the tube is necessary at all times. The tube should be lightly taped to the patient so that immediate extubation is possible (42). Chest x-ray and bronchoscopy are performed to reveal the extent of injury.

The lung as well as the trachea may be injured extensively either by smoke inhalation or a direct ther- mal burn (blowtorch effect) (50). Should such com- plications occur, steroids, humidification of inhaled gases, tracheostomy, and assisted ventilation may be necessary for long periods of time. Tracheal stenosis can be a late complication.

In addition to igniting the endotracheal tube, the laser beam can perforate the cuff with a resulting large air leak and inadequate ventilation (28,40,69). This complication may be avoided by placing saline-soaked cottonoids or gauze between the vocal cords and the cuff. Case reports have described cottonoids inad- vertently left in the airway causing respiratory ob- struction (69). An attached suture or wire will help

ANESTH ANALG 1983;62218-29

226 HERMENS ET AL.

prevent failure to remove the cottonoid. If cuff per- foration occurs and ventilation becomes inadequate, the tube should be replaced with a previously-wrapped second tube kept available in the operating room.

Noncombustible Tube. SPONTANEOUS OR CONTROLLED

TENANCE. Motivated by the tube ignition problem, Norton and de Vos developed a flexible noncom- bustible metal tube (70) (Fig. 6). This tube is com- mercially available but, in our experience, only after long delays as it must be specially ordered. Moreover, the user must sign a new device form releasing the manufacturer from any liability arising from use of the tube.

These tubes have thick walls. The internal diam- eters range from 2.14 mm to 6.14 mm with external diameters ranging from 6-9 mm. They are uncuffed. Norton recommends that if a cuff is necessary, AirLon cuffs (Ditmar Penn, Philadelphia, PA) are more re- sistant to combustion than other brands (71). A major disadvantage is the large external diameter that pre-

VENTILATION USING INHALATION ANESTHESIA FOR MAIN-

Porch tube (Fig. 11) is not commercially available but is easily consiructed from readily available materials. In contrast to the Norton tube, the Porch tube can only be used during jet ventilation. The internal di- ameter of 3 mm provides too great a resistance to allow its use during spontaneous or controlled ven- tilation. Its large external diameter of 7 mm precludes its use in pediatric patients. These tubes are placed below the level of the vocal cords assuring the integ- rity of the airway and adequacy of ventilation without the necessity of a cuff. They have the added advan- tage of stabilizing the vocal cords and vestibular folds in the abducted position for surgery of the anterior two-thirds of the larynx. This technique shares all of the problems of Venturi ventilation. Because passive exhalation must occur around the tube, the leak around the tube must be sufficiently large to prevent over- distension and pneumothorax.

Other Considerations. All oil-based ointments used

eludes use of the tube in pediatric patients and in patients with tracheal stenosis.

The ideal tube for C 0 2 laser surgery of the airway would be a noncombustible plastic tube with a cuff that could safely be used in 100% oxygen. Develop- ment and testing of tubes is currently under way. Figure 10 is one such example (45). The Porges Mil- haud tube is another example (72). We have not eval- uated these tubes.

LEVEL OF THE LARYNX. Two metal tubes, the Norton tube and the Porch tube (73), can be attached to a Venturi apparatus and used for Venturi ventilation. The Norton tube has been discussed previously. The

Figure 11. Porch tube with chrome male Luer-Lok connector, plas- tic tubing, pressure reducing valve, jet injector, and nitrous oxide blender. (Reprinted with permission of the publisher from Anesth Analg 1980;59:789-91.)

JET VENTILATION USING A TUBE PLACED BELOW THE

Figure 10. A prototype of a noncombustible plastic tube with a cuff. (Reprinted with permission of the publisher from Anesthesia 1981:36:411-5.)

LASER SURGERY 227 ANESTH ANALG 1983;62:21&29

Figure 12. Oil-based local anesthetic ointment ignited by a CO, laser pulse.

to lubricate endotracheal tubes are combustible and can be ignited by a laser beam (Fig. 12). Water-soluble solutions of local anesthetics are not combustible and can be used safely.

In patients with tracheostomies, anesthesia can be induced via the tracheostomy tube or with intrave- nous agents. Resection of lesions at the larynx pre- sents no special problems as a metal tracheostomy tube can be used. However, the metal tracheostomy tube should not be fenestrated as this might allow the laser beam to travel down into the trachea. Any plastic or rubber connection may ignite (45).

Resection of tracheal papillomata presents prob- lems. Many patients with papillomata, mostly chil- dren, have tracheostomies. Therefore, induction is easily accomplished. However, if laser resection is carried out through the stoma after the tracheostomy tube is removed, ventilation during laser resection becomes a problem.

Our preferred way of handling ventilation in this situation is via a metal tube inserted through the vocal cords and connected to a Venturi apparatus. Exhal- ation occurs via the tracheostoma. This allows ade- quate ventilation and continuous surgical access. However, the child must be large enough to allow the use of a metal tube with at least a 6-mm external diameter. Wrapped tubes are extremely hazardous in this situation as laser resection is occurring in the path of gases that readily support combusion and sparks may ignite the inside surface of the wrapped tube.

In smaller children, we rely upon spontaneous ven- tilation via the tracheostomy during resection of tra- cheal lesions. An endotracheal tube is placed in the tracheal stoma. A deep plane of inhalation anesthesia is established while maintaining spontaneous respi-

ration. The tube is withdrawn while laser resection occurs. When additional anesthesia is necessary, the tube is replaced in the stoma and deep anesthesia is reestablished. This sequence is repeated until all le- sions are treated. The drawbacks of this technique are prolonged surgery time, hypotension from deep anesthesia, and cardiac arrhythmias from hypoven- tilation and hypercapnia.

Muscle Relaxation The surgical field should ideally be immobile. A coughing or bucking patient predisposes to laser dam- age of normal tissue. To minimize this possibility, most authors recommend using muscle relaxants. For short procedures, intermittent injections or continu- ous infusion of succinylcholine can be used. Atropine or glycopyrrolate is given to decrease secretions and prevent vagally mediated bradyarrhythmias. For longer procedures, pancuronium or d-tubocurarine is used. Obviously, neuromuscular blockade is avoided in pa- tients in whom spontaneous ventilation is desired. Lawson et al. (74) used a continuous infusion of pro- caine at a rate of 1 mg/kg/min to decrease the need for muscle relaxation in children undergoing laser surgery.

Postoperative Considerations The patient is extubated in the operating room when- ever possible. The endotracheal tube is inspected to ascertain that all the adhesive wrapping is intact. If tape is missing, laryngoscopy and bronchoscopy are performed.

Postoperative bleeding is usually not a problem with laser surgery. Early postoperative laryngeal edema can occur and is usually manifested in the recovery room by inspiratory stridor and retractions. To reduce laryngeal edema, humidified oxygen is given. Topical anesthesia of the larynx reduces the incidence of laryngospasm.

It should be remembered that jet ventilation can cause a pneumothorax that may first become evident in the recovery room.

Summary Laser surgery offers several advantages to the sur- geon and patient: microscopic precision, a bloodless operative field, and complete sterility. While the ma- jority of procedures pose few problems beyond pro- tection of the eyes of operating room personnel and patients, microlaryngeal surgery with the CO, laser requires very careful anesthetic management. A pre-

HERMENS ET AL.

operative visit to determine the degree of existing airway obstruction is mandatory in deciding the safest anesthetic technique. Continued communication and cooperation between the surgeon and anesthesiolo- gist throughout the procedure will help minimize the conflicting needs for airway access and ventilation.

We feel the best approach to the anesthetic man- agement of patients undergoing laser airway surgery is to have several alternatives available at the time of induction of anesthesia. For adult patients wrapped tubes, metal tubes, and a jet injector should be on hand. The options are more limited in children. The smallest metal tubes available have external diameters of 6 mm (Norton tube) or 7 mm (Porch tube), which are too large to use in these younger patients. Small wrapped, uncuffed tubes or Venturi ventilation through a small-gauge needle are most often used. Regardless of the technique, constant vigilance throughout the procedure is required to detect complications early. Wrapped tubes, metal tubes, insufflation using no tube, and jet ventilation using a needle or metal tube reduce the fire hazard but each method substitutes its own set of problems. Before adopting any ap- proach, we strongly recommend that the equipment selected be tested for flammability with the laser be- fore its use in patients. If, in spite of precautions, ignition of equipment does occur, immediately inter- rupt the flow of oxygen and nitrous oxide as most materials do not burn readily in air. Then remove the offending material.

We have reviewed selected aspects of the manage- ment of the patient undergoing laser surgery, out- lined the principles of laser technology, and listed the many surgical procedures employing lasers. Also, recommendations on anesthetic management of mi- crolaryngeal surgery with the C02 laser with empha- sis on currently available measures to prevent prob- lems were reviewed in light of our own experience with this technique along with a summary of the lit- erature on laser surgery. An understanding of the fundamental principles and applications of lasers will hopefully lead to safer patient care.

The authors thank Edwin Everts, MD and James Smith, MD of the Department of Otolaryngology at The Oregon Health Sciences Uni- versity for their support, and Frances Cetinich for typing and ed- iting the manuscript.

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