Oxford Handbook of Anaesthesia (3 edn)
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General principles
Laser is an acronym for Light Amplification by Stimulated Emission of Radiation. Laser light is an intense beam of energy capable of vaporising tissues. Lasers have numerous medical and surgical applications, but also create unique hazards to patients and staff.
Light is a form of radiant energy that spans the mid-range of the electromagnetic spectrum. It is released as photons and travels as a wave.
In a laser tube, the application of an energy source on a lasing medium creates stimulated emissions of photons. These bounce back and forth between carefully aligned mirrors, and are focused into a high-intensity beam. The light produced is monochromatic (all the same wavelength) and coherent (all the wave peaks moving synchronously at the same amplitude).
Lasers are defined by their wavelengths, which also determine their colour. Some lasers are outside the visible spectrum and require a light guide to direct the laser beam to the surgical site.
Fibreoptic bundles can be used to transmit visible and near-infrared wavelength lasers. Wavelengths out of this range usually require an articulated arm.
Laser wavelength and colour
| Laser type | Wavelength (nm) | Colour |
|---|---|---|
Dye laser | 360–670 | Blue to red |
Argon | 488–515 | Blue/green |
Helium–neon | 633 | Red |
Ruby | 694 | Red |
Nd–YAG | 1064 | Near-infrared |
Carbon dioxide | 10 600 | Far-infrared |
| Laser type | Wavelength (nm) | Colour |
|---|---|---|
Dye laser | 360–670 | Blue to red |
Argon | 488–515 | Blue/green |
Helium–neon | 633 | Red |
Ruby | 694 | Red |
Nd–YAG | 1064 | Near-infrared |
Carbon dioxide | 10 600 | Far-infrared |
Laser light striking a tissue surface may be:
Reflected. Reflection off shiny surfaces may damage the eyes of staff in the vicinity.
Transmitted to deeper layers. Lasers pass through tissues to a variable depth, which is partially determined by the wavelength.
Scattered. Shorter wavelengths induce greater scattering.
Absorbed. This produces the clinical effect, when the absorbed light is converted to heat. Organic tissue contains various substances capable of absorbing light. These are termed chromophores, and include haemoglobin, collagen, and melanin. Each substance has a particular absorption spectrum, which is determined by its chemical structure. For example, oxyhaemoglobin, which is targeted in vascular lesions, has absorption peaks at 418, 542, and 577nm. Laser light at or close to these frequencies will be the most effective.
Safety aspects
A designated laser safety officer should be present at all times when a laser machine is in use. An illuminated light should display outside the theatre when the laser is in operation.
Laser light can be reflected off mirror-like surfaces. Medical instruments used with lasers should have matt rather than shiny surfaces.
The eye is the most susceptible tissue to injury. Retinal and corneal damage can occur, depending on the frequency of the beam. All operating room personnel must wear safety glasses appropriate for the laser in use. These should have side shields to protect the lateral aspect of the eye. If an anaesthetised patient is receiving laser radiation near the eyes, protective matt metallic eye covers can be applied.
Damage to skin can occur, depending on the type of laser in use. Anaesthetised patients must have all exposed skin covered with drapes. These should be made of absorbable material and not plastic, which is potentially combustible. Tissue adjacent to the lesion can be protected with moistened pads or swabs. In all cases, the eyes should be taped closed and covered with moist swabs. Plastic tape is combustible and should be avoided.
Some skin preparation fluids are flammable and should not be used during laser surgery.
Laser light can ignite plastic and rubber materials. Carefully consider the optimum method of airway maintenance if lasers are employed within the airway. The simplest approach is to use a Venturi system (Sanders injector). This uses a high-pressure oxygen source and entrainment of atmospheric air. The injector is placed in the lumen of a rigid laryngoscope or bronchoscope which is open at both ends, and permits entrainment of oxygen-enriched air during inspiration and escape of carbon dioxide and exhaust gases during expiration. This system of a ‘tube within a tube’ is safe, and reduces the chances of barotrauma-induced pneumothorax or pneumomediastinum. IV anaesthesia is usually employed to ensure an adequate depth of anaesthesia. It is also important to prevent the patient from moving or coughing, so a suitable muscle relaxant should be administered and neuromuscular transmission monitored with a nerve stimulator.
If the use of an endotracheal tube is required, unmodified conventional tubes cannot be used because they support combustion and can potentially cause airway fires. Specifically prepared non-flammable laser tubes are available, e.g Laser-Trach™ (Sheridan) and Laser-Shield™ (Medtronic Xomed). The cuffs of these tubes are vulnerable and should be protected by damp pledgets. The cuff should be filled with saline, which can be mixed with methylene blue so that cuff puncture is obvious.
If laser surgery is needed to the oropharynx a standard nasal ETT can be employed but must be protected with saline-soaked gauze packing.
Both nitrous oxide and oxygen support combustion. If using a circuit rather than an oxygen injector, 30% oxygen and air is a sensible choice. If air is not available, oxygen and nitrous oxide can be used, but take special care to protect the tube cuff.
A laser plume, composed of smoke and gas, accompanies the use of medical lasers. Efficient smoke evacuation must be maintained close to the operative site.
If a fire occurs in an airway during laser surgery, the flow of anaesthetic gases (including oxygen) should be stopped and the tube removed. The area should be flooded with saline, followed by ventilation with 100% oxygen. Spontaneous breathing in a sitting position should be aimed for in recovery, but reintubation may be necessary if airway oedema is severe. A tracheostomy may also be necessary.
Examples of medical lasers
Pulsed dye laser
This uses light at a wavelength that targets red blood cells within blood vessels. The energy is dissipated within the dermis and causes only minimal epidermal scarring. This is used mainly for treating port wine skin lesions. Children requiring laser therapy to these lesions will often be subjected to multiple treatments, usually under general anaesthesia. Postoperative pain may be a problem, particularly if large areas are treated. Combinations of paracetamol and NSAIDs may be effective, but occasionally opioid analgesics are required.
Carbon dioxide laser
These lasers have a long wavelength (10 600nm, outside the visible spectrum) and are preferentially absorbed by water. Target cells are heated to the point of vaporisation by the beam. They penetrate to only a very shallow depth, so tissue damage can be directly observed. They are used in aesthetic facial surgery, to reduce the wrinkling associated with ageing, and in ENT practice to vaporise vocal cord and airway lesions. Care must be taken to avoid eye and airway injury (see above).
Nd–YAG laser
This laser is also outside the visible range and, unlike the CO2 laser, is transmitted through clear fluids and absorbed by dark matter. It can penetrate to a depth of 1cm. It has multiple applications, including airway neoplasms, vascular malformations, and ophthalmic surgery.
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