Principles

The increasing surgical use of lasers, with their inherent potential hazards to patients and operating room staff, mandates an understanding of their physical principles by anaesthetists.¹

The word laser is an acronym for ‘light amplification by stimulated emission of radiation’. The laser produces an intense beam of pure monochromatic light (one wavelength: one colour), in which all of the waves are in phase (coherent). The output beam is likely to be of a very small cross-sectional area and is virtually a non-divergent parallel (collimated) beam. These properties mean that energy may be delivered to very small areas of tissue with great accuracy, and the intense parallel beam of light constitutes a very large amount of power per unit area of tissue. The wavelength of a laser is determined by the lasing medium used. Although described as monochromatic, most laser media produce light within a narrow waveband consisting of a number of discrete frequencies. In order to come up with the concept of a laser, which was first described in 1958 and first demonstrated in 1960, scientists had to understand the notion of quantum physics and of Niels Bohr’s model of the atom, with its orbital discrete energy levels (see below).

Although there are many more complex laser systems,² the basic components of a laser are shown in Fig. 26.1. The lasing medium may be a solid, liquid or gas. The atoms of a lasing medium are excited to high energy levels by a ‘pumping’ source, which may be a high-voltage discharge in the case of a gas, an intense flash of light from a flashtube or the energy from a radio frequency power source. Fig. 26.2 shows the excitation and emission process possible in a gaseous lasing medium. A photon of energy from the pumping source may be absorbed by a stable atom in its so called ‘ground state’, which then becomes an atom in an excited state, with an electron or electrons in an orbital shell at a higher energy level. Spontaneous emission of a photon of energy occurs as the electrons fall back to shells of a lower energy state and the excited atom reverts to the ground state.

Figure 26.1 Basic components of a laser.

Figure 26.2 Absorption, excitation and emission processes.

If a further photon of pumping energy, at the correct wavelength, is applied to an atom in its excited state, it will fall to its ground state and two photons of energy will be emitted instead of one. This is known as stimulated emission, originally described by Einstein in 1917 as the basis for laser technology³ and the inversion of the energy states is referred to as population inversion. The emitted photons thus produced are in phase with, have the same polarization, and travel in the same direction, as the stimulating radiation. This mechanism is amplified by many of the escaping photons being reflected back into the lasing medium by the mirrors. Thus a chain reaction occurs, and this can be thought of as a positive feedback system. The process produces an intense source of light energy, some of which is allowed to escape through the partially reflecting mirror at the output end of the lasing medium. The output beam of the laser is usually directed to the tissues through a fibre-optic light guide. However, the wavelength of the carbon dioxide laser is so long, at 10.6 µm, that there is no fibre currently available to transmit energy, so that it has to be directed by a series of mirrors instead.

Clinical applications

The clinical use of lasers depends on a compromise between:

The extent of the effect of a laser on human tissues depends primarily on the intensity and the frequency of the light being used.⁵ At low light intensity, stimulation occurs within the cell and this is exploited in physiotherapy applications. At slightly higher intensity, attenuation of cellular activity occurs. At still higher intensity levels, about 40 J cm⁻², sensitizing agents in tissue become activated (this is the basis of protecting the skin from the sun’s ultraviolet rays using suntan lotions). By the time the light intensity has risen to 400 J cm⁻², the tissue temperature has risen to 60^oC and protein denaturation and photocoagulation predominate. Further large increases in light intensity result in a tissue temperature rise to 100^oC, vaporization of tissue fluids and destruction of cell structures. In order to control the destructive power of a laser, most systems can be pulsed; the light is emitted in short bursts, to allow heat dissipation between bursts and reduction of thermal damage to neighbouring tissues. ‘Q switching’ of a laser refers to a device which allows aliquots of laser light to be stored and released in bursts of even higher energy and shorter duration. Such a technique is used in ophthalmology to cause photoablation and to minimize thermal damage to the eye. With Q switching, while collateral thermal damage may be reduced, the frequency of switching is such that tissue vibration and, therefore, mechanical damage may predominate.

The penetration of light energy into body tissues depends on the wavelength of the light. Far infrared and ultraviolet light has little penetration because it is rapidly absorbed near the surface of the tissue, by tissue water. Maximum penetration of light occurs at the red end of the spectrum.

The graph in Fig. 26.3 shows the spectrum of absorption of light at different wavelengths by haemoglobin, melanin and water. Monochromatic light energy is absorbed by tissue of complementary colour (opposite colour) and reflected by substances of the same colour.

Figure 26.3 Absorption characteristics of tissue constituents.

Carbon dioxide laser energy at 10.6 µm is absorbed by water within 1 mm depth, causing rapid vaporization of intracellular water. The main use of the carbon dioxide laser is, therefore, as a bloodless cutter and vaporizer. The blue-green argon laser beam penetrates to about 2 mm depth and is maximally absorbed in the 500 nm waveband by substances of a complementary colour (red), such as haemoglobin. Thus, the argon laser is used to coagulate blood in small vessels with very little effect on other more transparent tissues, for example the retina. The Nd:YAG (neodymium: yttrium-aluminium garnet) laser has a solid lasing medium and produces energy in the near infrared region of the spectrum, which has maximum penetration, being absorbed at 3–5 mm depth, by haemoglobin, melanin and water. When invisible infrared lasers are used (e.g. CO₂ laser), it is common practice to make use of a low powered, visible-light laser, such as a helium:neon laser at the same time, in order to aid in aiming the therapeutic laser accurately. Table 26.1 shows the currently available lasers in medical use.

Safety aspects

Apart from the danger to the patient from the beam of laser energy if it is misused, there is a risk to the operator and other persons in the operating environment.⁶ This is because of the long range of laser light due to the virtual non-divergence of the beam; thus, in contrast to a collimated X-ray beam for example, increased distance from the source has very little safety benefit. Even reflected laser light may be very dangerous to the eyes. Visible laser light transmitted to the retina of the eye may burn it irreparably, leaving a blind spot in the field of vision. A similar lesion over the optic nerve may result in total blindness of that eye. The cornea, lens and aqueous and vitreous humours partially or totally absorb far-infrared laser radiation; these tissues, therefore, are more susceptible to damage than the retina.

Laser radiation on the skin may be felt as a burning sensation, which is, therefore, self-protective, provided that the victim is conscious and has not received analgesia.

In terms of the danger that lasers pose to humans, there is a complex relationship between power, frequency and time of exposure. There is an international classification for lasers, shown in Table 26.2, where Class I lasers are inherently safe and Class IV lasers are, broadly speaking, hazardous if misused. Most lasers in medical use are in Class IV. No one should use a laser who is not trained to do so, and everyone who is working in the vicinity of a laser should be trained in the safety aspects of its use. This includes the anaesthetist, who is often standing in the line of fire of the laser.

Table 26.2 International classification of continuously working lasers