Читать книгу Complications in Equine Surgery - Группа авторов - Страница 183

Surgical lasers produce a range of wavelengths (Figure 12.1) with varying tissue interactions, the understanding of which is required to predict the laser's effect upon tissue. Many surgical wavelengths are invisible. Interactions are determined by the degree to which the tissue absorbs the particular wavelength of laser energy (Figure 12.2). The more a tissue absorbs laser energy, the less it penetrates into the tissue, thereby concentrating the surface effect. Whereas deeper penetration allows controlled coagulation (denaturation of protein) of a larger volume of tissue but may put associated deeper structures at risk of being injured. Complete lack of absorption of a wavelength by a tissue allows complete passage affecting only a deeper tissue. Interaction between laser light and a tissue that preferentially absorbs that wavelength apart from surrounding tissue, allowing selective coagulation/necrosis of that tissue, characterizes the principle of selective photothermolysis [3–7].

The laser tissue effect is due to optical and thermal interactions [8]. Optical interaction is the result of true absorption of electromagnetic energy and usually results in a thermal effect once absorbed by tissue. Depending upon the amount, heat may “boil” the cytosol thereby vaporizing the tissue into a smoke plume or simply denature tissue proteins. When the optical interaction does not achieve the desired effect (e.g. Nd:YAG/GAA diode lasers on pale surfaces), the irradiation is sometimes “artificially” converted into heat by the delivery device before applying it to the tissue, thereby causing the energy to be absorbed at the tip of the fiber, producing heat and a profound surface effect on the tissue while minimizing penetration to deeper structures. Means to transform irradiation into heat include blackening the tip of the bare quartz fiber or using a sapphire tip on a gas‐cooled quartz fiber [2].

Figure 12.1 Wavelengths of surgical lasers. Wavelengths in common veterinary use are in gray. The surgical lasers are generally not in the visible range.

Figure 12.2 Tissue absorption common surgical laser wavelengths. The visible spectrum is shown on the horizontal axis. The near‐infrared GAA Diode and Nd:YAG lasers are highly absorbed by dark pigment. However, note the increased absorption of the GAA Diode laser on the water curve compared to the Nd:YAG laser. The Ho:YAG and CO₂ lasers are both highly absorbed by water.

Lasers are rated by power, the rate at which they can deliver energy. Power is expressed in watts (W) (1 W = 1 joule/second). Energy is measured in joules or calories (1 joule = 0.24 calories). The total amount of energy delivered per unit area is fluence expressed in joules/cm², which depends upon time of exposure as well as power density. Power density (PD) (W/cm²) is a critically important value that expresses the amount of energy delivered per unit area of tissue. Similar to water at a constant flow through a hose, laser energy delivered through a wider aperture will have a less profound effect than the same amount of laser energy delivered through a narrower aperture (Figure 12.3). Power density is varied by adjusting the output of the laser, by varying spot size of the laser beam on the tissue (Figure 12.4), by changing the distance from the delivery device to the tissue (Figure 12.5), or by changing the delivery device. Power density varies with the square of the spot size and is calculated by the following formula where s = spot size in mm, and W = power setting of the laser [2].

Figure 12.3 Power density profoundly affects rate of tissue effect and collateral heating of tissue. Both water hoses transmit identical flows of water. The wider aperture of delivery in the top image produces no mechanical effect on the flower, whereas the narrower aperture in the lower image produces a jet of water that can disrupt the flower.

Source: Kenneth E. Sullins.

Figure 12.4 Power density decreases with the square of the increase in spot size, which in turn increases with distance from the surface. The beams depicted are all CO ₂ laser beams from machines set to 50 W. The power densities shown below each demonstrate the profound reduction in tissue effect by increasing spot size. Moving the handpiece away from the tissue increases spot size and decreases power density.

Delivering identical power density values over different periods of time produces different results. If an acceptable full‐thickness skin incision could be created with a 10‐W laser beam delivered as a 0.4‐mm spot size advanced along the skin for 5 sec just penetrating the skin completely, doubling the rate of advancement (total time halved), the incision would be shallower because the total laser energy (fluence) has been halved. Conversely, if the original time were doubled (advancement slowed), the depth of the incision would increase beyond the skin and damaging collateral heating of adjacent skin would increase. Furthermore, varying spot size or increasing distance from tissue dramatically changes power density. Using a single power setting, the power density (tissue effect) can regress from focal incision/ablation (vaporization) to coagulation to negligible by simply moving the delivery device away from the tissue surface. This is described further below under Carbon Dioxide Laser [2].

Figure 12.5 CO₂ laser handpiece with focusing lens. The stylus indicates the point of maximum focus (power density) for incision. Slight increase of distance widens the spot size and tissue can still be vaporized. More distance from the tissue further increases the spot size and reduces the effect on tissue to coagulation.

The objectives of laser surgery fall broadly into three categories: incision/excision, ablation and coagulation of tissue. Which of these occurs depends upon power density and absorption length of the laser, which in turn influence the rate of heat generation in tissue (Figure 12.6) [8]. Incision/excision and ablation result in cell disruption and “vaporization” of tissue into smoke. Coagulation here refers to denaturing of tissue proteins, which grossly appears as blanching and tissue contraction [2].

Excision is simply incising or dissecting tissue, whereas ablation refers to vaporization of tissue. An incision creates tissue loss the width of the laser beam (usually ≥0.16 mm). Highly concentrated laser energy (i.e. high‐power density) is required to efficiently cut tissue with minimal heating of surrounding tissue. Since laser energy has no mass (i.e. steel blade) to separate tissue, tension on tissue is absolutely required so the incised surfaces separate. Without tension, excess heat will accumulate and the margins will be jagged and eventually necrotic. Collateral heating of tissue can be a substantial contribution to wound dehiscence, because it produces a zone of necrosis along the wound margin. A small zone of necrosis has no effect on an open wound after resecting a mass, but it profoundly affects healing of a primarily sutured incision. Therefore, adequate power density to incise quickly is critical to create a precise incision with healthy adjacent tissue to achieve primary wound healing [9]. Ablation also requires a relatively high power density but laser energy is moved over a surface to “paint” tissue away [2].

With a small spot size, a single efficient pass across tissue with adequate tension on the tissue, 5,000 W/cm², is a minimally sufficient power density to avoid collateral thermal necrosis (Figure 12.7) [10]. While learning, the tendency is to reduce the power setting and move tentatively or in multiple passes causing the laser to remain on the tissue longer while increasing the width of the wound and collateral heating. Incisions may dehisce due to thermal necrosis of the margins [11]. Experienced surgeons apply a significantly higher power density and work efficiently with a single pass of the laser (Table 12.1) [9]. A carbon dioxide (CO₂) laser in continuous mode at 50 W delivered with a 0.16‐mm focused spot size yields a power density of 248,880 W/cm²; a waveguide‐delivered CO₂ laser at 8 W through a 0.4‐mm ceramic tip delivers approximately 6,300 W/cm². The former will produce an incision more efficiently, but should be moved quickly across the tissue to limit penetration beyond the skin. The latter will produce an acceptable incision if tension is adequate to separate tissue and the waveguide is passed once and quickly across the skin. The skin defect will be 0.24 mm wider than the former with a perfect incision, which is clinically insignificant.

Figure 12.6 Absorption length of various wavelengths of surgical lasers in unpigmented skin. Wavelengths commonly used in veterinary medicine are in dark gray; wavelengths (nm) are stated beside the names. The far‐infrared Ho:YAG and CO₂ lasers are highly absorbed by water so penetrate minimally into skin, whereas the near‐infrared Nd:YAG or GAA Diode lasers are absorbed more by the darker pigments of the deeper layers [8].

Figure 12.7 Range of tissue changes from laser beam. With sufficient power density, a laser beam has a central area of tissue vaporization/ablation shown by the crater in this drawing. A layer of carbonization occurs when tissue that has been significantly heated cools to produce char. The area of thermal necrosis is where tissue is heated beyond physiological limits and sloughs later. The goal of incisive surgery is to use adequate power density to create as little carbonization and thermal necrosis as possible [10].

Reports of laser research should be examined closely to detect flawed methods [2]. Incisions created with the CO₂ laser were reported to have reduced tensile strength upon healing, with more necrosis and inflammation compared to steel (scalpel) incisions, but the laser incisions were created using a power density of 1,990 W/cm² which resembles comparing a steel scalpel to a hobbyist’s wood burning set [11].

Laser energy can be delivered to the tissue in a noncontact or contact manner. As the term implies, with noncontact delivery, nothing but the laser light touches the tissue, thus imparting a purely optical interaction. Carbon dioxide laser energy is reflected by mirrors or down a highly polished waveguide and delivered in noncontact fashion. Lasers delivered by quartz fibers (Nd:YAG and GAA diode lasers) can deliver energy either way.

Laser energy is often delivered in continuous mode, i.e. uniform throughout application of the energy to tissue; some lasers have no other mode available. However, pulsed modes tremendously increase efficiency and minimize collateral heating of tissue. The principle is that spikes of laser energy at ≥200 Hz increase power density substantially while the interruptions allow tissue to cool slightly, minimizing diffusion of heat into adjacent tissues [12–14]. A CO₂ laser in continuous mode at 50 W delivered with a 0.16‐mm focused spot size yields a power density of 248,880 W/cm². In its pulsed mode, 400‐W power spikes provide intermittent power densities of 1,990,446 W/cm² while producing identical total fluence (Figure 12.8). The technique depends upon the interval between laser exposure not exceeding the thermal relaxation time of the tissue, which is the time required to cool 50% of the heat applied without conducting heat to the surrounding tissue. By supplying a second pulse before the tissue cools further, potential char is vaporized and tissue debris is evacuated as smoke or steam. This feature produces a cleaner skin incision with less collateral thermal injury than from a continuous wave [15, 16]. The same principle applies to ablating tissue/masses with a computerized scanner on a CO₂ laser. Pulsed mode should not be confused with simple gaited mode, which simply turns the laser delivery off and on at specified intervals, which may be useful to prevent overheating of the quartz fiber) [2].

Table 12.1 Common laser techniques and considerations [9].

Laser	Description	Capacity	Accessories	Preference for skin incision	Comments
GAA Diode Laser	Quartz fiber delivery	25–50 W	600 and 1,000 micron quartz fibers Handpiece to hold fibers	1,000 micron fiber sculpted down to approximately 600 micron at the tip	25 W is insufficient for noncontact vaporization 600 micron fiber too fragile for general surgery. Excellent for endoscopic surgery Sterilize fibers for aseptic procedures.
Nd:YAG Laser	Quartz fiber delivery	100 W	Gas cooled	Conical sapphire tip	Gas cooled fiber excellent for noncontact ablation
CO2 Laser Articulated Arm Delivery	125‐mm focusing handpiece. Minimum spot size 0.16 mm	Minimum 30 W	Computerized pattern scanner very useful for partial thickness ablation of skin tumors or corneal tumors	30–50 W pulsed mode. Better hemostasis in continuous mode if wound is to be left open	Sterilize handpiece and use sterile sleeve for aseptic procedures
CO2 Laser Waveguide Delivery	0.25–4.0mm (spot size) tips	15–40W		Super pulse available	No lens focus of laser beam. Power density varied with distance, power setting or changing diameter of tip
Laser Smoke Evacuator	Many brands available		Spare filters.		Performance drops off quickly when filter fills. Sterilize hose for aseptic procedures