Complications in Equine Surgery. Группа авторов

Figure 12.8 Pulsed laser energy compared to continuous laser energy. Pulsing higher power densities for short durations (vertical bars) produces a more efficient tissue effect with less collateral tissue heating compared to a continuous beam (horizontal bar) emitting the same average power (fluence). The tissue cools slightly between the pulses.

When 50 W of energy is administered through a 125‐mm hand piece to focus through a lens to a 0.16‐mm spot size, the power density is 248,680 W/cm², which incises skin with 0.1 mm of collateral tissue effect. The “incision” actually has removed tissue; the narrower the spot size, the more natural the closure. Without changing settings, the hand piece can be retracted to defocus the laser beam to a 2‐mm spot (1,592 W/cm²) or a 4‐mm spot (398 W/cm²), substantially changing the laser effect. The power density changes with the square of the spot size (Figure 12.4). The surgeon must acquire the experience to achieve the spectrum of incision, ablation or coagulation [2]. Hemostasis during CO₂ laser surgery is significant but less profound than with lasers that penetrate tissue more deeply, even though lack of penetration is one advantage of using this laser. Hemorrhage from vessels ≤0.5 mm in diameter and lymphatic drainage will largely be eliminated [17, 19]; larger vessels or visible lumens should be ligated [2].

      Carbon dioxide lasers transmit the energy by reflection through mirrors in an articulating arm to a lens in the handpiece to focus the beam (Figure 12.9a). Some models deliver the laser beam through a highly polished flexible waveguide with a hand piece. Interchangeable tips instead of a lens determine spot size (Figure 12.9b). Carbon dioxide lasers are often equipped with pulsed modes (described above), making incisional surgery similar to that of a steel scalpel possible.

      Some CO₂ lasers can be fitted with semiflexible waveguides to access deeper surgical sites. Waveguides are actually tubes and are not as flexible as quartz fibers. Some waveguides can be passed through the biopsy channel of some endoscopes, but they are fragile. Excessive bending will reduce the laser energy or damage the waveguide leading to a burn out; these should be kept relatively straight [2].

Computerized pattern scanners are accessories that manipulate the focused (high‐power density) laser beam across a preset scan size at a constant velocity to ablate tissue. Without a scanner, a slightly defocused beam is used to create a manual crosshatch pattern to vaporize a surface lesion, but char must be periodically scrubbed with a gauze sponge to proceed. The manual technique is workable but generates more heat and is less uniform than with the scanner (Figures 12.10 a–c). The difference between manual delivery of a slightly defocused beam and computer scanning is that scanners deliver focused laser energy, which ablates tissue completely. The beam moves away before collateral heating occurs and returns before the tissue cools sufficiently for char to form; less heating of deeper tissue occurs. The surgeon must acquire the “feel” of the scanner and keep it moving appropriately or it removes excessive tissue. The power settings should be kept low until the proper technique is acquired. Since this is focused laser energy, reducing the power simply slows the rate of surgery and produces no detrimental effect [2].