Laser source

TTTS. Laser source

The physical characteristics of the Nd:YAG laser make it particularly suitable for use within the amniotic cavity and for coagulation of blood vessels. The CO2 laser (wavelength 10 600 nm), which is the most commonly used laser in gynecology, is highly absorbed by water.10 Thus, it cannot be used within the amniotic fluid. In contrast, the Nd:YAG laser (wavelength 1064 nm) can transmit energy through clear fluid. 

The laser is a solid crystal of yttrium–aluminum– garnet (YAG) that contains a dopant of the rare earth element of neodymium (Nd), which actually makes the light. When applied through a bare fiber, the Nd:YAG causes deep tissue coagulation. Protein coagulation of 1–2 mm can be achieved with a power of 20 watts for 1–3 seconds. 

The pure color aspect of the laser is only marginally important, because it is heat that determines the surgical effect. However, color specificity of lasers facilitates its effect on target tissue (hemoglobin-containing tissue) while sparing adjacent normal tissues. Most of the desired effect can be achieved with 15–30 watts, but occasionally a power as high as 40 watts may be necessary, particularly with larger blood vessels. 

The greater the blood flow through a vessel, the more it acts as a heat sink, and the greater the difficulty in achieving hemostasis.11 Diode lasers (wavelength 980 nm) have also been recently used in TTTS and can also deliver energy within a fluid environment. 

They are typically less costly and less bulky, but may be limited in terms of the amount of energy they can provide (Figure 9.7a,b). The Nd:YAG laser energy is delivered into the amniotic cavity by means of quartz fibers 400–600 ?m in diameter. The fibers allow the beam to diverge 10–15° once emitted from the tip of the naked fiber, so the beam is no longer collimated. Thus, the smallest spot occurs at the tip and continuously enlarges as the fiber tip is moved away from the target. 

The fiber tip is typically placed within 1 cm of the target vessels for maximum safe effect.10 Most laser fibers are end-firing. However, side-firing laser fibers, such as those used in urology, can be particularly useful in the treatment of patients with anterior placentas. Side-firing laser fibers are of a larger diameter such that they cannot be used through the operating channel of the current endoscope. Instead, they must be inserted in the amniotic cavity through an accessory port (Figure 9.8a,b). 

Endoscopic monitoring of the coagulation effect is based on the known visual properties of proteins under different temperature conditions: 

• at 60–65° C, blanching of the vessel occurs, which is indicative of adequate coagulation 

• at 65–90° C, the tissue turns white-gray, indicative of protein denaturation 

• at 90–100° C, puckering of the tissue occurs, consistent with drying of the tissue, 

• finally, at 100° C, vaporization and carbonization of the tissue occurs.