Fundamentals of Energy Sources
The evolution of laparoscopic instrumentation has included the use of various energy sources for coagulation and dissection. The use of heat to achieve hemostasis in surgery was first employed in ancient times by the use of fire-treated iron rods to create vessel thrombosis through coagulation necrosis of body tissues (Ghazanfar 1995). During the nineteenth century, electricity-based surgical devices were introduced to the surgical theater. These electrocautery devices used electricity to heat wire loops that were placed in contact with tissues to coagulate tissue on contact. Later, spark generators were developed to create high frequency oscillating currents that caused coagulation by the application of electric current to tissue. The direct application of electric current to tissue distinguished electrosurgical devices from electrocautery instruments (O’Connor et al. 1996).
Ultrasonic technology was introduced in the 1990s and uses high frequency sound waves to cut and coagulate tissues without electric current. While low frequency ultrasound waves are useful for diagnostic imaging, high frequency ultrasound generators apply electromagnetic current to a piezoelectric transducer that converts that energy to mechanical vibration waves that convert mechanical energy into heat at the blade–tissue interface (McCarus 1996).
Laser is an acronym for light amplification by stimulated emission of radiation. In its pure form, laser light is unique because it is monochromatic (one wavelength), coherent (waves in phase), and collimated (waves are parallel). These devices apply light energy directly to tissues to cut, coagulate, or vaporize tissues with a thermal effect (Fisher 1992).
All of these technologies have been used to perform laparoscopic procedures in man and in animals. In this chapter, we will examine each of these energy sources as they may be used to accomplish laparoscopic surgical procedures.
Thermal Effects in Tissue
Each of these technologies achieves its surgical effect by the transfer of energy to the tissue. The resistance to passage of electric current through the tissue is inversely proportional to the water content of that tissue. As thermal energy is applied to the tissue, the tissue is modified in direct proportion to the amount of energy applied to the tissue. As the tissue is heated above 60°C, irreversible protein denaturation occurs. This results in coagulation and blanching of the tissue. At this temperature, hemostasis is achieved. As the tissue temperature exceeds 80°C, carbonization occurs. This effect is easily detected by observing the formation of black char at the tissue surface. Above 100°C, vaporization of tissue occurs as the tissue is turned to steam.
When considering the use of electrosurgical instruments, ultrasonic generators, or lasers for laparoscopic procedures, it is appropriate to review how the thermal energy is generated, how that energy is transferred to the tissue, and how the energy interacts with both the target tissue and adjacent tissues in the surgical field or in remote parts of the body.
Electrocautery units use high frequency electric current to heat a metal probe that is placed directly in contact with the tissue to increase tissue temperature, causing cutting, coagulation, and tissue ablation. The clinical effects of electrocautery result from conduction of heat from the probe to the tissue. Electrocautery is primarily used to coagulate small vessels and for dissection of vascular tissue with good hemostasis in soft tissue surgery.
Electrosurgical devices apply alternating electric current directly to the tissue to cut, coagulate, or fulgurate the tissue (see Figure 5.24). Electrosurgical generators produce electric current in the range of 300–3800 kHz. While electrocautery devices use heat conduction to achieve their thermal effects in the tissue, electrosurgical devices generate heat within the tissue by actually passing the high frequency electric current through the target tissue. Waveform devices may be adjusted to create cutting, coagulation, or a “blend” of the two waveforms. The amount of surface area of the electrosurgical probe in contact with the tissue, along with the power setting of the unit, will determine the tissue effect. The smaller the contact surface area, the higher the energy density that is applied to the tissue. In general, the use of a small contact point creates maximum cutting effect with less coagulation (hemostasis) effect than can be achieved with use of a larger probe.
Monopolar electrosurgical units employ a small active electrode to deliver highly concentrated energy to the target tissue. This high energy density causes tissue vaporization, which results in both cutting and coagulation effects. The energy passes through the body to a larger “return” or ground electrode, which has a greatly reduced or negligible thermal effect due to the dispersal of energy over its larger surface area.