Know when to terminate current to prevent vital-tissue damage. Although the flow of current and primary thermal effects are restricted to the tissue between the poles, this does not eliminate the risk of thermal injury to tissue that is distant from the site of directed hemostasis. As current is applied between the poles, the intervening tissue gradually desiccates until it becomes thoroughly dehydrated. Desiccation is complete when the tissue whitens and visible steam emission stops. If the application of current continues, the heat spreads well beyond the electrical limits of the instrument.
This secondary thermal bloom is caused by the bubbling of steam into the tissue parenchyma as heat is rapidly generated (due to dry tissue’s high resistance to the flow of electrical current). This explains why structures such as the ureter or bowel may suffer irreversible thermal damage despite being at some distance from an operative or bleeding site.
Use of an in-line ammeter does not prevent this problem. Rather, it tends to promote the flow of energy well beyond desiccation. Consequently, an ammeter should not be used to determine the treatment endpoint for coagulation of blood vessels in the vicinity of vital tissue.
Whenever bipolar electrosurgery is used for hemostasis, unwanted thermal damage can be minimized by:
- terminating the flow of current at the end of the visible vapor phase,
- applying current in a pulsatile fashion to permit tissue cooling,
- avoiding the use of an in-line ammeter to determine the coagulation endpoint, and
- securing pedicles by a stepwise process that alternates between partial desiccation and incremental cutting (TABLE 1).
Since the rate of temperature generation is a direct function of the volume of tissue being desiccated, thermal spread can also be reduced by using the sides or tips of a slightly open forceps to press or lift, rather than coapt for hemostasis (FIGURE 4).
Free adherent tissue gently. As with contact monopolar coagulation, tissue between the electrodes of a bipolar instrument may become adherent during desiccation. Repeated attempts to shake the tissue free may lead to traumatic avulsion of a key vascular pedicle. A stuck vascular pedicle can usually be unglued by energizing the opened device while immersing it in a conductive irrigant, such as saline. Once the solution is boiled by the high current density between the electrodes, the mechanical action of bubbling is usually sufficient to atraumatically free the pedicle.
FIGURE 4 Minimizing bipolar thermal damage
Using contact rather than coaptation is one way to limit thermal injury during bipolar electrosurgery.TABLE 1
Minimizing bipolar thermal damage
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Ultrasonic energy
Mechanism of action. Ultrasonic shears produce mechanical energy to cut and coagulate tissue. Housed in the hand piece of this laparoscopic device is a piezoelectric crystal that vibrates a titanium blade 55,500 times per second over a variable excursion of 50 microns to 100 microns. As energy is transmitted to tissue, hydrogen bonds of tissue proteins are ruptured, leading to a denatured protein coagulum without significant charring. The tissue cutting that occurs is secondary to mechanical vibration and cavitational fragmentation of tissue parenchyma.
Since ultrasonic energy does not generate the high temperatures created by electrosurgery, it is less dependable for deep-tissue coagulation. Rather, thermal change is gradual, requiring a modicum of patience.
Available instrument configurations include 5-mm curved or hook blades. In addition, 5-mm or 10-mm ligating and cutting shears coaptively desiccate and cut the tissue by securing it between a grooved plastic pad and the vibrating blade.
Tissue effects depend on interplay of factors. By using various combinations of blade shapes, blade excursions, and tissue tensions, surgeons can accomplish a variety of specific effects. Cutting velocity is directly proportional to blade excursion, tissue traction, and blade surface area (energy density), and is inversely related to tissue density and elasticity. Thus, the fastest cutting with the least amount of coagulation occurs when tissue is placed on tension and firmly squeezed, lifted, or rotated with the sharpest side of a blade set at maximum excursion.
Coagulation is the obverse of cutting: It is inversely related to tissue tension, blade sharpness, blade excursion, and cutting speed. Therefore, coagulation is best achieved by relaxing tension, minimizing blade excursion, and using a blunt edge or flattened blade surface.
A stuck vascular pedicle can usually be unglued by energizing the opened bipolar device while immersing it in a conductive irrigant.
Avoid excessive traction and torsion. Be mindful of the potential for premature incision of an incompletely coagulated tissue pedicle when excessive traction or torsion is applied. When used to coaptively desiccate and incise a vascular pedicle, ultrasonic energy should be applied patiently, taking great care to minimize tissue tension while using the broadest blade surface set to the lowest excursion (TABLE 2). Hemostatic incision is best ensured by first coagulating several overlapping areas along an untracted pedicle, limiting each application to the point of tissue blanching and initial vapor emission. Only then should the pedicle be incised by gradually lifting, squeezing, or rotating the distal device. In this fashion, ultrasonic energy can be successfully used to secure both the ovarian and uterine vessels.