4 JULY - SEPT 2006

NewsScope

c l i n i c a l o p i n i o n

hile it is not necessary to be an electrical engineer to perform laparoscopic or hysteroscopic surgery with radiofrequency (RF) energy, it is important to know some basic tenets of electricity as it applies to

surgical instruments.Basic Principles of Electricity

Electrosurgery is performed using RF alternating current (AC), derived from a wall source and modified by the electrosurgical generator or unit (ESU) to an output with a frequency of about 500 Kilohertz, in the range provided by am radio. Whereas all RF electrosurgery requires two poles interacting with the patient (and is therefore “bipolar”), surgical instrumentation is designed as being either monopolar (unipolar) or bipolar. Bipolar instruments possess both electrodes and only the tissue interposed between the two electrodes is included in the circuit. In contrast, monopolar instruments comprise only the “active” electrode, while the inactive or dispersive electrode is located remotely with the entire patient interposed between the two electrodes (Figure 1). Alternating currents result from a source of alternating polarity and therefore do not “flow” like direct currents so concepts such as “return” electrode do not apply.

Effect of RF Current on Cells and TissueHow does RF electricity cause a tissue effect?

The tissue effects of RF current are caused by an increase in intracellular temperature. With the application of RF current, the rapidly alternating polarity introduced across the cell causes intracellular cations and anions to rapidly oscillate. This mechanical energy is converted by friction into thermal energy thereby elevating intracellular temperature (Figure 2). If the intracellular temperature reaches about 60o to 90oC, both cellular dehydration and protein denaturation occur. When the tissue cools, molecular bonds reform in a random fashion turning the impacted tissues into a homogenous, undifferentiated mass. If, instead, the cytoplasmic temperature quickly reaches or exceeds 100oC, the intracellular water boils, and the subsequent formation of gas (steam) and intracellular expansion results in explosive vaporization of the cell.

What determines the amount of power transferred to the cells and tissue?

Power density is expressed in watts/cm2 (Figure 3) and is largely determined by the shape and size of the electrode. A very small electrode, such as a needle shaped device, or the edge of a blade, will concentrate current so that the power

density is high, allowing for the rapid elevation of cellular temperature and the creation of a narrow zone of vaporization. Given the same power, a wider, larger electrode, in contact with tissue, will dilute the power density preventing elevation of intracellular temperature to the boiling point, instead causing coagulation and desiccation. At the extreme, a very large electrode prevents cellular and tissue heating altogether – this is the principle around the dispersive electrode.

What are the tissue effects that can be created with RF electricity? Cutting and Vaporization

Vaporization of tissue is best achieved with a continuous, low voltage (“cutting”) current, using a pointed or thin edged (eg a blade) monopolar electrode held near to but not in contact with the tissue, thereby creating a zone of high power density (Figure 2). When vaporization occurs, the ionic products create a localized cloud of low impedance, sometimes called the steam envelope, within which the surgeon should seek to maintain the electrode tip. Electrosurgical cutting is really a process of

linear vaporization whereby the electrode is advanced in the desired direction, always maintaining the tip or edge within the continuously propagated steam envelope. To move too quickly results in contact with the tissue, a resulting reduction in the power density, and consequent propensity to coagulate and not vaporize. Properly performed, collateral damage to adjacent cells is minimal.

Desiccation and CoagulationLow voltage outputs (usually labeled cutting and blend)

are preferred to high voltage modulated outputs, usually labeled coagulation (Figure 3). There exist a number of

potential reasons for this apparent paradox. First, the highly modulated (interrupted) nature of the high voltage outputs contributes to uneven protein bonding that may prevent, for example, complete and secure occlusion of a blood vessel. In addition, such current may cause the more superficial layers of the tissue to become rapidly coagulated, increasing impedance, thereby inhibiting further transmission of current to the deeper layers. Finally,

the effect near the electrode causes rapid heating and adherence of tissue, allowing the eschar to be pulled off with removal of the electrode, a process that facilitates bleeding.

To perform coaptive coagulation of a vessel, it is necessary to have effective vascular compression, usually with forceps or a clamp, thereby preventing the heat sink effect of flowing blood, and allowing the broken molecular bonds to reform across the

Electrosurgery Basics

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Malcolm G. Munro, M.D. FRCS(c) FACOG

ProfessorDepartment of Obstetrics and GynecologyDavid Geffen School of Medicine at UCLA

Kaiser PermanenteLos Angeles Medical Center

Los Angeles, CaliforniaAAGL Advisor

The opinions, viewpoints, conclusions, recommenda-tions and statements in the Clinical Opinion column are solely those of the author(s) and are not-attributable to the sponsor, publisher, editor or editorial board of NewsScope, the AAGL, or any of its affiliates.

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Figure 1. Schematic depicting monopolar and bipolar instrumentation.

Figure 2. Cellular effects of RF currents.