Modern surgery has been enhanced by powered instruments that have revolutionized the delicacy, precision, and accuracy of operations. Yet many people who use these implements every day have very little understanding of the technology behind them. Although a complete treatise on the electromagnetic generation of heat and the physics of current generation are beyond the scope of this chapter (and are available elsewhere), understanding some fundamental rules governing the behavior of electrical currents and some relatively straightforward principles helps guide the use of these technologies.
Principles of Electricity
An electrical circuit is any pathway that allows the uninterrupted flow of electrons. Electrical current is the flow of electricity (the number of electrons) in a given circuit over a constant period of time and is measured in amperes (A). Current can be supplied either as direct current (DC), with constant positive and negative terminals, or as alternating current (AC), with constantly reversing poles. The electromotive force, or voltage, is a measurement of the force that propels the current of electrons and is related to the difference in potential energy between two terminals. The resistance is the tendency of any component of a circuit to oppose the flow of electrons, and applies to DC circuits. The equivalent of this tendency in an AC circuit is known as impedance. Any electromagnetic wave, from household electricity to radio broadcasts to visible light, can be described by three components: speed, frequency, and wavelength. These three characteristics are defined by the equation:
where c is the speed of light, 2.998 × 108 m/s.
Because the speed of light is constant, frequency (f) and wavelength (λ) are inversely related; that is, as frequency increases, wavelength decreases, and vice versa. Frequency is measured in cycles /second, reported as Hertz (Hz), and wavelength in meters. Although there are many electrical currents, including nerve impulses, in the human body, high-frequency current can pass through it easily. The ability of high-frequency current to pass through the human body without causing excess damage makes electrosurgery possible.
Electrosurgery is sometimes incorrectly called electrocautery, a separate technique. Electrocautery is a closed-circuit DC device in which current is passed through an exposed wire offering resistance to the current (Figure 7–1). The resistance causes some of the electrical energy to be dissipated as heat, increasing the temperature of the wire, which then heats tissue. In true electrocautery, no current passes through the patient. Electrocautery is primarily used in microsurgery, such as ophthalmologic procedures, where a very small amount of heat will produce the desired effect or where more heat or current may be dangerous. These devices also have use in customization of woven or knitted vascular grafts.
In electrocautery, current passes through a wire loop ...