Modern surgery has been redefined by powered instruments, technological tools that in many ways have revolutionized the delicacy, precision, and accuracy of the various operations performed. Yet many people who use these implements every day have very little understanding of the technology behind these tools. Although a complete treatise on 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 resist 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. Because all electromagnetic waves travel at the speed of light, which is a constant, these waves depend on the relationship between their frequency and wavelength. Since these three characteristics are defined by the equation:
c = f λ (where c is the speed of light, 2.998 × 108 m/s)
frequency (f) and wavelength (λ) are inversely related; that is, as frequency increases, wavelength decreases, and vice versa. The ability to pass high-frequency current through the human body without causing excess damage makes electrosurgery possible.
Electrosurgery is often incorrectly termed electrocautery, which is 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 applied for 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.
In electrocautery, current passes through a wire loop and heats it. This heat cauterizes tissue. No current passes through the patient.