Electrical Safety

Richard Botney

▪ INTRODUCTION

Electricity is used in the operating room (OR) to power a large variety of equipment, ranging from items associated with direct patient care to lighting, computers, and electronic devices that support patient care. Electrical power is essential for the performance of contemporary anesthesia and surgery, as well as nearly all functions throughout the hospital, yet remains mysterious and poorly understood by many users. Despite an excellent safety record, electricity is hazardous and poses a variety of risks. In order for electrical equipment to be used safely, it is necessary to understand the ways in which electricity can cause harm. This chapter reviews the risks associated with the use of electricity and methods for making it safer.

Harm related to the use of electricity can occur in four distinct ways (Fig. 51.1). Electrical currents flowing through the body can cause an electrical shock or result in skin or other tissues being burned. The loss of electrical power can imperil patients. Electricity can interfere with the function of implanted devices such as pacemakers or defibrillators. Finally, electricity can also ignite fires; fire safety is covered in Chapter 52.

While patient safety is a primary concern when considering electrical injuries, the OR staff is also at risk. As users of electrical equipment, anesthesiologists, surgeons, or other personnel can experience an electrical shock. For example, many surgeons have received an unpleasant jolt while using an electrosurgical device. As such, the considerations for protection from shock apply equally well to the OR staff.

▪ ELECTRICAL SHOCK

A shock is experienced when electric current passes through the body. The amount of current that flows will be a function of the voltage difference across the body and the resistance to current flow presented by the body. There must be a complete circuit for current to flow. In other words, there must be a continuous, unbroken path for current to flow from its point of origin through a circuit and back to its point of origin. Two points of contact must therefore exist for current to flow through the body. Oftentimes, one of these contacts is established as a result of standing on the ground, so only one other point of contact needs to be made in order for current to flow and a shock to occur. The primary objective in electrical safety is to prevent patients or staff from becoming a part of that complete circuit.

Ground or grounding can be difficult concepts to grasp, in part because they have several meanings and can be used in a number of different ways. For the purposes of this chapter, anything connected to ground, whether intentionally (e.g., an equipment case or the OR bed) or unintentionally (e.g., a patient or staff person contacting a source of electrical power) will be held at a reference voltage that is by definition 0 V. In addition, the connection to ground provides a low-resistance pathway that permits current to return to its source. Ground is also considered a current sink, meaning it can accept and carry virtually unlimited amounts of currents.

Ideally, patients (and other individuals) should never be grounded, thus removing any possibility of becoming part of the electrical circuit. However, this can be difficult and impractical to accomplish, so instead the entire OR is kept isolated from ground (see below for a discussion of isolated power). Conversely, electrical equipment should always be grounded, to provide a low-resistance pathway for current to return to its source, rather than through some alternate pathway, such as a human being. For example, if a piece of equipment was not grounded, but it had a fault such that electrical power was in
contact with the case, an individual coming into contact with that case would then serve as the sole pathway for the fault current to flow back to the source. By keeping the equipment grounded, the bulk of the fault current will be conducted by the ground connection and only a small portion will flow throw the person, thus significantly reducing the risk of shock.

▪ FIGURE 51.1 A framework for understanding the potentially harmful effects related to the use of electrical power in the operating room.

Below roughly 1 milliamperes (mA), current is not perceptible. At current levels between roughly 1 and 10 mA, current may be perceived as a tingling, warm sensation. Currents in the range of 10-20 mA produce muscle spasm, and the individual cannot let go of the conductor. This is known as the “can’t let go” current. As current reaches 100 mA in magnitude, ventricular fibrillation and death will occur.

Injuries that result from electric shock include burns and tissue damage, ventricular fibrillation, and death. The injury that occurs will depend on the magnitude and duration of current flow through the body, as well as the cross-sectional area through which it flows. This is embodied in the concept known as current density, which is defined as the amount of current flowing through a given cross-sectional area, and can be thought of as a measure of how “concentrated” the current is. Macroshock refers to currents on the order of 1 mA or larger that are applied externally to the skin, that is, currents that are perceptible. There is a second phenomenon known as microshock that involves currents below the threshold of perception.

Microshock occurs in what is known as an electrically susceptible patient, that is, a patient with a direct conductive connection to the heart (e.g., a temporary pacemaker wire or a saline-filled catheter) that bypasses the skin. This is important as the skin is normally a source of considerable resistance. Not only does the direct connection provide a low-resistance pathway to current flow, the connection contacts the heart in a very small area. As a result, despite the low current levels flowing (as low as 10-100 &mgr;A), the resulting current density is sufficient to cause ventricular fibrillation. Because the current levels associated with microshock are so low, below the threshold of perception, normal methods to detect hazardous situations and prevent shock don’t work. For example, line isolation monitors (LIMs) (see below) do not help to protect against microshock.

Many ORs utilize power sources that are isolated from ground, that is, isolated power supplies (Fig. 51.2). These differ from the type of power supplies used in the home and other hospital locations in several important ways. A grounded power supply will have one hot lead and one neutral lead, which is physically connected to the ground conductor. If a person
(who is typically going to be in contact with ground) should come into contact with the electric circuit, there is a potential for some current to flow from the point of contact through the individual to ground and thus back to its source. Because the equipment is grounded, however, most of the current will flow along established grounding pathways, and only a small fraction through the person.

▪ FIGURE 51.2 Schematic of the power supply. The power grid supplies grounded power, with the neutral lead physically connected to the ground conductor. In the OR, after passing through an isolation transformer, line 1 and line 2 supply electrical power, but there is no connection with ground. Consequently, contact with either line 1 or line 2 cannot result in current return via the ground conductor.

In contrast, isolated power supplies provide electrical power through two leads, line 1 and line 2, neither of which is connected to ground. The two lines are electrically isolated from ground by an isolation transformer located in the OR

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