∗ This chapter and the images contained herein were developed in whole from Clinical Anesthesia (ed. 7, Wolters Kluwer Health/Lippincott Williams & Wilkins; Barash PG, Cullen BF, Stoelting RK, et al., editors) and Anesthesia Equipment: Principals and Applications (ed. 2, Elsevier; Ehrenwerth J, Eisenkraft JB, Berry JM, editors) with permission of the editors and publishers.The myriad electrical and electronic devices in the modern operating room (OR) greatly improve patient care and safety. However, these devices also subject both the patient and OR personnel to increased risks. To reduce the risk of electrical shock, most ORs have electrical systems that incorporate special safety features. It is incumbent upon the anesthesiologist to have a thorough understanding of the basic principles of electricity and an appreciation of the concepts of electrical safety applicable to the OR environment.
Principles of Electricity
One basic principle of electricity is known as Ohm’s law, which is represented by the following equation, where E is electromotive force (in volts [V]), I is current (in amperes [A]), and R is resistance (in ohms [Ω]):
E = I × R
Ohm’s law forms the basis for the following physiologic equation:
BP = CO × SVR
Here, the blood pressure ( BP ) of the vascular system is analogous to voltage, the cardiac output ( CO ) is analogous to current, and the systemic vascular resistance ( SVR ) is analogous to the forces opposing the flow of electrons. Electrical power (p) is measured in watts. power is the product of the voltage ( E ) and the current ( I ), as defined by the formula:
P = E × I
The amount of electrical work done is measured in watts multiplied by a unit of time. The watt-second (a joule [J]) is a common designation for electrical energy expended in doing work. The energy produced by a defibrillator is measured in watt-seconds (joules). The kilowatt-hour is used by electrical utility companies to measure larger quantities of electrical energy.
Wattage can be thought of as a measure not only of work done but also of heat produced in any electrical circuit. Substituting Ohm’s law in the formula:
W = E × I
P = (I × R) × I
P = I 2 × R
Thus, power (in watts) is equal to the square of the current I (amperage) multiplied by the resistance, R. With these formulas, it is possible to calculate the number of amperes and the resistance of a given device if the wattage and the voltage are known. For example, a 60 W lightbulb operating on a household 120 V circuit would require 0.5 A of current for operation. Rearranging the formula so that I equals p/E:
I = 60W/120 V
I = 0 .5 A
Using this in Ohm’s law (R = E/I), the resistance can be calculated to be 240 Ω:
R = 120 V/0 .5 A
R = 240 Ω
It is obvious from the previous discussion that 1 V of electromotive force (EMF) flowing through a 1 Ω resistance will generate 1 A of current. Similarly, 1 A of current induced by 1 V of electromotive force will generate 1 W of power.
Direct and Alternating Currents
Any substance that permits the flow of electrons is called a conductor. Current is characterized by electrons flowing through a conductor. If the electron flow is always in the same direction, it is referred to as direct current (DC). However, if the electron flow reverses direction at a regular interval, it is termed alternating current (AC). Either of these types of current can be pulsed or continuous in nature.
The previous discussion of Ohm’s law is accurate when applied to DC circuits. However, when dealing with AC circuits, the situation is more complex because the flow of the current is opposed by a more complicated form of resistance, known as impedance.
Impedance, designated by the letter Z, is defined as the sum of the forces that oppose electron movement in an AC circuit. Impedance consists of resistance (ohms [Ω]) but also takes capacitance and inductance into account. In actuality, when referring to AC circuits, Ohm’s law states that voltage equals current multiplied by impedance:
E = I × Z
An insulator is a substance that opposes the flow of electrons. Therefore an insulator has a high impedance to electron flow, whereas a conductor has a low impedance to electron flow.
In AC circuits the capacitance and inductance can be important factors in determining the total impedance. Both capacitance and inductance are influenced by the frequency (cycles per second, or hertz [Hz]) at which the AC current reverses direction. The impedance is directly proportional to the frequency ( f ) times the inductance ( IND ):
Z α (f × IND)
In addition, the impedance is inversely proportional to the product of the frequency ( f ) and the capacitance ( CAP ):
Z α 1/(f × CAP)
As the AC current increases in frequency, the net effect of both capacitance and inductance increases. However, because impedance and capacitance are inversely related, total impedance decreases as the product of the frequency and the capacitance increases. Thus as frequency increases, impedance falls and more current is allowed to pass.
A capacitor consists of any two parallel conductors separated by an insulator ( Fig. 31-1 ). A capacitor has the ability to store charge. Capacitance is the measure of a substance’s ability to store charge. In a DC circuit the capacitor plates are charged by a voltage source (i.e., a battery), and there is only a momentary current flow. The circuit is not completed, and no further current can flow unless a resistance is connected between the two plates and the capacitor is discharged.
In contrast to DC circuits, a capacitor in an AC circuit permits current flow even when the circuit is not completed by a resistance. This is because of the nature of AC circuits, in which the current flow is constantly being reversed. Because current flow results from the movement of electrons, the capacitor plates are alternately charged—first positive and then negative, with every reversal of the AC current direction—resulting in an effective current flow as far as the remainder of the circuit is concerned, even though the circuit is not completed.
Because the effect of capacitance on impedance varies directly with the AC frequency in hertz, the greater the AC frequency, the lower the impedance. Therefore, high-frequency currents (0.5 to 2 million Hz), such as those used by electrosurgical units (ESUs), will cause a marked decrease in impedance.
Electrical devices use capacitors for various beneficial purposes. However, a phenomenon known as stray capacitance is capacitance that was not designed into the system but is incidental to the construction of the equipment. All AC-operated equipment produces stray capacitance. An ordinary power cord, for example, that consists of two insulated wires running next to each other will generate significant capacitance simply by being plugged into a 120 V circuit, even though the piece of equipment is not turned on. Another example of stray capacitance is found in electric motors. The circuit wiring in electric motors generates stray capacitance to the metal housing of the motor. The clinical importance of capacitance will be emphasized later in the chapter.
Whenever electrons flow in a wire, a magnetic field is induced around the wire. If the wire is coiled repeatedly around an iron core, as in a transformer, the magnetic field can be very strong. Inductance is a property of AC circuits in which an opposing EMF can be electromagnetically generated in the circuit. The net effect of inductance is to increase impedance. Because the effect of inductance on impedance also depends on AC frequency, increases in frequency will increase the total impedance. Therefore the total impedance of a coil will be much greater than its simple resistance.
Electrical Shock Hazards
Alternating and Direct Currents
Whenever an individual contacts an external source of electricity, an electric shock is possible. An electric current can stimulate skeletal muscle cells to contract and thus can be used therapeutically in devices such as pacemakers or defibrillators. However, casual contact with an electrical current, whether AC or DC, can lead to injury or death. Although it takes approximately three times as much DC as AC to cause ventricular fibrillation, this by no means renders DC harmless. Devices such as an automobile battery or a DC defibrillator can be sources of direct current shocks.
In the United States, utility companies supply energy in the form of alternating currents of 120 V at a frequency of 60 Hz. The 120 V of EMF and 1 A of current are the effective voltage and amperage in an AC circuit. This is also referred to as root-mean-square (RMS). It takes 1.414 A of peak amperage in the sinusoidal curve to give an effective amperage of 1 A. Similarly, it takes 170 V (120 × 1.414) at the peak of the AC curve to get an effective voltage of 120 V. The 60 Hz refers to the number of times in 1 second that the current reverses its direction of flow. Both the voltage and current waveforms form a sinusoidal pattern ( Fig. 31-2 ).
To have the completed circuit necessary for current flow, a closed loop must exist, and a voltage source must drive the current through the impedance. If current is to flow in the electrical circuit, there has to be a voltage differential, or a drop in the driving pressure across the impedance. According to Ohm’s law, if the resistance is held constant, the greater the current flow, the larger the voltage drop must be.
The power company attempts to maintain the line voltage constant at 120 V. Therefore by Ohm’s law, the current flow is inversely proportional to the impedance. A typical power cord consists of two conductors: one, designated as hot, carries the current to the impedance; the other is neutral , and it returns the current to the source. The potential difference between the two is effectively 120 V ( Fig. 31-3 ). The amount of current flowing through a given device is frequently referred to as the load. The load of the circuit depends on the impedance. A very high impedance circuit allows only a small current to flow and thus has a small load. A very low impedance circuit will draw a large current and is said to carry a large load. A short circuit occurs when there is a zero impedance load with a very high current flow.
Source of Shocks
Electrical accidents or shocks occur when a person becomes part of, or completes, an electrical circuit. To receive a shock, a person must contact the electrical circuit at two points, and there must be a voltage source that causes the current to flow through the individual ( Fig. 31-4 ).
When an individual comes in contact with a source of electricity, injury occurs in one of two ways. First, the electrical current can disrupt the normal electrical function of cells. Depending on its magnitude, the current can contract muscles, alter brain function, paralyze respiration, or disrupt normal heart function, leading to ventricular fibrillation. The second mechanism involves the dissipation of electrical energy throughout the body’s tissues. An electrical current passing through any resistant substance raises the temperature of that substance. If enough thermal energy is released, the temperature will rise sufficiently to produce a burn. Accidents involving household currents usually do not result in severe burns. However, in accidents involving very high voltages (i.e., power transmission lines), severe burns are common.
The severity of an electrical shock is determined by the amount of current (amperes) and the duration of the current flow. For the purposes of this discussion, electrical shocks are divided into two categories: macroshock refers to large amounts of current flowing through a person, which can cause harm or death; microshock refers to very small amounts of current. Concern for injury with microshock applies only to the electrically susceptible patient, an individual who has an external conduit that is in direct contact with the heart; this can be a pacing wire or a saline-filled catheter, such as a central venous or pulmonary artery catheter. In the case of the electrically susceptible patient, even minute amounts of current (microshocks) can cause ventricular fibrillation.
Table 31-1 shows the effects typically produced by various currents following a 1-second contact with a 60 Hz current. When an individual contacts a 120-V household current, the severity of the shock will depend on his or her skin resistance, the duration of the contact, and the current density. Skin resistance can vary from a few thousand to 1 million Ω. If a person with a skin resistance of 1000 Ω contacts a 120 V circuit, he or she would receive 120 mA of current, which would probably be lethal. However, if that same person’s skin resistance is 100,000 Ω, the current flow would be 1.2 mA, which would barely be perceptible.
I = E/R = 120 V/1000 Ω = 120 mA
I = E/R = 120 V/100,000 Ω = 1 .2 mA
|1 mA (0.001 A)||Threshold of perception|
|5 mA (0.005 A)||Accepted as maximum harmless current intensity|
|10 to 20 mA (0.01-0.02 A)||“Let go” current before sustained muscle contraction|
|50 mA (0.05 A)||Pain, possible fainting, mechanical injury; heart and respiratory functions continue|
|100 to 300 mA (0.1-0.3 A)||Ventricular fibrillation will start, but respiratory center remains intact|
|6,000 mA (6 A)||Sustained myocardial contraction followed by normal heart rhythm; temporary respiratory paralysis; burns if current density is high|
|100 μA (0.1 mA)||Ventricular fibrillation|
|10 μA (0.01 mA)||Recommended maximum 60 Hz leakage current|
The longer an individual is in contact with the electrical source, the more dire the consequences because more energy will be released, and more tissue will be damaged. Also, there will be a greater chance of ventricular fibrillation from excitation of the heart during the vulnerable period of the electrocardiogram (ECG) cycle.
Current density is a way of expressing the amount of current applied per unit area of tissue. The diffusion of current in the body tends to be in all directions. The greater the current or the smaller the area to which it is applied, the higher the current density. In relation to the heart, a current of 100 mA (100,000 μA) is generally required to produce ventricular fibrillation when applied to the surface of the body. However, only 100 μA (0.1 mA) is required to produce ventricular fibrillation when that minute current is applied directly to the myocardium through an instrument with a very small contact area, such as a pacing wire electrode. In this case, the current density is a thousandfold greater when applied directly to the heart; therefore only <SPAN role=presentation tabIndex=0 id=MathJax-Element-16-Frame class=MathJax style="POSITION: relative" data-mathml='11000′>1100011000
of the energy is required to cause ventricular fibrillation. An electrically susceptible patient can be electrocuted with currents well below 1 mA, which is the threshold of perception for humans.
The frequency at which the current reverses is also an important factor in determining the amount of current an individual can safely contact. Utility companies in the United States produce electricity at a frequency of 60 Hz, because higher frequencies cause greater power loss through transmission lines, and lower frequencies cause a detectable flicker from light sources. The “let go” current is defined as that current above which sustained muscular contraction occurs and at which an individual would be unable to let go of an energized wire. The let-go current for 60 Hz AC power is 10 to 20 mA, whereas at a frequency of 1 million Hz, up to 3 A (3000 mA) is generally considered safe. It should be noted that very high frequency currents do not excite contractile tissue; consequently, they do not cause cardiac dysrhythmias.
It can be seen that Ohm’s law governs the flow of electricity. For a completed circuit to exist, a closed loop must exist with a driving pressure to force a current through a resistance, just as in the cardiovascular system, in which BP must drive the cardiac output through the peripheral resistance. Figure 31-5 illustrates that a hot wire carrying a 120 V pressure through the resistance of a 60 W lightbulb produces a current flow of 0.5 A. The voltage in the neutral wire is approximately zero volts, whereas the current in the neutral wire remains at 0.5 A. This correlates with our cardiovascular analogy, in which a mean BP decrease of 80 mm Hg between the aortic root and the right atrium forces a cardiac output of 6 L/min through a systemic vascular resistance of 13.3 resistance units. However, the flow—in this case, the cardiac output, or in the case of the electrical model, the current—is still the same everywhere in the circuit. That is, the cardiac output on the arterial side is the same as the cardiac output on the venous side.
To fully understand electrical shock hazards and their prevention, the clinician must have a thorough knowledge of the concepts of grounding. These concepts of grounding probably constitute the most confusing aspects of electrical safety, because the same term is used to describe several different principles. In electrical terminology, the term grounding is applied to two separate concepts. The first is the grounding of electrical power, and the second is the grounding of electrical equipment. Thus the concepts that power can be grounded or ungrounded and that power can supply electrical devices that are themselves grounded or ungrounded are not mutually exclusive. It is vital to understand this point as the basis of electrical safety. Whereas electrical power is grounded in the home, it is usually ungrounded in the OR ( Table 31-2 ). In the home, electrical equipment may be grounded or ungrounded, but it should always be grounded in the OR.
|Home||Grounded||May or may not be grounded|
Electrical Power: Grounded
Electrical utilities universally provide power that is grounded (by convention, the earth-ground potential is zero, and all voltages represent a difference between potentials); that is, one of the wires supplying the power to a home is intentionally connected to the earth. The utility companies do this as a safety measure to prevent electrical charges from building up in their wiring during electrical storms. This also prevents the very high voltages used in transmitting power by the utility from entering the home in the event of an equipment failure in their high-voltage system.
The power enters the typical home via two wires attached to the main fuse box or circuit breaker box at the service entrance. The “hot” (electrified) wire supplies power to the hot distribution strip. The neutral wire is connected to the neutral distribution strip and to a service entrance ground (i.e., a pipe buried in the earth; Fig. 31-6 ). From the fuse box, three wires leave to supply the electrical outlets in the house. In the United States, the hot wire is color-coded black and carries a 120 V above-ground potential. The second wire is the neutral wire color-coded white; the third wire is the ground wire, which is either color-coded green or is uninsulated (bare wire). The ground and the neutral wires are attached at the same point in the circuit breaker box and are also connected to a cold-water pipe ( Figs. 31-7 and 31-8 ). Thus, this grounded power system is also referred to as a neutral grounded power system . The black wire is not connected to the ground, because this would create a short circuit. The black wire is attached to the hot distribution strip (i.e., 120 V above ground) on which the circuit breakers or fuses are located. From here, numerous branch circuits supply electrical power to the outlets in the house. Each branch circuit is protected by a circuit breaker or fuse that limits current to a specific maximum amperage. Most electrical circuits in the house are 15 or 20 A circuits. These typically supply power to the electrical outlets and lights in the house. Several higher amperage circuits are also provided for certain devices, such as electric stoves or clothes dryers. These devices are powered by 240 V circuits, which can draw from 30 to 50 A of current. The circuit breaker or fuse will interrupt the flow of current on the hot side of the line in the event of a short circuit, or if the demand placed on that circuit is too high. For example, a 15 A branch circuit is capable of supporting 1800 W of power.
P = E × I