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Electrical Safety in the Operating Room: Dry Versus Wet

Barker, Steven J. PhD, MD*; Doyle, D. John MD, PhD, FRCPC

doi: 10.1213/ANE.0b013e3181ac1d69
Editorials: Editorials
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From the *Department of Anesthesiology, University of Arizona College of Medicine, Tucson, Arizona; and Cleveland Clinic Lerner College of Medicine of Case Western Reserve University, Cleveland, Ohio.

Address correspondence and reprint requests to Steven J. Barker, PhD, MD, Department of Anesthesiology, University of Arizona College of Medicine, P.O. Box 245114, Tucson, AZ 85724-5114. Address e-mail to sjbarker@u.arizona.edu.

Accepted April 17, 2009

A case report in this issue of Anesthesia & Analgesia by Wills et al.1 demonstrates that even today, the operating room (OR) is not always a safe working environment. Despite years of education on electrical safety, combined with policies and precautions dictated by national watchdog agencies such as the National Fire Prevention Agency, a nurse sustained a serious, life-threatening electrical shock while performing her duties in the OR. How could this happen, given our advanced understanding of both the physics and hazards of electricity? To understand and learn from this important case report, we begin with a brief review of some relevant electrical safety principles.

Electricity can stimulate muscle cells to contract, and can thus be used therapeutically in devices such as pacemakers or defibrillators. Electrical current can also trigger grand mal seizures, and this effect is used beneficially in electroconvulsive therapy. However, contact with a large electrical current, whether alternating current (AC) or direct current, can lead to injury or death.

Utility companies supply electrical energy in the form of AC of 120 or 230 V at a frequency of 60 Hz (50 Hz in Europe). The 60 Hz refers to the number of times that the current cycles per second. A typical power cord consists of two conductors. One designated as “hot” carries the current to the load; the other is neutral or “ground” and it returns the current to the source. The root mean square potential difference between the two is 120 V.

To receive a shock, one must contact an electrical circuit at two points, and there must be a potential difference that causes current to flow between those points. When an individual contacts a source of electricity, damage usually 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, paralyze respiration, cause seizures, or lead to ventricular fibrillation. The second mechanism involves the dissipation of electrical energy throughout the tissues. An electrical current passing through any resistance increases the temperature of that substance. The power dissipated as heat equals the square of the current multiplied by the resistance (P = I2 × R). If enough thermal energy is released, the temperature will increase sufficiently to produce a burn or cause cellular death.

The severity of an electrical shock is determined by the amount of current (amperes, A) and the duration of the current flow. In medical terms, electrical shocks are usually divided into two categories. Macroshock refers to larger currents (typically more than 10 mA) flowing through a person, which can cause harm or death. Microshock refers to very small currents (as little as 10–50 μA) and applies only to the electrically susceptible patient, such as an individual who has an internal conduit that is in direct contact with the heart. This conduit can be a pacing wire or a saline-filled central venous or pulmonary artery catheter. In the electrically susceptible patient, even minute amounts of current (10 μA) may cause ventricular fibrillation.

To understand electrical shock hazards and their prevention, one must have knowledge of grounding. This concept is probably the most confusing aspect of electrical safety, because the same term is used to describe several different principles. By definition, a “ground” is an infinite source or sink for electrons. That is, a ground can absorb or provide infinite amounts of current in response to an applied electrical potential. In practical terminology, grounding refers to two separate principles. The first is the grounding of an electrical power supply and the second is the grounding of electrical equipment. Although electrical power is normally grounded in the home, it is usually ungrounded in the OR. In the home, electrical equipment may be grounded or ungrounded, but it should always be grounded in the OR.

What does all this mean? In an ungrounded or “floating” power supply, a specified potential difference or voltage (e.g., 120 V) is provided between the two power lines or “sides” of the supply. Neither side is connected to ground, and there is thus no potential difference between either side and ground. Conversely, in a grounded power supply, one of the two power lines is physically connected to ground and is thus maintained at zero potential. The “hot” side of the supply will then have the full specified voltage (120 V) between it and the other side, and also between it and any ground source. If one side of a floating power supply accidentally becomes grounded through an equipment fault, no current will flow, but the other side of the supply now becomes hot with respect to ground. In this situation, the unfortunate individual who comes into contact with the hot side of the supply while also being in contact with some sort of ground (standing in a puddle of water usually suffices) will receive a serious macroshock.

The line isolation monitor (LIM) is a device that continuously monitors the integrity of an isolated or floating power system. As noted above, in such a power supply there should be zero potential difference between either side of the supply and ground. The LIM effectively measures this potential difference (technically, it measures the leakage current through a fixed resistance), and gives an alarm when this is significantly greater than zero. If a faulty piece of equipment is connected to the isolated power system, this can ground one side of the supply and change the system back to a grounded system. The faulty piece of equipment will usually continue to function normally. Therefore, it is essential that a warning system be in place to alert the personnel that the power is no longer ungrounded.

Because all AC wiring and electrical devices have some internal capacitance (i.e., have the ability to hold an electrical charge), small leakage currents are normally present, and these will partially degrade the isolation of the system. (For those unfamiliar with capacitance, think of it as a resistance to ground that is infinite for direct current, but gradually decreases with increasing frequency of AC.) The meter of the LIM will indicate (in mA) the total amount of leakage current resulting from stray capacitance, electrical wiring, and any devices plugged into the isolated power system. The reading on the LIM meter does not mean that current is actually flowing; rather, it indicates how much current would flow in the event of a first fault. The LIM is generally set to alarm at 2 or 5 mA, depending on the age and brand of the system. Once this preset limit is exceeded, visual and audible alarms are triggered to indicate that the isolation from ground has been degraded beyond the predetermined limit. This does not necessarily mean that there is an immediate hazard, but rather that the system is no longer totally isolated from ground. It would require a second fault to create a dangerous situation. A ground fault circuit interrupter (GFCI) is a simpler device, and can be thought of as a “poor man's LIM.” Rather than actually measuring the leakage current, the GFCI will trip and shut off all power in that circuit when the leakage current to ground exceeds a critical value.

Thus, armed with knowledge, what can we conclude about the case reported here by Wills et al.? The exact mechanism of the macroshock to this OR nurse may never be known, but we agree with the authors' conclusion that she was “grounded” by kneeling in puddled water on the OR floor. Any faulty piece of equipment or power cord that compromised the isolation of the power supply would provide the remainder of the equation for shock in this operating suite with neither LIMs nor GFCIs. The crucial point here is that inadvertent grounding of both equipment and personnel will occur in an OR. One reason for this is the almost universal presence of liquids (water, blood, gastrointestinal contents, urine, etc.) on the floor. It seems to us that anyone (or any agency) who believes that the OR is a “dry location” has never spent any time in one. On this point, we fully agree with the authors. Yes, there may be ORs that can usually be kept dry. But “usually” is not good enough for this situation, wherein the hazard is great and the precautions for avoiding said hazard are rather simple and inexpensive. LIMs and/or GFCIs seem like inexpensive insurance to us, and yet many (most?) ORs built in the past 25 yr do not have them.

The American Society of Anesthesiologists House of Delegates is about to consider a recommendation from the Committee on Equipment and Facilities regarding this issue (Report 511-1.3). This report recommends to the National Fire Prevention Agency that ORs be considered “wet locations,” but it allows for “opt-out” exceptions if hospitals can offer evidence that their own ORs are dry. We support the primary recommendation to designate ORs as wet, but we are concerned about the opt-out provision, because it might allow hospitals to continue business as usual by claiming that their own ORs are dry.

We would like to see this discussion continue, and the Wills et al. case report provides a good starting point. Our bias at the outset of the discussion is that ORs are never dry locations. Disagree if you wish, but offer evidence to the contrary. And tell it to this unfortunate OR nurse.

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REFERENCE

1. Wills JH, Ehrenwerth J, Rogers D. Electrical injury to a nurse due to conductive fluid in an operating room designated as a dry location. Anesth Analg 2010;110:1647–9
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