B. Source of Shocks

1. Electrical accidents or shocks occur when a person becomes part of or completes an electrical circuit (Fig. 8-1).

2. Damage from electrical current is caused by disruption of normal electrical function of cells (skeletal muscle contracture, VF) or dissipation of electrical energy (burn).

3. The severity of an electrical shock is determined by the amount of current and the duration of current flow.

a. Macroshock describes large amounts of current flow that can cause harm or death.

b. Microshock describes small amounts of current flow and applies only to electrically susceptible patients (those with an external conduit that is in direct contact with the heart, such as a pacing wire or saline-filled central venous pressure [CVP] catheter) in whom even minute amounts of current (1 mA, which is the threshold of perception) may cause VF.

4. Very high-frequency current does not excite contractile tissue and does not cause cardiac dysrhythmias.

C. Grounding. To fully understand electrical shock hazards and their prevention, one must have a thorough knowledge of the concepts of grounding. In electrical terminology, grounding is applied to electrical power and equipment.

III. ELECTRICAL POWER: GROUNDED

A. Electrical utilities universally provide power to homes that are grounded. (By convention, the earth ground potential is zero.)

B. Electrical shock is an inherent danger of grounded power systems. An individual standing on ground or in contact with an object that is referenced to the ground needs only one additional contact point to complete the circuit.

C. Modern wiring systems have added a third wire (a low-resistance pathway through which the current can flow to ground) to decrease the severity of potential electrical shocks (Fig. 8-2).

IV. ELECTRICAL POWER: UNGROUNDED

A. The numerous electronic devices, along with power cords and puddles of saline-filled solutions on the floor, tend to make the OR an electrically hazardous environment for both patients and personnel.

FIGURE 8-2. When a faulty piece of equipment containing an equipment ground wire is properly connected to an electrical outlet with grounding protection, the electrical current (dashed line) will preferentially flow down the low-resistance ground wire. An individual touching the instrument case (A) and standing on the ground (B) still completes the circuit; however, only a small part of the current flows through the individual.

FIGURE 8-3. In an operating room, the isolation transformer converts the grounded power on the primary side to an ungrounded power system on the secondary side of the transformer. There is no direct connection from the power on the secondary side to ground. The equipment ground wire, however, is still present.

B. In an attempt to decrease the risk of electrical shock, the power supplied to most ORs is ungrounded (current is isolated from the ground).

C. Supplying ungrounded power to an OR requires the use of an isolation transformer (Fig. 8-3).

1. The isolated power system provides protection from macroshock (Fig. 8-4).

FIGURE 8-4. A safety feature of the isolated power system is illustrated. An individual contacting one side of the isolated power system (A) and standing on the ground (B) will not receive a shock. In this instance, the individual is not contacting the circuit at two points and thus is not completing the circuit.

2. A faulty piece of equipment plugged into an isolated power system does not present a shock hazard.

V. THE LINE ISOLATION MONITOR

A. The line isolation monitor is a device that continuously monitors the integrity of an isolated power system (i.e., it measures the impedance to ground on each side of the isolated power system).

B. If a faulty piece of equipment is connected to the isolated power system, it will, in effect, change the system to a conventional grounded system, yet the faulty piece of equipment will continue to function normally.

1. The meter of the line isolation monitor indicates the amount of leakage in the system resulting from any device plugged into the isolated power system.

2. Visual and audible alarms are triggered if the isolation from the ground has been degraded beyond a predetermined limit (Fig. 8-5).

C. If the line isolation monitor alarm is triggered, the first step is to determine if it is a true fault.

1. If the gauge reads between 2 and 5 mA, there probably is too much electrical equipment plugged into the circuit. All alternating current-operated devices have some capacitance and associated leakage current.

FIGURE 8-5. When a faulty piece of equipment is plugged into the isolated power system, it decreases the impedance from line 1 or line 2 to the ground. This is detected by the line isolation monitor (LIM), which sounds an alarm. The faulty piece of equipment does not present a shock hazard but converts the isolated power system into a grounded power system.

2. If the gauge reads above 5 mA, it is likely that a faulty piece of equipment is present in the OR. This equipment may be identified by unplugging each piece of equipment until the alarm is silenced.

3. If the faulty piece of equipment is not essential, it should be removed from the OR for repair. If it is a vital piece of life support equipment, it can be safely used, but no other piece of electrical equipment should be connected during the remainder of the case or until the faulty piece of equipment can be removed.

4. The line isolation monitor is not designed to provide protection from microshock.

VI. GROUND FAULT CIRCUIT INTERRUPTER

A. A ground fault circuit interrupter (circuit breaker) is used to prevent individuals from receiving an electrical shock in a grounded power system. It monitors both sides of the circuit for equality of current flow, and if a difference is detected, the power is immediately interrupted.

B. The disadvantage of using a ground fault circuit interrupter in the OR is that it interrupts the power without warning. A defective piece of equipment can no longer be used, which might be a problem if it were necessary for life support.

VII. DOUBLE ISOLATION. This applies to equipment that has a two-prong plug (infusion pumps) and is permissible to use in the OR with an isolated power system.

VIII. MICROSHOCK

A. In an electrically susceptible patient (one who has a direct external connection to the heart such as a CVP catheter or transvenous pacing wires), VF can be produced by a current that is below the threshold of human perception (1 mA).

B. The stray capacitance that is part of any alternating current–powered electrical instrument may result in significant amounts of charge build-up on the case of the instrument.

1. An individual who simultaneously touches the case of this instrument and an electrically susceptible patient may unknowingly cause a discharge to the patient that results in VF.

2. An intact equipment ground wire provides a low-resistance pathway for leakage current and constitutes the major source of protection against microshock in electrically susceptible patients.

3. The anesthesiologist should never simultaneously touch an electrical device and a saline-filled CVP catheter or external pacing wires. Rubber gloves should be worn.

4. Modern patient monitors are designed to electrically isolate all direct patient connections from the power supply of the monitor by placing a very high impedance between the patient and the device (this limits the amount of internal leakage through the patient connection to <0.01 mA).

C. The objective of electrical safety is to make it difficult for electrical current to pass through people. Patients and anesthesiologists should be isolated from the ground as much as possible.

1. The isolation transformer is used to convert grounded power to ungrounded power. The line isolation monitor warns that isolation of the power from the ground has been lost in the event that a defective piece of equipment has been plugged into one of the isolated circuit outlets.

2. All equipment that is plugged into the isolated power system has an equipment ground wire that provides an alternative low-resistance pathway enabling potentially dangerous currents (macroshock) to flow to the ground. The ground wire also dissipates leakage currents and protects against microshock in electrically susceptible patients.

3. All electrical equipment must undergo routine maintenance, service, and inspection to ensure that it conforms to designated electrical safety standards. Records of the routine maintenance service must be kept.

4. Electrical power cords should be located overhead or placed in areas of low traffic because they are subject to being crushed if they are left on the floor.

5. Multiple-plug extension boxes should not be left on the floor where they can come in contact with electrolyte solutions.

IX. ELECTROSURGERY

A. The electrosurgical unit (ESU), invented by Professor William T. Bovie, operates by generating high-frequency currents (radiofrequency range). Heat is generated whenever a current passes through a resistance. By concentrating the energy at the tip of the “Bovie pencil,” the surgeon can accomplish either therapeutic cutting or coagulation.

FIGURE 8-6. A properly applied electrosurgical unit (ESU) return plate. The current density at the return plate is low, resulting in no danger to the patient.