Handbook of Clinical Anesthesia
Electrical Fire and Safety
The myriad electronic devices in the modern operating room greatly improve patient care and safety but also subject patients and operating room personnel to increased risks (Ehrenwerth J, Seifert HA: Electrical and fire safety. In Clinical Anesthesia. Edited by Barash PG, Cullen BF, Stoelting RK, Cahalan MK, Stock MC. Philadelphia: Lippincott Williams and Wilkins, 2009, pp 165–191).
- Principles of Electricity
A basic principle of electricity is known as Ohm's law and is represented by the equation E = I ÷ R (electromotive force in volts = current in amperes times resistance in ohms). Ohm's law forms the basis for the physiologic equation in which the blood pressure is equal to the cardiac output times the systemic vascular resistance (BP = CO ÷ SVR). Electrical power is measured as watts (voltage ÷ amperage). The amount of electrical work done (watt-second or joule) is a common designation for electrical energy expended. (Energy produced by a defibrillator is measured in joules.)
- Direct and Alternating Currents.The flow of electrons (current) through a conductor is characterized as direct current (electron flow is always in the same direction) or alternating current (electron flow reverses direction at a regular interval).
- Impedanceis the sum of forces that oppose electron movement in an alternating current circuit.
- Capacitanceis the ability of a capacitor (two parallel conductors separated by an insulator) to store charge.
- In a direct current circuit, the charged capacitor plates (battery) do not result in current flow unless a resistance is connected between the two plates and the capacitor is discharged.
- Stray capacitance is capacitance that is not designed into the system but is incidental to the construction of the equipment. (All alternating current operating equipment produces stray capacitance even while turned off.)
- Electrical Shock Hazards
- Alternating and Direct Currents
- Whenever an individual contacts an external source of electricity, an electrical shock is possible. (It requires approximately three times as much direct current as alternating current to cause ventricular fibrillation [VF].)
- A short circuit occurs when there is zero impedance with a high current flow.
- Source of Shocks
- Electrical accidents or shocks occur when a person becomes part of or completes an electrical circuit (Fig. 8-1).
- Damage from electrical current is caused by disruption of normal electrical function of cells (skeletal muscle
contracture, VF) or dissipation of electrical energy (burn).
Figure 8-1. When a faulty piece of equipment without an equipment ground wire is plugged into an electrical outlet not containing a ground wire, the instrument case becomes energized (“hot”). If an individual touches the case (A), he or she will receive a shock (dashed line depicts path of electrical current) because he or she is standing on the ground (B) and completes the circuit.
- The severity of an electrical shock is determined by the amount of current and the duration of current flow.
- Macroshock describes large amounts of current flow that can cause harm or death.
- 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.
- Very high-frequency current does not excite contractile tissue and does not cause cardiac dysrhythmias.
- 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
- Electrical utilities universally provide power to homes that are grounded. (By convention, the earth ground potential is zero.)
- 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.)
- 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).
- Electrical Power: Ungrounded
- The numerous electronic devices, along with power cords and puddles of saline-filled solutions on the floor, tend to make the operating room an electrically hazardous environment for both patients and personnel.
- In an attempt to decrease the risk of electrical shock, the power supplied to most operating rooms is ungrounded (i.e., current is isolated from the ground).
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.
- Supplying ungrounded power to the operating room requires the use of an isolation transformer (Fig. 8-3).
- The isolated power system provides protection from macroshock (Fig. 8-4).
- A faulty piece of equipment plugged into an isolated power system does not present a shock hazard.
Figure 8-3. In the 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.
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.
- The Line Isolation Monitor
- The line isolation monitor is a device that continuously monitors the integrity of the isolated power system (i.e., it measures the impedance to ground on each side of the isolated power system).
- 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.
- 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.
- Visual and audible alarms are triggered if the isolation from the ground has been degraded beyond a predetermined limit (Fig. 8-5).
- If the line isolation monitor alarm is triggered, the first step is to determine if it is a true fault.
- 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.)
- If the gauge reads above 5 mA, it is likely that a faulty piece of equipment is present in the operating room. This equipment may be identified by unplugging each piece of equipment until the alarm is silenced.
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 ground. This is detected by the line isolation monitor, 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.
- If the faulty piece of equipment is not essential, it should be removed from the operating room 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.
- The line isolation monitor is not designed to provide protection from microshock.
- Ground Fault Circuit Interrupter
- The 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.)
- The disadvantage of using a ground fault circuit interrupter in the operating room 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 operating room with an isolated power system.
- 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).
- 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.
- 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.
- 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.
- The anesthesiologist should never simultaneously touch an electrical device and a saline-filled CVP catheter or external pacing wires. Rubber gloves should be worn.
- 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).
- 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.
- 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.
- 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.
- 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.
- 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.
- Multiple-plug extension boxes should not be left on the floor where they can come in contact with electrolyte solutions.
- 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.
- High-frequency currents have a low tissue penetration and do not excite contractile cells. (Direct contact with the heart does not cause VF.)
- High-frequency electrical energy generated by the ESU interferes with signals from physiologic monitors.
- The ESU cannot be safely operated unless the energy is properly routed from the unit through the patient and back to the unit via a large surface area dispersive electrode (often erroneously referred to as the “ground plate”) (Fig. 8-6).
- Because the area of the return plate is large, the current density is low, and no harmful heat or tissue destruction occurs.
- If the return plate is improperly applied to the patient or if the cord connecting the return plate to the ESU is broken, the high-frequency electrical current will seek an alternate return path (electroencephalographic leads, temperature probe), possibly resulting in a serious burn
to the patient at this return site because high-current density generates heat (Fig. 8-7).
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.
- In most modern ESUs, the power supply is isolated from the ground to protect the patient from burns by eliminating alternate return pathways. The isolated ESU does not protect the patient from burns if the return electrode does not make proper contact with the patient.
Figure 8-7. An improperly applied electrosurgical unit (ESU) return plate. Poor contact with the return plate results in a high current density (heat) and a possible burn to the patient.
Table 8-1 Proper Application of the Electrosurgical Unit Dispersive Return Plate
- The most important factor in preventing patient burns from the ESU is proper application of the return plate (Table 8-1).
- The need for higher than normal settings should initiate an inspection of the return plate and cable.
- In a bipolar ESU, the current passes only between the two blades of a pair of forceps.
- Because the active and return electrodes are the two blades of the forceps, it is not necessary to attach another dispersive electrode to the patient.
- The bipolar ESU generates less power than the unipolar ESU and is mainly used for ophthalmic and neurologic surgery.
- The use of a unipolar ESU (also electroconvulsive therapy) may cause electrical interference, which may be interpreted by an automatic implantable cardioverter-defibrillator as a ventricular tachydysrhythmia, resulting in delivery of a defibrillation pulse to the patient.
- In the presence of an oxygen-enriched environment, a spark from the ESU can serve as the ignition source for fuels (e.g., plastics such as anesthesia face masks, tracheal tubes), causing fires with associated injuries to patients and operating room personnel.
- The risk of surgical fires should be considered whenever the ESU is used in close proximity to oxygen-enriched environments.
- Tenting of the drapes to allow dispersion of any accumulated oxygen and its dilution by room air or use of a circle anesthesia breathing system with minimal to no leak of gases around the anesthesia mask will decrease the risk of ignition from a spark generated by an ESU.
- Conductive flooringis not necessary in anesthetizing areas where flammable anesthetics are not used.
- Environmental Hazards
- Potential hazards in the operating room include electrical shock to the patient and operating room personnel and the presence of cables and power cords to electrical equipment and monitoring devices (ceiling mounts).
- Modern monitoring devices include an isolated patient input from the power supply of the device.
- All health care facilities are required to have a source of emergency power (electrical generators; battery-operated light sources, including laryngoscopes).
- Electromagnetic Interference
- Wireless communication devices (cellular telephones, cordless telephones, walkie-talkies) emit electromagnetic interference (EMI).
- There is concern that EMI may interfere with implanted pacemakers or various types of monitoring devices in critical care areas.
- Cellular telephones should be kept at least 15 cm from a pacemaker.
- Cellular telephones do not seem to interfere with automatic implantable cardioverter-defibrillators.
XII. Construction of New Operating Rooms
- The National Fire Protection Association standards for health care facilities no longer require isolated power systems or line isolation monitors in areas designated for use of only nonflammable anesthetics.
- The decision to install isolated power is determined by whether the operating room is a wet location (presence of blood, fluid, saline solutions) and, if so, whether an interruptible power supply is acceptable,
- When power interruption is acceptable, a ground fault circuit interrupter is permitted as a protective means.
- When power interruption would be unacceptable, an isolated power system and a line isolation monitor are preferred.
XIII. Fire Safety
- Fires in the operating room are just as much a danger today as they were 100 years ago when patients were anesthetized with flammable anesthetic agents (Table 8-2). However, in contrast to situations during the era of flammable anesthetics, today there appears to be a lack of awareness of the potential for operating room fires (Table 8-3).
- Dangers of fires are burns to patients and operating room personnel and release of toxic compounds (e.g., carbon monoxide, ammonia, hydrogen chloride, cyanide) when plastics burn.
- Laser-resistant endotracheal tubes may act as a blowtorch type of flame, resulting in severe injury to the trachea and lungs.
- Leakage of gases around an uncuffed tube in the presence of the ESU can ignite flammable endotracheal tubes. The risk can be minimized by keeping the inspired oxygen concentration as low as possible.
- In critically ill patients requiring high concentrations of oxygen during a surgical tracheostomy, it may be prudent not to use the ESU.
Table 8-2 The Fire Triad: Elements Necessary for a Fire to Start
Table 8-3 Recommendations for the Prevention and Management of Operating Room Fires
- Diffusion of nitrous oxide into the abdomen may create a fire hazard during laparoscopic surgery; carbon dioxide will not support combustion.
- Fires on the patient are most likely during surgery in and around the head and neck where the patient is receiving monitored anesthesia care and supplemental oxygen (facemask or nasal cannulae).
- Oxygen should be treated as a drug and administered to provide optimum benefits (e.g., titrated to the desired level).
- Tenting the drapes and using adhesive sticky drape that seals the operative site from the oxygen flow reduce the risk of a fire.
- Pooling of prep solutions may result in alcohol vapors that are flammable.
- Desiccated carbon dioxide absorbents in the presence of volatile anesthetics (especially sevoflurane) may result in an exothermic reaction. (Fires may occur involving the breathing circuit.)
- If a fire occurs, the first step is to interrupt the fire triad by removing one of the components. This is best accomplished by removing the oxidizer (Table 8-3).
- Disconnecting a burning endotracheal tube from the anesthetic circuit usually results in extinguishing the fire. (It is not recommended to remove a burning endotracheal tube because doing so may cause even greater harm to the patient.)
- When the endotracheal tube fire has been extinguished, it is safe to remove the tracheal tube
and inspect the patient's airway by bronchoscopy followed by reintubation.
- If the fire is on the patient, extinguishing it with a basin of saline is a rapid and effective intervention.
- Paper drapes are impervious to water, so throwing water or saline on them is likely to be ineffective.
- After removing the burning drapes from the patient, the flame should be extinguished with a fire extinguisher.
- Prep solutions with alcohol should be dry before surgery begins.
Editors: Barash, Paul G.; Cullen, Bruce F.; Stoelting, Robert K.; Cahalan, Michael K.; Stock, M. Christine
Title: Handbook of Clinical Anesthesia, 6th Edition
Copyright ©2009 Lippincott Williams & Wilkins
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