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Electrical Safety Cedric Dupont-Eisner M.D.
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Electrical Safety Steven L. Shafer, M.D. Professor of Anesthesia Stanford University Adjunct Professor of Biopharmaceutical Sciences University of California at San Francisco
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Introductory Physics for Anesthesiologists
T. Turkstra, M. Eng, P. Eng, MD, FRCPC April
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Electrical Safety Electrical safety in the OR is often regarded as being of historical interest only Reality is that the OR environment is becoming more electrically complex by the year More complications arise with the networking of electronic equipment which may not conform to the rigid safety standards of conventional medical equipment device related injuries in USA every year Electrocution 5th leading cause of accidental death in US
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Why are patients at risk?
Placed on a metal table. Wet Surrounded by electrical devices. Connected to multiple electrical devices, typically by low resistance pads Can’t move away from current
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Principles
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Basics of Power Distribution
Service Entrance Hot 120 volts Neutral Examine power distribution in room
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Definitions Materials that will not transfer electrons under normal circumstances are termed insulators An excess of charge may be carried by some materials as a result of friction (static electricity) This may later be discharged by contact with a conductor, or if the potential is sufficiently high by jumping a gap as a spark.
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Principles Electricity is the flow of electrons Capacitance
Direct Current-electrons flow in 1 direction Alternating Current-electrons switch directions at regular intervals Capacitance The ability of a capacitor to store a charge. CAPAcitor - (two parallel conductors separated by an insulator-like a battery)
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Definitions When electrons move from one atom to another in a consistent direction, current is said to flow The applied force to do this is described as potential difference, and energy is used up by the process (volts) This energy can both fulfill its function or injure our patients if care is not taken Materials that permit easy transfer of their electrons from one atom to another are termed conductors Those that do so reluctantly are termed resistors
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Ohm’s Law V=I R Voltage (V) =Current (I) Resistance (R)
Variac example
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What is 120 Volts? 120 volts is the root mean square voltage
Actual peak voltage is about 150 volts
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Capacitative Linkage If a material carries a negative charge, other nearby electrons will tend to move away If the potential at that point varies from positive to negative, such as happens with all alternating current sources (most obviously with mains electricity) then the surrounding electrons will be attracted and repelled alternately In other words, an alternating current can be induced in a material without an electrical source being directly connected to it. This is termed capacitative linkage.
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Inductive Linkage Moving electrons generate a magnetic field
A moving magnetic field causes movement of electrons AC current source will produces a moving magnetic field and therefore induces secondary current in any nearby wires without the need for direct contact Inductive linkage is intentionally utilized in some devices e.g. transformers
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Inductive Linkage When two transformers are placed in series in a power supply it allows the power source for a medical device to be separated from any other parts of the circuitry Consequent reduction in the risk of direct transmission of mains energy to the patient This is known as a floating circuit—indicated by the surrounding box in the symbols and the letter F in the description of equipment
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Safety Standards As monitoring devices proliferated in OR’s, awareness of leakage currents grew Because of capacitative and inductive linkage within medical devices, there will virtually always be some tiny current floating down wires to patients Moderate currents are not a big issue, and the maximum permitted level is below that which can be sensed, or cause harm
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Pickle Physics Pickle is nearly saturated with Na Cl as part of pickling process Na Cl provides conducting path Na+ is reduced by electric current to Na0 When the Na0 is oxidized to Na+ by oxygen, a reaction giving off light at the frequency of Na. Same color as Na lights on highways
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Principles Electrical Shock
Body contact with 2 conductive materials may complete OR become part of a loop and cause a shock
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Electrical Shock If electrical systems are not properly wired, persons can be subjected to electric shock. Resulting in injury or death
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Electrical Shock People become injured and death occurs when voltage pushes electrons through the human body, particularly through the heart.
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Electrical Shock Damage from electrical current is due to disruption of normal electrical function of cells or dissipation of electrical energy into human bodies (increased temp->burn). Electrical function of cells – skeletal muscle contraction or v fib
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Electrical Shock Macroshock-large amount of current flow that can cause harm or death
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The Effects of Current On The Human Body - (Source -Hand)
Around 1mA Mild Tingling 1-5 mA Painful >15 mA Tonic Muscle Contraction - unable to release grip, risk of asphyxia 75-100mA Risk of VF >5000mA Tonic Ventricular Contraction, cardiac standstill and death
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Application: Electrical Shock
What levels of current (mA) is dangerous? <1 mA = imperceptable to touch 10 mA skeletal muscle shock (you can let go) V Fib can be induced by: 100 mA of macroshock 100 μA (microamperes)of microshock current
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Electrical Shock Microshock-small amounts of current flow. Dangerous only to electrically susceptible patients via artificial conductive pathways to the heart. (patients with external devices capable of conducting a charge-pacing wires, saline filled cvp catheters)
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Application: Electrical Shock
DC-is less dangerous AC-is more dangerous 3x DC is needed for VFib High frequency current have low tissue penetration and does not excite contractile cells Low frequency current penetrates more The larger the area exposed to current the better (think about huge Bovie pad and small bovie tip)
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Microshock Of the current passing through a human hand, less than 0.1% passes through the heart Therefore any cardiac effects result from tiny currents The implication is that if you passed a current directly through the heart, much smaller currents can cause injury 5 seconds of sustained 50 A AC current produces sustained VF This phenomenon is known as microshock.
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Microshock Anaesthetist can be earthing point for patient, and source for microshock - IF you touch a faulty apparatus and SG catheter at the same time, small leakage current from poorly grounded device can be sufficient to cause VF, even though you don’t feel a thing…
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Principles Stray Capacitance/Capacitive Coupling, the cause of “leakage” inherent in all electrical equipment. Not designed into the system, but incidental to the equipment construction
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Application: Electrical Shock
THE MAX LEAKAGE ALLOWED IN OR EQUIPMENT IS 10 μA (microamperes)
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Grounding Electrical Power Grounding can exist in two forms GROUNDED
UNGROUNDED Easy?
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Grounding Think about your house:
2 prong outlets = no ground 3 prong outlets = grounded Modern homes have a ground to reduce amount of shock
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Protecting the Operator
Adequate grounding of casing Don’t let operator touch casing Non-conductive casing Don’t assume all equipment is always in good shape - regular checks Extension cables are frowned upon - frayed from over use, on floor, exposed to saline etc…….
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Basic Toaster ______________________________ / \
______________________________ / \ Hot _______________________/_____________/\/\/\/\/\/\/\ \ | | | Neutral ______________________|______________/\/\/\/\/\/\/\| | | (filament) | Ground ______________________| | | T o a s t e r | |__________________________________| (Ground connects to chassis)
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Worst Case Scenario: Hot shorts to chassis, ground broken
(short circuit) ! ! ! ___!_______________ / | \ _____ _____Hot______/____|___/\/\/\/\ \ // \\ | | | Patient | * * | __Neutral____|_________/\/\/\/\| | wants >| ^ |< | | toast: \ O / / | | | \_____/ / | T o a s t e r | |_____|_____/ ___________|_______________________| | / | Patient Broken Ground ______________ /.\ is \ | O.R. table |-----towel-- / \ toast / \__________/ / \ / | / \ / | Ground wire _/ \_ \ ground | 10 amps 10 amps Notice role of ground. How is the circuit completed? Grounded vs. ungrounded demo
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Safe Scenario: Hot shorts to chassis, ground intact
(short circuit) ! ! ! ___!_______________ / | \ _____ _____Hot______/____|___/\/\/\/\ \ // \\ | | | Patient | * * | __Neutral____|_________/\/\/\/\| | wants >| ^ |< | | toast: \ / / | | | \_____/ / | T o a s t e r | |_____|_____/ ___________|_______________________| | / | Fuse | ______________ /.\ blows \ | O.R. table |-----towel-- / \ before / \__________/ / \ patient / | / \ / | Ground wire _/ \_ \ ground | 30 amps 30 amps Notice role of ground. How is the circuit completed?
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Microshock hazard: Neutral shorts, ground wire intact
___________________ / \ _____ _____Hot______/________/\/\/\/\ \ // \\ | | | Patient | * * | __Neutral____|_________/\/\/\/\| | wants >| ^ |< | | | toast: \ / / | | | | \_____/ / | | T o a s t e r | |_____|_____/ ___________|__|____________________| | / Short!! | Patient | /.\ is \ | O.R. table |-----towel-- / \ OK | /ft \__________/ / \ | | / \ / | Ground wire _/ \_ \ ground | 10 amps 5 amps 5 amps V=I R = 5 A = 320 mV, even though the chassis is properly grounded!
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Microshock Current delivered directly to heart
10 A will cause fibrillation The chassis voltage from a neutral-ground short carrying 5 amps, 320 mV, exceeds the microshock threshold. Since the line isolation monitor will sense this, the monitor is able to provide some protection against microshock.
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Microshock Voltage to induce VF through intracardiac pacing electrode: 5 mV Chassis voltage with neutral short to ground in conventional circuit: 320 mV Chassis voltage with neutral short to ground with functioning line isolation monitor: mV
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Grounded Power
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Grounded Electrocution if I have a CVP
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Why is equipment grounded?
Stray Capacitance/Capacitive Coupling: Defective appliance current diversion to ground wire. Remember all equipment leaks a small amount of current All OR equipment has 3 prong plug Equipment ground wire is attached to the case of the instrument to allow the small amount of current to have a low resistance path to ground and reduce risk of macroshock (think carpet static electricity)
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Ungrounding The OR has many perils that make grounding less ideal.
Saline puddles Power cords w/ tears in their insulation (colored part of cord) Numerous electronic devices that risk
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Ungrounding This is where the questions are derived:
OR uses ungrounded power that is derived from Grounded utilities ISOLATION TRANSFORMER is the answer…
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Isolated and Ungrounded
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Ungrounding: Isolated Power
Power source does not have a ground, the equipment is grounded Isolated Power System provides protection from Macroshock. Faulty equipment plugged into an isolated power system does not present a shock hazard.
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Summary Macroshock: Microshock: Electrocautery:
80 mA can induce VF across an arm-arm circuit Hard with dry skin, easier with wet skin Isolation transformer makes it impossible to complete circuit to ground in OR, provided ground wires are attached. Microshock: 10 A can induce VF on the heart Isolation monitor limits the ability of any grounded chassis to come up to adequate voltage to induce microshock Electrocautery: Correct placement and function of return electrode prevents burns at site where current exits the body.
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Ungrounded Safety
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LIM (Line Isolation Monitor)
Continuously monitors the potential for current flow from the isolated power supply to ground. Determines the degree of isolation b\w 2 power wires and the ground. Predicts the current flow.
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LIM (Line Isolation Monitor)
Alarm is activated if 2mA-5mA of current is detected. Remember Macroshock vs. Microshock?
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Figure A faulty piece of equipment plugged into the isolated power system does not present a shock hazard. It merely converts the isolated power system into a grounded power system. The figure insert illustrates that the isolated power system is now identical to the grounded power system. The dashed line indicates current flow in the ground wire.
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Figure When a faulty piece of equipment is plugged into the isolated power system, it will markedly decrease the impedance from line 1 or line 2 to ground. This will be detected by the LIM, which will sound an alarm.
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Application: Electrical Shock
What levels of current (mA) is dangerous? <1 mA = imperceptable to touch Ventricular Fibrillation can be induced by: 100 mA of macroshock 100 μA (microamperes)of microshock current
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Electrical Shock SO L.I.M. DOES NOT PROTECT AGAINST MICROSHOCK SINCE IT DETECTS 2mA-5mA
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LIM (Line Isolation Monitor)
Indicates the existence of a single problem (SINGLE FAULT) Namely there is a problem with the ungrounded system becoming grounded So we are back to regular home power No shock with single fault
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LIM (Line Isolation Monitor)
A second problem (TWO FAULTS) are required for SHOCK to occur. A faulty piece of equipment-very likely at HH Unsafe environment like electric device + pool of normal saline.
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Boards LIM DOES NOT protect from MICROSHOCK, it warns of a POTENTIAL problem If an LIM goes off, remove the last piece of equipment that was plugged in In OR power source is ungrounded and equipment is grounded
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QUESTIONS The isolation monitor (LIM) on the wall of the operating room is to alert one to Faulty drainage of static electricity from equipment to the conductive floor The presence of equipment with a broken ground wire connection An excessive load on the isolation transformer supplying the room An excessive electrical leakage from either side of the power line to ground Too low conductance in the floor.
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Answer The isolation monitor (LIM) on the wall of the operating room is to alert one to D - An excessive electrical leakage from either side of the power line to ground
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QUESTION Electrocautery machines do not cause VF because the current they deliver differs from the electric current supplied by wall electrical outlets primarily by being: DC instead of AC Lower in voltage Lower in frequency Higher in frequency
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Answer Electrocautery machines do not cause VF because the current they deliver differs from the electric current supplied by wall electrical outlets primarily by being: D - Higher in frequency
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QUESTION The LIM (line isolation monitor)
Measures leakage current flowing from patient to ground Measures leakage current flowing from the electrical equipment to the patient Sounds an alarm if the leakage current exceeds 50 mA Measures the impedance between AC wiring and ground Cuts off power to the circuit if a faulty piece of equipment is connected
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Answer D The LIM measures the impedance from the AC wiring of an isolated power system to ground and only sounds an alarm if current in excess of 2-5 mA could flow to ground.
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QUESTION The isolated power supply system used in OR’s requires that:
The metal portions of the OR table be connected to earth ground The patient be insulated from the metal portions of the OR table Conductive flooring be used in the OR A transformer be connected between electrical equipment in the OR and the electric power supplied by the utility company
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Answer The isolated power supply system used in OR’s requires that:
D - A transformer be connected between electrical equipment in the OR and the electric power supplied by the utility company
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QUESTION Microshock hazard is increased in patients:
With a temporary pacing wire in place In electrically operated beds Receiving TPN via CVC Who are in rooms with isolation transformers
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Answer B Both 1 and 3 can conduct electricity close to the heart and bypass the high resistance of the skin 100 mA for VF in MACROSHOCK 100 μA (microamperes) for VF in MICROSHOCK
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QUESTION Operating rooms use an isolated power supply because:
Grounding cannot occur Contact with both wires of the isolation transformer would cause no shock Leakage current is zero It affords protection against high amperage electrocution
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Answer Operating rooms use an isolated power supply because:
4 - It affords protection against high amperage electrocution Remember LIM’s detect 2-5 mA (milli Ampres), a small amount of current
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QUESTION A ground fault circuit interrupter:
Is almost never used in OR Is foolproof method for preventing serious electrical shock May be used in a grounded electrical system Has both audio and visual alarms to indicate the presence of a ground fault
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Answer B A GFCI prevents current from flowing in a circuit when there is an imbalance in the current flowing in the two sides of the circuit. Not used in OR b\c it would turn off life support equipment
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QUESTION Leakage current: Is harmless
Occurs b\c of the magnitude of the inductance b\w electrical conductors supplying equipment Is of the DC type Is unintentional flow of current from the internal wiring of a device largely a result of capacitance and AC current
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Answer Leakage current
4 - Is unintentional flow of current from the internal wiring of a device
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QUESTION The current delivered to a pt by an electrocautery device differs from the current supplied by an electrical utility in its Capacitance Frequency Amperage Voltage Power
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Answer B USA electricity is 60 Hertz and electrocautery is > 10,00000 Hertz Hertz = frequency-remember bovie uses high frequency to prevent tissue damage
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Electrocautery
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Electrocautery with Poor Contact
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Electrocautery Return Electrodes
If the return electrode is broken, current will find alternative path back to electrocautery unit. ECG Electrodes Twitch Monitor (case of 3rd degree facial burn at VA) Be suspicious if standard settings are not producing an adequate spark Electrocautery example
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Electrocautery Return Electrodes
Place the return electrode close to surgical site If an ECG electrode is closer, it may still take some of the current Don’t place over metal implants Function as internal lightening rods, focusing current on a small area near the implant, and the return electrode
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Bipolar electrocautery
No return electrode, current just flows between two poles Useful for laparoscopy, because current doesn’t flow through bowel, risking burning the intestine
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The hospital-grade plug pattern conforms to the NEMA 5-15 standard; however, (1) the blades are usually solid instead of folded brass, (2) the blades are normally nickel-plated, and (3) the plug includes a cable retention device or strain relief to prevent any stress to the plug's internal connections. It is imperative that the ground connection be reliably maintained to protect the patient and medical staff. Although many hospitals prefer that the plug be clear so that internal connections can be inspected visually .
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Summary Macroshock: Microshock: Electrocautery:
80 mA can induce VF across an arm-arm circuit Hard with dry skin, easier with wet skin Isolation transformer makes it impossible to complete circuit to ground in OR, provided ground wires are attached. Microshock: 10 A can induce VF on the heart Isolation monitor limits the ability of any grounded chassis to come up to adequate voltage to induce microshock Electrocautery: Correct placement and function of return electrode prevents burns at site where current exits the body.
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