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Physics for Anaesthetists > Electrical Safety > Flashcards

Electrical Safety Flashcards

1
Q

How does electricity cause morbidity and mortality

A
  1. Electrocution
  2. Burns
  3. Ignition of flammable material –> fire or explosion
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2
Q

Why is there a substation between the power station and the hospital

A

A transformer reduces the extremely high voltage from the power station

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3
Q

What wires deliver the current from the substation to the hospital. What is the difference between these wires

A

The live wire and the neutral wire

The neutral wire is connected to earth at the substation

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4
Q

What do wires do most plug points contain at the hospital

A

Live wire
Neutral wire
Earth wire (earthed at hospital)

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5
Q

What happens if the anaesthetist touches the live wire

A

Circuit is completed to the substation via the anaesthetist, through is antistatic shoes, through the antistatic floor, into the earth and back to the substation. Hence current flows through the anaesthetist

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6
Q

What happens to the anaesthetist if a current of 1mA flows through him?

A

Tingling sensation

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7
Q

What is the Mains electricity supply voltage in RSA and the UK and North America

A

RSA
- 220/230 V AC at 50 Hz

UK
- 240 V AC at 50Hz

North America
- 110 V

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8
Q

Write the formula that defines the current that would flow through the anaesthetist if he were to touch the live wire. What would be the current that flows through him the impedance of his footware and the antistatic floor is 240 kOhms

A

Current = Potential / Impedance

Current = 240/240 000 Ohms

= 1 mA (tingling)

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9
Q

What is the recommended impedance of antistatic shoes

A

75kOhms - 10 megaOhms

Low enough for safe dissipation of electrostatic charges but high enough to give some protection against electric shock

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10
Q

What is the tissue impedance of the anaesthetist

A

a few hundred ohms –> most of the impedance occurs at points of contact with the skin (a few kOhms)

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11
Q

What factors increase the risk of ventricular fibrillation during electric shock

A
  1. Lower impedance + higher current + higher voltage
  2. Lower frequency (50 Hz more likely than above 1kHz)
  3. Timing of the shock: R on T phenomena
  4. Lower current required in patients with myocardial disease
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12
Q

What are the classes of medical electrical equipment according to the International Electrotechnical Commission Standard 601

A 
Class I (Earth wire + fuse)
- Metal case connected to earth wire. If fault occurs connecting case to live wire then immediately current flows via earth wire which then melts a protective fuse which breaks the current. This breaks the live potential from the equipment case

Class II (Double insulated equipment)

  • Double layer insulation or reinforced insulation –> no chance of fault causing potential in equipment casing
  • No earth wire required.

Internally Powered Equipment
- risk of electric shock still present but very low potential (batteries)

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13
Q

Why should space blankets not be used in theater

A

Made of metal coated plastic –> electrical hazard if they come into contact with earthed metal apparatus.

They can also cause burns

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14
Q

What is current density and why is it relevant

A

If an anaesthetist touches a live wire and the current flows fromhand to feet, a small proportion of the 24 A travelling through him will travel through the heart. The remainder will travel through the other tissues.

The portion of current traveling through a specific tissue (e.g. the heart) is called the current density and it is the current density that will determine whether ventricular fibrillation will occur

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15
Q

What is a microshock

A

Faulty intracardiac catheter passes from an item of monitoring equipment into the heart itself, and this catheter touches the wall of the heart itself, any electric current flowing through the catheter will pass through a very small area of the heart. The current density will therefore be very high. E.g. a current of 150 uA may cause the same current density as in the case of a 24 mA current flowing from the hands to the feet.

Hence ventricular fibrillation may be induced by this microshock

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16
Q

Is microshock more likely with DC or AC

A

DC

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17
Q

Is microshock (and gross electric shock) more likely to occur at high or low frequencies

A

Much more likely at lower frequency –> like mains frequency 50Hz.

Much less likely with frequencies > 1 kHz

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18
Q

What may act as an earthing point for a patient with a faulty intracardiac catheter at risk of microshock

A

The anaesthetist

Contact with conductive theatre equipment that is earthed.

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19
Q

List equipment which risks microshock

A
  1. CVC
  2. IABP
  3. Intracardiac pacemaker with external lead
  4. Temp probe at lower 3rd of oesophagus
20
Q

What is an isolated patient circuit?

A

For equipment that requires electrical connections to be made to the patient (E.g. ECG) a deliberate effort is made to reduce the impedance at the skin. This decreases the protection of skin impedance.

To counteract this problem, most equipment that requires electrical connections be made to the patient uses an isolated patient circuit = floating circuit.

A floating circuit is created by connecting this floating circuit to the mains circuit via and isolation transformer. The principle of a transformer is that current

21
Q

What are the advantages and disadvantages of AC versus DC current

A 
AC current (Home plug points)
1. Power transmission lines: Efficient to transport over long distances as voltage can easily be increased or decreased over long distances.

DC current (Laptop and electrical devices. And batteries)

  1. Easier to control
  2. allows circuits to be smaller and more compact

Some devices use both (washing machine)

22
Q

What is a rectifier

What is an inverter

A

Rectifier: converts AC to DC current

Inverter: Converts DC to AC current

23
Q

What six factors determine the effects produced by electrocution

A
  1. Current magnitude
  2. Current location (pathway)
  3. Current density
  4. Current type (AC vs. DC)
  5. Current duration
  6. Current timing
24
Q

Define an ampere

A

1 Ampere represents a flow of 6.24 x 10^18 electrons (1 coulomb of charge) past a specific point in 1 second.

25
Q

What determines the size of current flow

A

Current = Voltage / Impedence (resistance)

26
Q

How does current pathway determine tissue damage

A
  1. Current through chest –> VF / Asphyxia d/t tetany resp muscles
  2. Current through body vertically –> Loss of consciousness or spinal cord damage
27
Q

Define current density

A

The amount of current flowing per unit area

28
Q

Describe the effects of AC passing from hand to hand at the following:

  1. 1 mA
  2. 15 mA
  3. 75 mA
A
  1. 1 mA —-> Tingling
  2. 15 mA —> Muscle tetany, pain, asphyxia
  3. 75 mA –> VF
29
Q

Why do electrical burns usually occur on the skin despite current flowing through multiple tissues

A

Heat dissipation increases at area of highest resistance. skin is a tissue with relatively high resistance

30
Q

How can sparks be prevented when plugs are removed

A

Spark proof switches –> plugs cannot be removed whilst turned on.

31
Q

What are leakage currents

A

Leakage currents arise because electrical equipment is at a higher potential than earth. Even if the equipment is well insulated and since there is no such thing as perfect insulation or infinite resistance, some current will flow to earth.

32
Q

What are the different ways that electricity could flow through a patient’s body. Describe these

A

RESISTIVE COUPLING
Either:
1) Faulty equipment
- live wire touching equipment casing

2) Leakage currents
- Electrical equipment at a higher potential than earth

These currents are normally small BUT they can be fatal (microshock)

CAPACITIVE COUPLING
1) Body acts as one plate of a capacitor (e.g. in an MRI scanner –> alternating magnetic field can induce current in wires or metal within pulse oximeter. Although the patient is not in direct contact with this conductors, capacitative coupling allows the patient to become part of an electrical circuit which may cause a burn.

33
Q

Define capacitance

A

Capacitance is the storage of electrical charge. The amount of charge a capacitor can store is called the Farad. A capacitor is an electrical component that consists of two conducting plates separated by a layer of insulation. When a current is applied to a capacitor, the negative charge collects within one of these conductors. These electrons have flowed from the positive side of the capacitor. There for the opposing conducting plate is devoid of electrons and oppositely charged. The charge cannot be transferred due to the insulating plate and the absence of a continuous circuit. The positive and negative charged sides of the capacitor hold the charge in here. When a circuit is conneceted,

34
Q

What is the equation for the impedance of a capacitor

A

Impedance = Distance between plates
_______________________
Current frequency x plate area

35
Q

List 5 methods of reducing the risk of electrocution

A
  1. General measures
  2. Equipment design
  3. Equipotentiality
  4. Isolated circuits
  5. Circuit breakers
36
Q

What are the general measures to reduce the risk of electrocution in theatre?

A
  1. Adequate maintenance and regular testing of electrical equipment
  2. Ensure patient is not in contact with earthed objects
  3. Wear appropriate antistatic shoes
37
Q

What are the elements of equipment design that protect against electrocution

A

As defined by the International Electrotechnical Committee standard in IEC 601

Class 1

  • Metal casing is connected to earth by an earth wire
  • If live wire contacts casing –> current flows in lower resistance earth wire connected to a fuse which blows and discontinues the current
  • SYMBOL: Vertical line with shortening horizontal lines below)

Class 2

  • Double or re-inforced insulation
  • Earth wire not required
  • SYMBOL (Box within a Box)

Class 3

  • Use SELV = Safety Extra Low Voltages (Max 25 V AC and 60 V DC)
  • Battery operated equipment
  • Unlikely that these voltages will cause electrocution
  • However, risk of microshock remains so this class is excluded from safety standards.
38
Q

What is the difference between Class designation and Type designation with regards to equipment design in the prevention of electrocution in theatre

A

Class design 1, 2, 3 as described above refers to earthed (1), Double insulated (2) or Low voltage (3).

Type designation is based on the maximum permissible leakage currents:

Type B

  • May be class 1, 2 or 3
  • BUT MAXIMUM leakage current must not exceed 100uA
  • Symbol: just a man

Type BF

  • Same as type B but uses a ‘F’loating circuit
  • Symbol: man inside a box

Type CF
- Provide highest degree of protection
- Using floating (isolated) circuits
- Maximum leakage current allowed < 10 uA
- Suitable for direct cardiac connection:
E.g. ECG leads, pressure transducers, thermodilution computers
- Symbol: A heart in a box

39
Q

What is the symbol for equipment type designations B, BF and CF

A

B - Man
BF - Man in box
CF - Heart in box

40
Q

What is equipotentiality

A

Connect different pieces of equipment together bringing them all to the same potential to avoid multiple different potentials relative to earth

41
Q

What is an isolated (floating) circuit

A

This provides a circuit whereby a connection between the electrical source and the earth does NOT allow current to flow

  • Isolation transformer used
  • -> This is two coils electrically isolated from one another. When alternating current flows through the primary (or mains coil) it generates a magnetic field around it. This induces a current in the patient or secondary coil.

Mains circuit is earthed
Patient circuit is not earthed (hence floating)

42
Q

What is the down side to a floating circuit

A

If the floating circuit supplies the entire theatre, and a fault occurs in one piece of equipment, the power may be lost to the entire theatre

43
Q

How does a circuit breaker work

A

Current operated earth leakage circuit breakers (COELCB)

  1. Live wire wound around core of transformer
  2. Neutral wire wound around core of transformer

If current in these windings is the same –> the magnetic fluxes cancel each other out.

If the current in these windings are different (i.e. excessive current leakage) there is a resultant magnetic field. –> This induces a current in a third winding causing a relay to break the circuit

A difference of as little as 30 mA can trip the COELCB in a very short time period (milliseconds). This greatly reduces the possibility of serious electrical shock

44
Q

What is surgical diathermy. Classify and describe the different types

A

Surgical diathermy equipment uses the heating effects of high frequency (kHz - MHz) electrical current to coagulate and cut tissues.

There are two types:

  1. Monopolar
    - 200 kHz - 6MHz
    - Energy is applied between two electrodes: neutral and active.
    - Neutral electrode: large conductive surface area producing a large current density with no measurable heating effect.
    - Active electrode has a very small contact area resulting in a very high current density
  2. Bipolar
    - The output is applied between the points of a pair of specially designed forceps producing high local current density.
    - No current passes through the rest of the body
45
Q

How can the risk of burns during use of surgical diathermy be reduced

A
  1. keep forceps in protective quiver
  2. Audible buzzer during use
  3. Alarm to indicate inadequate neutral plate contact
46
Q

Classify the two major types of pacemaker

A

Temporary (A few days)
- external lead into endocardium via IJ or SCV into RV: 4 V + pulse duration < 1 ms

Long-term (implanted)

  • Battery powered
  • Cardiac surgery embed cables in myocardium
47
Q

What is the problem with demand mode pacemakers

A

External interference from

  1. Microwave oven
  2. Electric motors
  3. Antitheft devices in shops and libraries
  4. MRI
  5. Physiotherapy equipment

The electromagnetic fields from these devices may cause misinterpretation and the sensing function might think that this is a QRS complex, so no pacing when patient requires it. Most have safety function to change automatically to fixed-rate mode when this interference is detected.

Physics for Anaesthetists (26 decks)

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