Electrical Safety in the Operating Theatre

Question 1

How does mains electricity travel from the substation?

Answer 1

In two conductors - live wire (potential of 240V) - neutral wire (connected to earth and thus at the same potential as earth) This is the same as the positive and negative wires used in DC

Question 2

What happens if a connection is made between the live wire and earth?

Answer 2

The current will flow through that connection - including if this connection is a patient

Question 3

How can electricity cause damage to the body?

Answer 3

Electrocution Burns Ignition of a flammable material

Question 4

What are the factors affecting the damage caused by electrocution?

Answer 4

The amount of electricity that flows (current) Where the current flows (current pathway) Density of current The type of current (AC vs DC) Current duration

Question 5

What is current?

Answer 5

A flow of electrons, measured in amps (SI unit)

Question 6

What would a current passing through the chest cause?

Answer 6

VF or asphyxia due to tetany of the respiratory muscles

Question 7

What would a current passing through the body vertically cause?

Answer 7

Loss of consciousness and spinal cord damage

Question 8

What would a 1 mA of 50Hz current flowing between each hand cause?

Answer 8

Tingling

Question 9

What would a 15 mA of 50Hz current flowing between each hand cause?

Answer 9

Muscle tetany, pain, asphyxia

Question 10

What would a 75 mA of 50Hz current flowing between each hand cause?

Answer 10

VF

Question 11

What current applied directly to the heart (or in close proximity) can cause VF?

Answer 11

50 milliamps at 50Hz (microshock)

Question 12

What equipment might allow microshock?

Answer 12

CVCs Intracardiac pacemakers with external lead Temperature probe placed in the oesophagus immediately behind the left atrium

Question 13

Why is alternating current at 50Hz the most dangerous current?

Answer 13

The myocardium is most susceptible to the arrythmogenic effects of electric currents at this frequency and the muscle spasm prevents the victim letting go of the source

Question 14

How are sparks prevented in theatre?

Answer 14

Spark proof switches and electric socket outlets which prevent the plug being withdrawn whilst the switch is turned on.

Question 15

What are the ways in which electricity can flow through the body?

Answer 15

Resistive or capacitive coupling

Question 16

What is resistive coupling?

Answer 16

When the body provides a direct physical connection to let electricity flow through the body. This happens if the body comes into contact with electricity and the earth directly eg. faulty equipment

Question 17

What is capacitive coupling?

Answer 17

When the body forms a connection between an electrical source and earth by acting as one plate of a capacitor.

Question 18

What is the formula for impedance in a capacitor?

Answer 18

impedance = distance between plates / (current frequency x plate area)

Question 19

What happens if direct current is applied to a capacitor?

Answer 19

Current flows only until the positive plate is charged to the same potential as the electrical source, then the current stops

Question 20

Give an example of capacitive coupling in the clinical setting

Answer 20

MRI scanner - the scanner creates a changing electromagnetic field that can induce currents in conductors such as the wire or metal of a standard pulse oximeter probe - capacitive coupling allows the patient to become part of an electrical circuit which may cause a burn

Question 21

What methods can we use to reduce the risk of electrocution?

Answer 21

General measures Equipment design Equipotentiality Isolated Circuits Circuit Breakers

Question 22

What are the general measures to reduce risk of electrocution?

Answer 22

Adequate maintenance Regular testing of equipment Ensuring patient not touching earthed objects Wearing antistatic shoes

Question 23

What are the equipotentiality to reduce risk of electrocution?

Answer 23

If different piece of equipment have different potentials relative to earth and are in close proximity - a connection may be made between them by the user. A current would then flow from higher to lower potential via the user. To avoid this - the terminals of each piece of equipment in a stack can be connected to each other bringing them all to the same potential.

Question 24

What are the isolated circuits measures to reduce risk of electrocution?

Answer 24

This provides a circuit where the connection between the electrical source and earth does not allow current to flow. It consists of an isolating transformer which is 2 coils electrically insulated from each other. The mains circuit is earthed but the patient circuit is not (floating).

Question 25

What is class 1 equipment?

Answer 25

Any part of it which the user can touch is connected to earth. If a fault occurs the current flows down the earth wire. This new circuit has a lower resistance - so this melts the protective fuses and breaks the circuit.

In addition to the fuse in the mains socket it should also have fuses at the equipment end of the mains supply lead, in both live and neutral conductors.

Question 26

What is class II equipment?

Answer 26

Any accessible conducting parts are protected from the live supply by either double or re-inforced insulation. Therefore an earth wire is not required.

Question 27

What is class III equipment?

Answer 27

It provides protection against electric shock by using voltages no higher than safety extra low voltage (SELV).

The equipment is either battery operated or supplied by a SELV transformer.

The danger of microshock persists.

Question 28

What is SELV?

Answer 28

Safety extra low voltage

Not > 25V AC or 60V DC

Question 29

What is Type B equipment?

Answer 29

Can be Class I, II, or III

Maximum leakage current must not exceed 100 microamps

NOT suitable for direct connection to the heart

Question 30

What is type BF equipment?

Answer 30

Same as type B but uses a floating circuit

(can be Class I, II, or III)

Question 31

What is class CF?

Answer 31

These provide the highest degree of protection, using isolated circuits and having a maximum leakeage current of < 10 micro amps

They can be used for direct cardiac connection (eg ECG leads, pressure transducers, thermodilution computers)

Question 32

What is a circuit breaker?

Answer 32

COELCB (current-operated earth leakage circuit breakers) consist of a live and neutral wire with the same number of windings around the core of a transformer. A third winding connects these to the coil of a relay that operates the circuit breaker.

If the current in the live and neutral conductors is the same, the magnetic fluxes cancel themselves out. BUT - if they are different (due to excessive current leakage) there is a resultant magnetic field. This induces current in the third winding causing the relay to break the circuit. A difference of 30mA can trip the COELB in millisecs.

Question 33

What is monopolar diathermy?

Answer 33

Generates electrical energy at 200kHz to 6MHz

Energy is applied between 2 electrodes (neutral and active)

Neutral has a large conductive surface area producing a low current density with no heating effect.

Active has a v. small contact area resulting in a very high current density - produces heating and deliberate tissue damage.

Question 34

What do cutting diathermy and coagulation use?

Answer 34

Monopolar diathermy.

Cutting diathermy employs a sine waveform.

Coagulation uses a modulated waveform.

Question 35

What is bipolar diathermy?

Answer 35

It operates with a much lower power output. The output is applied between the points of a pair of specially designed forceps producing high local current density.

No current passes throughout the rest of the body.

Question 36

What accidents can occur with diathermy?

Answer 36

Burns

• due to inadvertent depression of the foot switch
• poor contact between the neutral plate and the patient resulting in increased current density
• if the electrical circuit is completed via the operating table, or other points through which the patient may be earthed, a burn may result at this site

Fires

• may be caused by sparks igniting flammable materials eg skin cleaning solutions, bowel gas

Pacemakers

• unipolar diathermy can inhibit or permanently damage pacemakers
  
  • bipolar should be used if diathermy is essential but applied well away from pacemaker