Electrical Injury
27/12/10
- FANZCA Part II Notes
- SP Notes
- Boumphrey, S et al (2003) “Electrical Safety in the Operating Theatre” CEPD Volume 3,
Number 1, pages 10-14
- Magee et al (2005) “The physics, clinical measurement and equipment of anaesthetic
practice” Electricity Chapter, page 167
Electrical Supply
- AC current
- 50Hz
- travels from substation via conductors (live wire = 230 V and neutral wire = earth)
- if connection made then electricity will flow through the connection to the earth
- a problem occurs if the connection is a patient or a staff member
- patients are often unconscious and unable to respond normally to electric current + there
are many solutions that can conduct electricity (blood, saline, H2O)
- high frequency current has low tissue penetration and does not excite contractile cells
- low frequency current penetrate more
Definitions
Flow = change in P/resistance
Change in P = flow x resistance
V = IR
Voltage = the tendency of electrons to move through a conductive system (volts)
Resistance = tendency of a material to limit flow of electrons through itself
Current = number of electrons/second that move (amps)
Power = voltage x current (watts)
Earth referenced supplies = power that seeks return to substation via ground
Floating power supplies = supplies that do not seek return to substation via ground
Macroshock = large amount of current flow that produces harm or death
Microshock = small current @ high density close to myocardium causing VF (CVL, intra
cardiac pacemakers with an external lead, PAC, oesophageal temperature probes)
0.1mA – to myocardium = VF
<1mA – nothing
1mA – pain (withdraw)
10mA – skeletal muscle spasm (tetany)
100mA – VF
1A or 1000mA – muscular burn
>1000A – severe burn, loss of limb (high tension injury)
>12,000A – lightening (coma, severe burns, loss of limb)
AC = alternating current -> electrons switch direction @ regular intervals
Jeremy Fernando (2010)
DC = electrons flow in one direction (safer than AC)
Capacitance = ability of a capacitor to store charge
Isolating transformer = floating transfer that isolates mains supply from earth
Isolating monitor = monitors potential for current to flow from isolated current supply to
ground.
How might electricity flow through the body?
1. Resistive coupling
- body can complete the circuit by connecting electricity with earth or by touching an earthed
object (ie. source of electricity -> body -> drip stand that is touching earth)
- sources of electricity = faulty equipment (ie. live wire touching casing) or current
leakage (ie. electrical equipment @ higher potential than earth despite adequate insulation
some current will flow to earth if connection made -> these can produce microshock
2. Capacitive coupling
- body can act as one plate of a capacitor
- a capacitor has two plates separated by insulating material -> stores electrical charge
(Farads)
- if current applied, electricity flows for brief moment until positive plate has same charge as
the electrical source.
- if AC current applied the process will continue
- ie. MRI and pulse oximetry -> changing electromagnetic field causes induction of currents in
probe -> capacitive coupling allows patients finger to become part of the circuit -> burn
Electricity and Damage
1. Burns – when current flow through substance with any resistance -> heat produced
2. Ignition of a flammable substance -> explosion/fire – sparks produced from plugs being
withdrawn or switches being turn off
3. Electrocution – when current passes through a person and disrupts normal electrical
function of cells -> damage determined by 4 factors:
(i) amount of electricity that flow (current) – flow of electrons (dependent on V=IR)
(ii) current path & density – through which tissues
(iii) type of current (AC or DC) – AC @ 50 Hz is most dangerous as myocardium most
sensitive here and muscle spasm prevents victim letting go.
(iv) duration – the shorter duration the higher the current flow must be before damage is
done
Injuries
Jeremy Fernando (2010)
- depolarisation of muscle cells: VF, sustained asystole, arrhythmia, myocardial damage, LV
dysfunction, tetanic contraction -> fractures
- vascular injuries: thrombosis, compartment syndrome -> rhabdomyolysis
- neurological injuries: peripheral nerve injury, coma, encephalopathy, autonomic dysfunction
- renal injuries: myoglobinuria
- other injuries: traumatic, fire, ruptured ear drum, cataracts
MANAGEMENT
Goals
(1)
(2)
(3)
(4)
turn power off
ACLS protocol
trauma protocol
burn protocol
Resuscitation
- may require intubation if obtunded, received major burns or to allow appropriate treatment
(suxamethonium safe for 48 hours)
- ventilation: lung protective strategy, rule out life threatening chest injury as also trauma
patient
- circulation: aim to restore normal circulating volume, prevent effects of rhabdomyolysis,
may require inotropes/vasopressors if there is significant myocardial dysfunction or SIRS
response, Parkland’s formula
- disability: may have an associated TBI, aim to prevent secondary brain injury,
normoglycaemia
- exposure: quantify severity of injuries, burn (depth, TBSA involved and type) -> cool burn
for 10 min
Electrolytes and Acid Base
- features of rhabdomyolysis – hyperkalaemia, hypocalcaemia, hyperphosphataemia,
metabolic acidosis
- supportive management
Specific Therapy
-
burns: early debridement and grafting, fasciotomies +/- amputations
ischaemic/necrotic tissue: debridement
tetanus
antibiotics if indicated
Underlying Cause
- prevention (see below)
PREVENTION OF ELECTROCUTION
Jeremy Fernando (2010)
1. General measures
-
maintenance and testing
making sure patient isn’t in contact with earthed objects
anti-static shoes (high impedance -> current can’t flow through)
no extension cords that can reach from an unprotected area
no double adaptors (may not sit well and can leak current)
all equipment should meet Australasian Standards for Safety
personnel education
2. Class of Equipment
- Class 1: third pin of plug (direct earthing) + metal casing -> this circuit should be low
resistance and if live wire comes in contact with an accessible part should be linked to a fuse
that breaks the circuit
- Class 2: double or reinforced insulation (non-conductive plastic), earthing wire not needed
- Class 3: provides protection by stepping down main power to a safety extra low voltage
(SELV, <25 V AC), either battery operated or connected to a SELV transformer
3. Type of Equipment allowed to be connected to patient
- based on maximal permissible leakage currents
- Type B: may be class 1, 2 or 3 but maximum leakage must not exceed 100microamps (thus
must not be directly connected to heart)
- Type BF: as for type B but uses a isolated (or floating circuit) -> see below
- Type CF: these provide the highest degree of protection using isolating circuits and having a
maximum current leakage of <10microamps (these can be connected directly to heart – ECG
leads, pressure transducers and thermodilution computers)
4. Equipotentiality
-
connections of equipment to make them all the potential differences the same
connections of equipment by low impedance green cables
includes anaesthetic machines and IV poles with pumps on them
prevents devices leaking charge to a user or patient
5. Isolating (floating) Circuits
- patient circuit not earthed by using an isolated transformer
- see diagram
- can be used to isolate an entire theatre -> if leaves entire power susceptible to failure
- used frequently to isolate individual instruments
- line isolation monitor attached that continuously monitors the potential for current to flow
from isolated current supply to ground (alarm activated if 2mA of current detected)
6. Circuit breakers (RCD’s)
Jeremy Fernando (2010)
- live and neutral wire wrapped around the core of a transformer and then around a circuit
breakers (normally the magnetic fluxes cancel each other out)
- if excess current is leaked -> magnetic field is produced between the transformer that
triggers a current causing break in circuit
- will protect against macroshock
7. Surgical Diathermy
- the use of the heating effects of high frequency (kHz -> MHz) electrical to coagulate or cut
tissue
- can cause burning, explosions or muck up pacemakers
Monopolar diathermy
-
200 kHz -> 6 MHz
neutral and active
neutral = large conductive area -> low current density and minimal heat
active = small contact area + high current density
Bipolar diathermy
- lower power output
- output between 2 points on forceps -> high local current density
- no current passes through rest of body
8. Area Classification
Body protected areas
-
patients connected to equipment that lowers the natural resistance of the skin
examples: electrode gels, conductive fluids, metal needles and catheters
protection from macroshock is goal
RCD’s, line isolation transformers and monitors used
Cardiac protected areas
- this is where procedures are performed within or near the heart -> protection from
microshock is required
- examples: intracardiac pacing electrodes, intracardiac ECG electrodes, intracardiac catheters
- equipotential earthing, RCD’s and line isolation monitors used
Jeremy Fernando (2010)