Electrical Injury

OVERVIEW

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
1000A – severe burn, loss of limb (high tension injury)
>12,000A – lightning (coma, severe burns, loss of limb)

  • AC = alternating current -> electrons switch direction @ regular intervals
  • 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.

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

HOW ELECTRICITY FLOWS 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

ELECTRICAL 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

  • 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) turn power off
(2) ACLS protocol
(3) trauma protocol
(4) 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

PREVENTION OF ELECTROCUTION

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, see below

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 if leaves entire power susceptible to failure

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
  • 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)

  • 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

References and Links

  • Boumphrey, S et al (2003) “Electrical Safety in the Operating Theatre” CEPD Volume 3, Number 1, pages 10-14 [Free Full Text]
  • Magee et al (2005) “The physics, clinical measurement and equipment of anaesthetic practice” Electricity Chapter, page 167

CCC 700 6

Critical Care

Compendium

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