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

Chris is an Intensivist and ECMO specialist at the Alfred ICU in Melbourne. He is also the Innovation Lead for the Australian Centre for Health Innovation at Alfred Health and Clinical Adjunct Associate Professor at Monash University. He is a co-founder of the Australia and New Zealand Clinician Educator Network (ANZCEN) and is the Lead for the ANZCEN Clinician Educator Incubator programme. He is on the Board of Directors for the Intensive Care Foundation and is a First Part Examiner for the College of Intensive Care Medicine. He is an internationally recognised Clinician Educator with a passion for helping clinicians learn and for improving the clinical performance of individuals and collectives.

After finishing his medical degree at the University of Auckland, he continued post-graduate training in New Zealand as well as Australia’s Northern Territory, Perth and Melbourne. He has completed fellowship training in both intensive care medicine and emergency medicine, as well as post-graduate training in biochemistry, clinical toxicology, clinical epidemiology, and health professional education.

He is actively involved in in using translational simulation to improve patient care and the design of processes and systems at Alfred Health. He coordinates the Alfred ICU’s education and simulation programmes and runs the unit’s education website, INTENSIVE.  He created the ‘Critically Ill Airway’ course and teaches on numerous courses around the world. He is one of the founders of the FOAM movement (Free Open-Access Medical education) and is co-creator of litfl.com, the RAGE podcast, the Resuscitology course, and the SMACC conference.

His one great achievement is being the father of two amazing children.

On Twitter, he is @precordialthump.

| INTENSIVE | RAGE | Resuscitology | SMACC

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