ORT21BMI

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ORT21BMI

• Week 3--Electrical Safety

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Clinical context for Week 3

• You need to record an electroretinogram from your
  patient. This means forming a direct electrical
  contact with her via a conductor sitting on her
  eye or beneath her eyelid.
• Are there any risks in this procedure?
• What hazards could arise if something
  malfunctions in the recording system?
• What sorts of mistakes do you need to watch for
  in setting up the recording?
• Sometimes visual evoked potentials are recorded
  during surgery. Are there any additional hazards
  to worry about if youre doing a recording in an
  operating theatre?
• What sorts of external interference can lead to
  poor quality or inaccurate recordings?
• Youve set up the patient but get no signal.
  Besides swearing softly, what else is there to
  do?

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Learning goals

• What are the relevant features of the mains
  supply in Australia?
• How does this differ from a battery?
• What are the major hazards from the mains supply
  that you might encounter?
• What is the earthing connection for?
• Why dont fuses make everything safe?
• What precautions should you take with clinical
  equipment?
• What hazards does your mobile phone present to
  patients?
• What should you do when something doesnt work
  (before ringing the service rep)?

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Primum non nocere

• The first rule for all clinicians--first, do no
  harm.
• Orthoptics would seem pretty safe for you and
  your patients, as opposed to, say, neurosurgery
• Much equipment you use is electrically operated,
  much of it from the 240 v, 50 Hz mains supply.
• Today well look at what hazards this implies

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Whats the danger?

• A battery-powered retinoscope poses no risk to
  the patient, unless you poke them in the eye with
  it
• Mains-operated apparatus is another story
• Why?
• The answer lies in the difference between direct
  current (DC) and alternating current (AC)
• The risks have been known since people first
  decided to wire up buildings for electrical power

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AC vs. DC--which to use?

• In the early 20th century, Thomas Edison and
  Nikola Tesla fought over mains distribution lines
  should use DC or low frequency AC
• In ACs favour was the ability to transmit it
  over long distances efficiently (Tesla)
• In DCs favour was its safety (Edison). Edison
  favoured AC only for execution by electrocution
• Tesla won.
• Until the mid-20th century, though, a few areas
  had DC distribution, but thats all gone now

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An extreme case
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Why is AC more dangerous?

• Pass enough current through someone and theyll
  be cooked (this was a popular way to cook hot
  dogs when I was a student)
• This isnt the usual way for someone who comes
  into contact with AC supplies to die
• They usually die from a loss of heart function

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What does AC do to you?
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Whats going on in that last slide?

• Note first the low threshold to feel the current
• Note how about at 10x that current you cant let
  go that is, your muscles spasm and you cant
  release the energised conductor
• Then notice that another 10x increase fatally
  brings about ventricular fibrillation
• If enough current goes through the sino-atrial
  node, normal ventricular conduction is disrupted
  and the left ventricle just sits there and
  quivers, instead of pumping blood
• Then you die.

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AC powers unique hazard

• Notice the particular risk from the mains supply
• First, 50 Hz is right within the most dangerous
  frequency band
• Next, you can get trapped into gripping a
  conductor as it kills you

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AC hazards, continued

• This isnt just a problem in clinics--it can
  happen anywhere mains power is used
• at home
• even on the street--recall the bus shelter
  story in your handout
• The universality of hazard posed by AC mains
  power is why Edison opposed it so strongly
• Engineering advantages have made it universal,
  however, so the lessons of this class may be
  useful in many places

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Consider a piece of equipment
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And if the active and earth leads were switched
and you completed the circuit between earth and
device case?
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How realistic is this possibility?

• Not all that implausible--people often rewire
  plugs (and sometimes power points)
• There was even a manufacturers recall recently
  for power boards with misconnected pins
• Why wouldnt either the buildings or devices
  fuses protect you?
• But is this the only way one could come into
  contact with the mains supply?
• No! Even unbelievable mistakes can, and are,
  made--consider the matter of electrode leads...

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FDA Proposed Standard Lead Wires DEPARTMENT OF
HEALTH AND HUMAN SERVICES
Food and Drug Administration 21 CFR Parts
895 and 897 Medical Devices Proposed
Performance Standards for Electrode Lead Wires
and Proposed Banning of Unprotected Electrode
Lead Wires AGENCY Food and Drug
Administration, HHS. ACTION Proposed rule.
-------------------------------------------------
SUMMARY The Food and Drug Administration
(FDA) is proposing to establish a performance
standard for electrode lead wires. The agency is
taking this action because it has determined that
a performance standard is needed to prevent
hazardous electrical connections between patients
and electrical power sources. FDA is also
proposing to make unprotected electrode lead
wires a banned device upon the effective date of
the standard for the device. FDA has determined
that unprotected electrode lead wires and patient
cables present an unreasonable and substantial
risk of illness or injury, and that the risk
cannot adequately be corrected or eliminated by
labeling or a change in labeling.
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FDA rule, contd
SUPPLEMENTARY INFORMATION I. Background
FDA's records of incidents with unprotected
electrode lead wires and patient cables reveal
the following Between 1985 and 1994, 24 infants
or children received "macro-shock" (large,
externally applied currents) from electrode lead
wires or cables, including 5 children who died by
electrocution (Ref. 2). The most recent death
(1993), which occurred in a hospital, involved a
12-day old infant. The apnea monitor involved in
the incident had been sold with safety protected
electrode lead wires and patient cable, but an
unprotected patient cable from another
manufacturer of an ECG monitor and unprotected
prewired electrodes from a third manufacturer
were being used when the infant was
electrocuted...In 1986, for example, a death
occurred when the ECG lead wires were plugged
into an infusion pump power cord in a hospital
environment.
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Is this relevant to orthoptists?

• Remember the electroretinograms from last year?
• Did that apparatus differ much from ECG machines?
• You could never make a mistake like that, but
  someone less-talented might!
• Are there any other shock hazards you could
  encounter in your professional life?
• Some orthoptists assist in surgery or may be
  involved in other tests in the operating theatre

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In surgery...

• Consider a catheter in an artery or vein
• What body system to they give access to?
• What are they often filled with?
• What are the electrical characteristics of these
  contents?
• Does this environment present any special risks?

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Macroshock vs. microshock

• Normal, intact bodies receive macroshock, where
  current has to penetrate the skin
• Macroshock threshold for fatality lies in the
  100-300 mA range
• If the skin is punctured and current given a low
  resistance, direct route to the heart, microshock
  occurs
• Microshock threshold for fatality lies at the
  5-10?A range--10,000 times less!
• Why?

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Macroshock vs. microshock, contd

• Note the increased current density when the
  catheter is present
• This more readily disrupts cardiac conduction,
  leading to fibrillation

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Electrical safety summary

• If somethings safe only if everything works
  correctly and everyone is intelligent, then
  things are unsafe!!!
• Use properly and professionally tested equipment
• Use only clinically approved equipment on
  patients
• Dont override safety features
• If using several pieces of equipment, be sure
  that theyre all connected to the same power point

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Interference hazards

• Electrocution not the only thing that can go
  wrong
• A safe recording may still be inaccurate
• It may not be detecting the signal you want
• It may be contaminated with other signals
• Physiological signals are tiny and electrode
  leads make excellent antennas
• This lets them pick up nearby radio stations
  quite nicely
• Mobile phones can also interfere--notice those
  signs in hospitals asking you to turn them off?
• You may be interfering with not only your own
  recording but someone elses

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Troubleshooting--what to do when things dont work

• Is it plugged in? Are you sure?
• Is the power point switched on?
• Is the mains cord any good?
• Is the instrument switched on?
• This includes all the individual components
• If battery operated, is the battery charged?
• Are the cables routed to the right places?

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Troubleshooting, continued

• Are the cables any good?
• A common problem in electrophysiology!
• Try a spare if youve got one.
• Wiggle connections gently if the signal comes
  and goes, youve found the problem
• If the system comes on but doesnt run and
  contains a computer, has it booted up?

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More troubleshooting

• In general, start with most likely point of a
  problem first that is, if the power on light
  doesnt come on, its likely that the instrument
  isnt getting power (or that the light is bad),
  so check the mains related bits first.
• For example, if youre using a slit lamp and it
  doesnt light up, is the globe any good?
• Common sense may not be as common as wed like,
  but its a great asset!

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Troubleshooting--a final thought

• Whilst you may be tempted to whack the machine to
  teach it a lesson/get it to work, this delicate
  technique (known to us techies as percussive
  maintenance) should be left to professionals!