Principles of Biomedical Systems

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Principles of Biomedical Systems Devices
Safety in Clinical Environment
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Safety in Clinical Environment

• Definitions
• Safety freedom from unacceptable risk of harm.
• Basic Safety Protection against direct physical
  hazards when medical electrical equipment is used
  under normal or other reasonably foreseeable
  conditions.
• Hazard A situation of potential harm to people
  or property.
• Risk The probable rate of occurrence of a
  hazard causing harm and the degree of severity of
  the harm.
• Effectiveness the ability of an item to meet a
  service demand of given quantitative
  characteristics.
• Safety Integrity Level A qualitative measure of
  the level of assurance that a component or system
  will function as intended or, if it malfunctions,
  will do so in a safe manner

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Types of Hazards

• Electrical hazards
• Electrical shocks (micro and macro) due to
  equipment failure, failure of power delivery
  systems, ground failures, burns, fire, etc.
• Mechanical hazards
• mobility aids, transfer devices, prosthetic
  devices, mechanical assist devices, patient
  support devices
• Environmental hazards
• Solid wastes, noise, utilities (natural gas),
  building structures, etc.
• Biological hazards
• Infection control, viral outbreak, isolation,
  decontamination, sterilization, waste disposal
  issues
• Radiation hazards
• Use of radioactive materials, radiation devices
  (MRI, CT, PET), exposure control

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Types of Hazards

• Many sources of energy, potentially hazardous
  substances, instruments and procedures
• Use of fire, compressed air, water, chemicals,
  drugs, microorganisms, waste, sound, electricity,
  radiation, natural and unnatural disaster,
  negligence, sources of radiation, etc.
• Medical procedures expose patients to increased
  risks of hazards due to skin and membranes being
  penetrated / altered
• 10,000 device related injuries in the US every
  year! Typically due to
• Improper use
• Inadequate training
• Lack of experience
• Improper (lack of) use of manuals
• Device failure

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Electrical Safety

• Safety in the clinical environment Electrical
  safety
• Physiological effects of electricity
• Susceptibility parameters
• Distribution of electrical power
• Isolated power systems
• Macroshock hazards
• Microshock hazards
• Electrical safety codes and standards
• Protection
• Power distribution
• Ground fault circuit interrupters (GFCI)
• Equipment design
• Electrical safety analyzers / Testing electrical
  systems

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Physiological Effects of Electricity

• For electricity to have an effect on the human
  body
• An electrical potential difference must be
  present
• The individual must be part of the electrical
  circuit, that is, a current must enter the body
  at one point and leave it at another.
• However, what causes the physiological effect is
  NOT voltage, but rather CURRENT.
• A high voltage (K.103V) applied over a large
  impedance (rough skin) may not cause much (any)
  damage
• A low voltage applied over very small impedances
  (heart tissue) may cause grave consequences
  (ventricular fibrillation)
• The magnitude of the current is simply the
  applied voltage divided by the total effective
  impedance the current faces skin largest.
• Electricity can have one of three effects
• Electrical stimulation of excitable tissue
  (muscles, nerve)
• Resistive heating of tissue
• Electrical burns / tissue damage for direct
  current and high voltages

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Physiological Effects of Electricity
The real physiological effect depends on the
actual path of the current
Dry skin impedance93 kO / cm2 Electrode gel on
skin 10.8 kO / cm2 Penetrated skin 200 O / cm2
Physiological effects of electricity. Threshold
or estimated mean values are given for each
effect in a 70 kg human for a 1 to 3 s exposure
to 60 Hz current applied via copper wires grasped
by the hands.
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Physiological Effects of Electricity

• Threshold of perception The minimal current that
  an individual can detect. For AC (with wet hands)
  can be as small as 0.5 mA at 60 Hz. For DC, 2 10
  mA
• Let-go current The maximal current at which the
  subject can voluntarily withdraw. 6 100 mA, at
  which involuntary muscle contractions, reflex
  withdrawals, secondary physical effects (falling,
  hitting head) may also occur
• Respiratory Paralysis / Pain / Fatigue At as low
  as 20 mA, involuntary contractions of respiratory
  muscles can cause asphyxiation / respiratory
  arrest, if the current is not interrupted.
  Strong involuntary contraction of other muscles
  can cause pain and fatigue
• Ventricular fibrillation 75 400 mA can cause
  heart muscles to contract uncontrollably,
  altering the normal propagation of the
  electrical activity of the heart. HR can raise up
  to 300 bpm, rapid, disorganized and too high to
  pump any meaningful amount of blood ? ventricular
  fibrillation. Normal rhythm can only return using
  a defibrillator
• Sustained myocardial contraction / Burns and
  physical injury At 1 6 A, the entire heart
  muscle contracts and heart stops beating. This
  will not cause irreversible tissue damage,
  however, as normal rhythm will return once the
  current is removed. At or after 10A, however,
  burns can occur, particularly at points of entry
  and exit.

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Important Susceptibility Parameters

• Threshold and let-go current variability

Distributions of perception thresholds and let-go
currents These data depend on surface area of
contact, moistened hand grasping AWG No. 8 copper
wire, 70 kg human, 60Hz, 13 s. of exposure
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Important Susceptibility Parameters

• Frequency
• Note that the minimal let-go current happens at
  the precise frequency of commercial power-line,
  50-60Hz.
• Let-go current rises below 10 Hz and above
  several hundred Hz.

Let-go current versus frequency Percentile
values indicate variability of let-go current
among individuals. Let-go currents for women are
about two-thirds the values for men.
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Important Susceptibility Parameters

• Duration
• The longer the duration, the smaller the current
  at which ventricular fibrillation occurs
• Shock must occur long enough to coincide with the
  most vulnerable period occurring during the T
  wave.
• Weight
• Fibrillation threshold increases with body weight
  (from 50mA for 6kg dogs to 130 mA for 24 kg dogs.

Fibrillation current versus shock duration.
Thresholds for ventricular fibrillation in
animals for 60 Hz AC current. Duration of current
(0.2 to 5 s) and weight of animal body were
varied.
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Important Susceptibility Parameters

• Points of entry
• The magnitude of the current required to
  fibrillate the heart is far greater if the
  current is not applied directly to heart
  externally applied current loses much of its
  amplitude due to current distributions. Large,
  externally applied currents cause macroshock.
• If catheters are used, the natural protection
  provided by the skin (15 kO 2 MO) is bypassed,
  greatly reducing the amount of current reqd to
  cause fibrillation. Even smallest currents (80
  600 µA), causing microshock, may result in
  fibrillation. Safety limit for microshocks is 10
  µA.
• The precise point of entry, even externally is
  very important If both points of entry and exit
  are on the same extremity, the risk of
  fibrillation is greatly reduced even at high
  currents (e.g. the current reqd for fibrillation
  through Lead I (LA-RA) electrodes is higher than
  for Leads II (LL-RA) and III (LL-LA).

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Important Susceptibility Parameters

• Points of entry

Effect of entry points on current distribution
(a) Macroshock, externally applied current
spreads through-out the body. (b) Microshock, all
the current applied through an intracardiac
catheter flows through the heart.
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Distribution of Electrical Power
(230 V)
Simplified electric-power distribution for 115 V
circuits. Power frequency is 60 Hz
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Distribution of Power

• If electrical devices were perfect, only two
  wires would be adequate (hot and return), with
  all power confined to these two wires. However,
  there are two major departures from this ideal
  case
• A fault may occur, through miswiring, component
  failure, etc., causing an electrical potential
  between an exposed surface (metal casing of the
  device) and a grounded surface (wet floor, metal
  case of another device etc.) Any person who
  bridges these two surfaces is subject to
  macroshock.

• Even if a fault does not occur, imperfect
  insulation or electromagnetic coupling
  (capacitive or inductive) may produce an
  electrical potential relative to the ground. A
  susceptible patient providing a path for this
  leakage current to flow to the ground is subject
  to microshock.
• The additional ground line provides a good line
  of defense! (how / why?)

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GROUND!

• The additional line that is connected directly to
  the earth-ground provides the following
• In case of a fault (short circuit between hot
  conductor and metal casing), a large current will
  use the path through the ground wire (instead of
  the patient) and not only protect the patient,
  but also cause the circuit breaker to open. The
  ability of the grounding system to conduct high
  currents to ground is crucial for this to work!
• If there is no fault, the ground wire serves to
  conduct the leakage current safely back to the
  electrical power source again, as long as the
  grounding system provides a low-resistance
  pathway to the ground
• Leakage current recommended by ECRI are
  established to prevent injury in case the
  grounding system fails and a patient touches an
  electrically active surface (10 100 µA).

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Isolated Power Distribution
Not grounded !

• In fact, in such an isolated system, if a single
  ground-fault occurs, the system simply reverts
  back to the normal ground-referenced system.
• A line isolation monitor is used with such
  system that continuously monitors for the first
  ground fault, during which case it simply informs
  the operators to fix the problem. The single
  ground fault does NOT constitute a hazard!

• Normally, when there is a ground-fault from hot
  wire to ground, a large current is drawn causing
  a potential hazard, as the device will stop
  functioning when the circuit breakers open !
• This can be prevented by using the isolated
  system, which separates ground from neutral,
  making neutral and hot electrically identical. A
  single ground-fault will not cause large
  currents, as long as both hot conductors are
  initially isolated from ground!

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Macroshock

• Most electrical devices have a metal cabinet,
  which constitutes a hazard, in case of an
  insulation failure or shortened component between
  the hot power lead and the chassis. There is then
  115 230 V between the chassis and any other
  grounded object.
• The first line of defense available to patients
  is their skin.
• The outer layer provides 15 kO to 1 MO depending
  on the part of the body, moisture and sweat
  present, 1 of that of dry skin if skin is
  broken,
• Internal resistance of the body is 200O for each
  limb, and 100O for the trunk, thus internal body
  resistance between any two limbs is about 500O
  (somewhat higher for obese people due to high
  resistivity of the adipose tissue)!
• Any procedure that reduces or eliminates the skin
  resistance increases the risk of electrical
  shock, including biopotential electrode gel,
  electronic thermometers placed in ears, mouth,
  rectum, intravenous catheters, etc.
• A third wire, grounded to earth, can greatly
  reduce the effect of macroshock, as the
  resistance of that path would be much smaller
  then even that of internal body resistance!

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Macroshock Hazards

• Direct faults between the hot conductor and the
  ground is not common, and technically
  speaking, ground connection is not necessary
  during normal operation.
• In fact, a ground fault will not be detected
  during normal operation of the device, only
  when someone touches it, the hazard becomes
  known. Therefore, ground wire in devices and
  receptacles must be periodically tested.

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Macroshock Hazards
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Microshock Hazards
Small currents inevitably flow between adjacent
insulated conductors at different potentials ?
leakage currents which flow through stray
capacitances, insulation, dust and
moisture Leakage current flowing to the chassis
flows safely to the ground, if a low-resistance
ground wire is available.
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Microshock Hazards

• If ground wire is broken, the chassis potential
  rises above the ground a patient who has a
  grounded connection to the heart (e.g. through a
  catheter) receives a microshock if s/he touches
  the chassis.
• If there is a connection from the chassis to the
  patients heart, and a connection to the ground
  anywhere in the body, this also causes
  microshock.
• Note that the hazard for microshock only exists
  if there is a direct connection to the heart.
  Otherwise, even the internal resistance of the
  body is high enough top prevent the microshocks.

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Microshock via Ground Potentials
Microshocks can also occur if different devices
are not at the exact same ground potential. In
fact, the microshock can occur even when a device
that does not connected to the patient has a
ground fault! A fairly common ground wire
resistance of 0.1O can easily cause a a
500mVpotential difference if initiated due to
a, say 5A of ground fault. If the patient
resistance is lessthen 50kO, this would cause an
above safe current of 10µA
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Microshock Via Ground Potentials
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Safety Codes Standards

• Limits on leakage current are instituted and
  regulated by the safety codes instituted in part
  by the National Fire Protection Association
  (NFPA), American National Standards Institute
  (ANSI), Association for the Advancement of
  Medical Instrumentation (AAMI), and Emergency
  Care Research Institute (ECRI).

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Basic Approaches to Shock Protection

• There are two major ways to protect patients from
  shocks
• Completely isolate and insulate patient from all
  sources of electric current
• Keep all conductive surfaces within reach of the
  patient at the same voltage
• Neither can be fully achieved ? some combination
  of these two
• Grounding system
• Isolated power-distribution system
• Ground-fault circuit interrupters (GFCI)

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Grounding Systems

• Low resistance (0.15 O) ground that can carry
  currents up to the circuit-breaker ratings
  protects patients by keeping all conductive
  surfaces and receptacle grounds at the same
  potential.
• Protects patients from
• Macroshocks
• Microshocks
• Ground faults elsewhere (!)
• The difference between the receptacle grounds and
  other surface should be no more then 40 mV)

All the receptacle grounds and conductive
surfaces in the vicinity of the patient are
connected to the patient-equipment grounding
point. Each patient-equipment grounding point is
connected to the reference grounding point that
makes a single connection to the building ground.
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Isolated Power Systems

• A good equipotential grounding system cannot
  eliminate large current that may result from
  major ground-faults (which are rather rare).
• Isolated power systems can protect against such
  major (single) ground faults
• Provide considerable protection against
  macroshocks, particularly around wet conditions
• However, they are expensive !
• Used only at locations where flammable
  anesthetics are used. Additional minor protection
  against microshocks does not justify the high
  cost of these systems to be used everywhere in
  the clinical environment

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Ground Fault Circuit Interrupters (GFCI)

• Disconnects source of electric current when a
  ground fault greater than about 6 mA occurs!

When there is no fault, IhotIneutral. The GFCI
detects the difference between these two
currents. If the difference is above a threshold,
that means the rest of the current must be
flowing through elsewhere, either the chassis or
the patient !!!. The detection is done through
the monitoring the voltage induced by the two
coils (hot and neutral) in the differential
transformer!
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GFCI
The National Electric Code (NEC - 1996) requires
that all circuits serving bathrooms, garages,
outdoor receptacles, swimming pools and
construction sites be fitted with GFCI. Note that
GFCI protect against major ground faults only,
not against microshocks. Patient care areas are
typically not fitted with GFCI, since the loss of
power to life support equipment can also be
equally deadly!
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Protection through Equipment Design

• Strain-relief devices for cords, where cord
  enters the equipment and between the cord and
  plug
• Reduction of leakage current through proper
  layout and insulation to minimize the capacitance
  between all hot conductors and the chassis
• Double insulation to prevent the contact of the
  patient with the chassis or any other conducting
  surface (outer case being insulating material,
  plastic knobs, etc.)
• Operation at low voltages solid state devices
  operating at lt10V are far less likely to cause
  macroshocks
• Electrical isolation in circuit design

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Electrical Isolation

• Main features of an isolation amplifier
• High ohmic isolation between input and output
  (gt10MO)
• High isolation mode voltage (gt1000V)
• High common mode rejection ration (gt100 dB)

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Transformer Isolation Amplifiers
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Optical Isolation Amplifier
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Electrical Safety AnalyzersWiring / Receptacle
Testing

• Three LED receptacle tester
• Simple device used to test common wiring problems
  (can detect only 8 of possible 64 states)
• Will not detect ground/neutral reversal, or when
  ground/neutral are hot and hot is grounded (GFCI
  would detect the latter)

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Electrical Safety AnalyzersTesting Electrical
Appliances

• Ground-pin-to-chassis resistance Should be
  lt0.15O during the life of the appliance

Ground-pin-to-chassis resistance test
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Electrical Safety AnalyzersTesting Electrical
Appliances

• Chassis leakage current The leakage current
  should not exceed 500µA with single fault for
  devices not intended for patient contact, and not
  exceed 300 µA for those that are intended for
  patient contact.

Appliance power switch (use both OFF and ON
positions)
Open switch for appliances not intended
to contact a patient
Grounding-contact switch (use in OPEN position)
Polarity- reversing switch (use both positions)
Appliance
H (black)
To exposed conductive
H
surface or if none, then 10 by
Internal Circuitry
N
120 V
20 cm metal foil in contact
N (white)
with the exposed surface
G
Insulating surface
G (green)
I
H hot
Building ground
Current meter
N neutral (grounded)
G grounding conductor
Test circuit
This connection
is at service
I
lt 500 µA for facility
Ð
owned housekeeping and maintenance appliances
entrance or on
I
gt 300 µA for appliances intended for use in
the patient vicinity
supply side of
separately derived
system
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Electrical Safety AnalyzersTesting Electrical
Appliances

• Leakage current in patient leads
• Potentially most damaging leakage is the one with
  patient leads, since they typically have low
  impedance patient contacts
• Current should be restricted to 50µA for
  non-isolated leads and to 10 µA for isolated
  leads (used with catheters / electrodes that make
  connection to the heart)
• Leakage current between any pair of leads, or
  between a single lead and other patient
  connections should also be controlled
• Leakage in case of line voltage appearing on the
  patient should also be restricted.

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Leakage current Testers
Test for leakage current from patient leads to
ground
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Leakage Current testers
Test for leakage current between patient leads
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Leakage Current Testers
Test for ac isolation current
Isolation current is the current that passes
through patient leads to ground if and when line
voltage appears on the patient. This should also
be limited to 50µA