Transcript hazard of medical instrument

HAZARD OF MEDICAL INSTRUMENT
by
Mohd Yusof Baharuddin
Objectives
Explain and discuss on the main
hazard connected with the used of
medical instruments.
Discuss occurrence between
macroshock and microshock
Introduction
Electrical shock involves electrical
stimulation of tissue
Effects range from tingling sensation
to the violent reactions of muscle
tetanus to ventricular fibrillation
Measured in terms of current
intensity at specific frequency
Macroshock
Define as high value current level (mA)
which passes arm to arm through body
by skin contact with a voltage source
Must be 2 points of body contact
Resulting current eventually passes
through heart & may cause ventricular
fibrillation or death
Macroshock
Sometime define as the
undesirable effect of a current
greater than 5 mA at 60 cycles
applied to the surface of the
body
Microshock (Cardiac Shock)
Define as low level current (uA)
which passes directly through the
heart via a needle or catheter in
artery or vein
The catheter may touch the interior
surface of the heart where blood
pressure is measured or cardiac
pacing is effected.
Microshock (Cardiac Shock)
Sometime define as undesirable
effect of a current greater than
10 uA applied directly to the
heart
Hazard only to patients who are
in a critical care situation
because the current must be
applied directly to the heart
Shock
Define in term of current because
the voltages produce the current are
highly variable
Variance in voltage caused by wide
variation in skin resistance among
individuals and clinical situation
Example : skin resistance @ 60
cycles may vary from 93 kΩ down to
200 Ω.
Skin resistance @ 60 Hz
Condition
Skin resistance per square
centimeter of electrode
93 kΩ
Dry Skin
Electrode gel on skin
10.8 kΩ
Penetrated skin
200 Ω
Table 1
Example 1
Using the skin resistance in Table 1,
compute the voltage levels that
would deliver a macroshock current
of 5 mA, Is between two surface
electrodes for each case : dry skin,
electrode gel treated skin and
penetrated skin. Given the electrode
area is 15.5 cm2.
Macroshock Hazard
 Occurs more often with 2 wire system than 3
wire system
 2 wire equipment
 Dangerous to get between hot H and neutral N
wires.
 Touching H & N simultaneously with two limbs
can direct currents through vital organs of
circulation & respiration
 Because N are internally grounded, touching H &
G can produced macroshock
 Example : Inexpensive AC / DC radio
Two-Wire Macroshock
Situations
Two-wire, power-cord-energized
equipment that is not double-insulated,
and on which the plug is reversible in
its receptacle, is extremely hazardous.
– Unfortunately, much commercial
equipment falls into this category.
Two-Wire Macroshock
Situations
The macroshock situations that can
develop with this equipment are
illustrated by the following situations.
Two-Wire Macroshock
Situations
In part (a) of the figure, a conductive
fault has developed between the H
lead and the P lead connected to the
patient.
– When the patient completes the circuit
by touching the chassis, which is
connected to the N lead, the patient
receives a hair-raising macroshock.
The same thing happens in part (b),
except this time the patient completes
the circuit by touching the radiator.
– The radiator is grounded because it is
metal and filled with water. The N wire is
also attached to ground at the power
line service box; this completes the
circuit and gives the patient a
macroshock.
In part (c), the patient is shocked
because the plug happens to be
reversed in its socket and the H lead
gets connected to the chassis that
the patient is touching while holding
the radiator at the same time, which
completes the circuit to ground.
In part (d), the patient is in the same
position and gets shocked because
the H wire has a conductive fault to
the chassis.
– The fuse did not blow out in this case
because the N wire is not connected to
the chassis, completing the fault circuit
to the fuse.
In part (e), the patient gets shocked
because, with the same kind of
conductive fault, the patient
completes the circuit between the N
wire and the chassis.
In part (f), the patient gets shocked
because the patient gets across the
H wire and the chassis, which is
connected to the N wire, completing
the circuit through the patient.
In part (g), the macroshock is
delivered as the patient touches the
H wire and ground through the
radiator.
Three-Wire Macroshock
Situations
Macroshock situations are fewer and
more improbable when the
equipment has a three-wire plug.
Picture retrieved from Wikipedia – AC power plugs & socket
• Part (a) illustrates a shock being
delivered when the H wire and the N
wire are touched simutaneously.
• Likewise, in part b, the person
receiving a macroshock is on the H
wire and the grounded chassis.
Such situations could result from a
frayed power cord.
• Part (c) illustrates an H wire
conductive fault to the chassis that
does not cause a macroshock
because both the chassis and the
radiator are grounded and no
potential appears across the person.
– If such a fault were a short circuit, a
circuit breaker would trip, or a fuse
would blow out, removing the high
voltage from the chassis.
• In part (d), the same situation as in
part (c) only with the G wire also
open in a fault results in a
macroshock.
– Notice that two failures had to occur to
induce a macroshock in this case,
lowering the probability of this
happening.
• In part (e), a conductive fault to a
patient lead connected to a patient
introduces a macroshock, when the
patient touches ground in the
radiator.
• In part (f), the macroshock comes
when the patient touches the
chassis, which is grounded.
• Notice how the three-wire power
cord gives more protection against
macroshock than the two-wire cord.
– It protects against conductive faults to
the chassis.
• It also prevents faults due to reversing the
plug in the receptacle, because it can be
inserted in only one way.