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

IntroductionElectrical 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 contactResulting 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.

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

Microshock (Cardiac Shock)

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

ConditionSkin resistance per square

centimeter of electrode

Dry Skin

Electrode gel on skin

Penetrated skin

93 kΩ

10.8 kΩ

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.

The macroshock situations that can develop with this equipment are illustrated by the following situations.

Two-Wire Macroshock 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 per-son. – 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.