HAZARD OF MEDICAL INSTRUMENT by Mohd Yusof Baharuddin

HAZARD OF MEDICAL INSTRUMENT by Mohd Yusof Baharuddin

Objectives Ø Explain and discuss on the main hazard connected with the used of medical instruments. Ø Discuss occurrence between macroshock and microshock

Introduction Ø Electrical shock involves electrical stimulation of tissue Ø Effects range from tingling sensation to the violent reactions of muscle tetanus to ventricular fibrillation Ø Measured in terms of current intensity at specific frequency

Macroshock Ø Define as high value current level (m. A) which passes arm to arm through body by skin contact with a voltage source Ø Must be 2 points of body contact Ø Resulting current eventually passes through heart & may cause ventricular fibrillation or death

Macroshock ØSometime define as the undesirable effect of a current greater than 5 m. A at 60 cycles applied to the surface of the body

Microshock (Cardiac Shock) Ø Define as low level current (u. A) which passes directly through the heart via a needle or catheter in artery or vein Ø The catheter may touch the interior surface of the heart where blood pressure is measured or cardiac pacing is effected.

Microshock (Cardiac Shock) ØSometime define as undesirable effect of a current greater than 10 u. A applied directly to the heart ØHazard only to patients who are in a critical care situation because the current must be applied directly to the heart

Shock Ø Define in term of current because the voltages produce the current are highly variable Ø Variance in voltage caused by wide variation in skin resistance among individuals and clinical situation Ø 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 m. A, Is between two surface electrodes for each case : dry skin, electrode gel treated skin and penetrated skin. Given the electrode area is 15. 5 cm 2.

Macroshock Hazard Ø Occurs more often with 2 wire system than 3 wire system Ø 2 wire equipment Ø Dangerous to get between hot H and neutral N wires. Ø Touching H & N simultaneously with two limbs can direct currents through vital organs of circulation & respiration Ø Because N are internally grounded, touching H & G can produced macroshock Ø Example : Inexpensive AC / DC radio

Two-Wire Macroshock Situations Ø 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 Ø The macroshock situations that can develop with this equipment are illustrated by the following situations.

Two-Wire Macroshock Situations

Ø 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.

Ø 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.

Ø 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.

Ø 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.

Ø 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.

Ø 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.

Ø In part (g), the macroshock is delivered as the patient touches the H wire and ground through the radiator.

Three-Wire Macroshock Situations Ø 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.