HAZARD OF MEDICAL INSTRUMENT II by Mohd Yusof Baharuddin

Three-Wire Microshock Situations

A microshock affects the patient when leakage from the H wire gets to the P line, either from a stray capacity, dirt, fluids, or bad insulation. This leakage current goes directly to the heart through an insulated catheter (C). In this case, the circuit is completed because the patient is contacting the chassis.

In part (b), the leakage current flows through the patient and back to ground through a second instrument.

In part (c), the H wire opens on one instrument, and the N wire opens on the other instrument. Microshock does not occur because the power is simply removed by these faults and no excessive leakage current is generated.

In part (d), an open G- wire in the instrument on the left causes an increase in P lead leakage and causes a microshock.

The three-wire power cord gives considerable protection against macroshock, but it is not so effective against microshock.

Two-Wire Microshock Situations

In figure (a), the patient coming in contact with the two grounded chassis with the two-wire plug receives a microshock because of voltage elevation due to high current in the N wire.

That voltage elevation does not exist in the three-wire case illustrated in figure (b) because the G wire does not normally carry a significant current. Thus, the patient does not receive a microshock due to the protection of the three-wire power cord.

Attendant-Mediated Microshock Microshock is insidious because it cannot be felt and leaves no tract in the affected tissue. It is not large enough to stimulate a perceptible number of pain cells to give warning. Therefore, an attendant can pass a microshock to a patient without being aware, except by observing the symptoms of cardiac arrhythmia in the patient.

In figure (a), the attendant completes the circuit to a leaky patient lead by holding it while touching the patient’s catheter.

In part (b), the attendant completes the circuit by touching a piece of equipment with a voltage elevation due to a faulty power cord.

In both cases, the microshock current would pass through the attendant without his or her awareness.

Figure (c) illustrates the case where the attendant provides the path for the leakage current by touching the patient’s body at a place other than the catheter. In this case, the attendant grounds the patient to complete the path for the leakage.

 The basic defense of the patient against attendant- mediated microshock is to have the attendant wear insulating gloves whenever touching a patient with a CVC (central vessel catheter), including an external pacemaker. Also, the attendant should touch a water pipe or a known grounding point before touching a patient with a CVC. The attendant should also touch the patient skin-to-skin at a site away from the catheter, in order to neutralize any electrostatic charge on either of them.

This action dissipates any electrostatic charge that may have accumulated. This precaution is made in addition to the use of antistatic garments, bed sheets, blankets, and sterile drapes.

Microshock for Ground Wire Currents  The three-wire plug on equipment protects patients against certain kinds of macroshock. However, it is not as effective in protecting against microshock.

The figure illustrates a case where the faulty equipment on the top causes a large current to flow in the G wire. That equipment may not even be in the same room. An air conditioner on the roof.

The large ground currents from that equipment may cause enough voltage elevation between the two devices connected to the patient to result in a microshock.

The defense against such microshock is to use a grounding strap between all pieces of equipment grounded to the patient. As an added precaution, the room may have its own electrical circuit to the service entrance of the power line. Any ground currents would be generated in the room only.

Summary Electrical Shocks Produced by current, not voltage Amount of current dependant on body resistance Human body resistance can range between 1000 ohms and 1,000,000 ohms, depending on body mass, moisture content, and area of contact

Macroshock vs Microshock Macroshock current is distributed somewhat evenly through body parts Microshock current path is through a single point, usually the heart Microshock can be fatal at levels that would be imperceptible if applied to skin

Macroshock Electrical current that leaks from a broken cord or piece of equipment When passing from hand to hand, only about 5% of the current passes through the heart When passing from leg to leg, no current passes through the heart

Microshock Term used to describe the very low level shocks that go undetected Dangerous to an “electrically sensitive” patient – patient with breaks in skin like abrasions, wet dressings, pacemakers, or monitoring lines connected to a transducer Path of current with an intra-cardiac electrode